Lecture 2 - Genes Flashcards
(199 cards)
0
Q
Repetitive Sequence DNA
A
Regions of non-coding DNA found only in euk.
1
Q
Gene
A
- Series of DNA nucleotides that generally codes for the production of a single polypeptide or mRNA, rRNA, or tRNA
- Referred to as a unique sequence DNA - unique sequence DNA dominates
2
Q
Eukaryotes and Genes
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Have more than one copy of some genes
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Q
Prokaryotes and Genes
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Have only one copy of each gene
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Q
Euchromatin
A
Regions of DNA associated with eukaryotic genes that are being actively transcribed by a cell
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Q
Heterochromatin
A
- Tightly packed regions of DNA associated with genes not being actively transcribed
- Repetitive sequence DNA is found mainly in heterochromatin
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Q
ONE GENE = ONE POLYPEPTIDE … What’s the exception?
A
Posttranscriptional processing RNA
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Q
Genome
A
- Entire DNA sequence of an organism
- Btwn 26,000 to 38,000 genes in the human genome
- Only a little over 1% of a human genome actually codes for protein
- Variation of nucleotide sequence among humans is small, 0.08%
8
Q
The Central Dogma of Gene Expression
A
- DNA is transcribed to RNA, which is translated to amino acids forming a protein
- DNA –> RNA –> Protein
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Q
DNA
A
- Deoxyribonucleic acid
- polymer of nucleotides
- DNA nucleotides differ only in their nitrogenous bases
- The end 3’ C is attached to an -OH group
- The end 5’ C is attached to a phosphate group
- Double stranded structure, antiparallel, bound by H bonds btwn nitrogenous bases
- 2 strands that match w/ correct b.p. are complementary strands and curl into a double helix
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Q
What are the nitrogenous bases that exist in DNA?
A
Adenine, guanine, cytosine, and thymine
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Q
How are the nucleotides bound in DNA?
A
Phosphodiester bond between the 3rd C of one deoxyribose and the 5th C of the other creating the sugar-phosphate backbone of a single strand of DNA w/ 5’ to 3’ directionality
12
Q
Base-pairing is referred to as…?
A
Hydrogen bonding
13
Q
How is the length of DNA measured?
A
In base-pairs
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Q
Double Helix
A
- Contain major and minor groove
- Each groove spirals once around double helix for every 10 b.p
- Diameter is ~ 2 nm or 13x the diameter of a C atom
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Q
Purines
A
- nucleotides
- 2 ring structures
- adenine and guanine
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Q
Pyrimidines
A
- nucleotides
- single ring structures
- thymine and cytosine
- uracil in RNA
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Q
Purine and Pyrimidine Pairs
A
- A forms 2 H-bonds with T
- G forms 3 H-bonds with C
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Q
Replication
A
- A cell replicates its DNA one time in each life cycle
- DNA replication is semi-conservatie
- Governed by a group of proteins called replisome
- Begins toward the middle of chromosome at site called origin of replication
- Bidirectional process
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Q
Semi-Conservative
A
When a new double strand is created, it contains one strand from the original DNA and one newly synthesized strand
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Q
Bidirectional Process
A
Proceeds in both directions from an origin, each direction produces a leading and lagging strand
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Q
Replication Fork
A
The point where a replisome is attached to the chromosome
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Q
Origins of Replication in Prokaryotes vs. Eukaryotes
A
- A single eukaryotic chromosome contains multiple origins, while replication in prokaryotes takes place for a single origin on a circular chromosome
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Q
DNA Helicase
A
As part of the replisome, it unwinds the double helix separating the two strands
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DNA Polymerase
- Enzyme that builds the new DNA strand
- Cannot initiate a strand from 2 nucleotides, but can only add nucleotides to an existing strand
* Requires an RNA primer to get started
- Adds deoxynucleotides to the primer and moves along each DNA strand creating a complimentary strand
* Reading DNA is 3' to 5' , synthesizing DNA is 5' to 3'
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Primase
- RNA polymerase
| - creates an RNA primer approx. 10 ribonucleotides long to initiate the strand
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Downstream
5' to 3'
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Upstream
3' to 5'
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Pyrophosphate Group
- Two phosphates bounded together
- Addition of each nucleotide to the new strand requires removal of a pyrophosphate group from a deoxynucleotide triphosphate
- Some of the energy from the hydrolysis of the pyrophosphate is used to drive replication
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Leading Strand
continuous new strand
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Lagging Strand
- Interrupted Strand, restarted with new primer
| - Made from series of disconnected strands called Okazaki fragments
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Okazaki Fragments
~ 100 to 200 nucleotides in eukaryotes and ~ 1000 to 2000 nucleotides in prokaryotes
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DNA Ligase
Moves along lagging strand & ties okazaki fragments together to complete polymer
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Semidiscontinuous
The process of replication is said to be since the formation of one strand is continuous and the other fragmented
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Telomeres
- Ends of eukaryotic chromosomal DNA possess them
- Repeated 6 nucleotide units from 100 to 1,000 units long
- Protect the chromosomes from being eroded through repeated rounds of replication
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Steps of Replication
1. Helicase unzips the double helix
2. RNA polymerase builds a primer
3. DNA polymerase assembles the leading & lagging strands
4. The primers are removed
5. Okazaki Fragments are joined
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RNA
- Ribonucleic acid
- Can move through nuclear pores and isn't confined to the nucleus, unlike DNA
- Exists in 3 forms: mRNA, rRNA, tRNA
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Structure of RNA
Identical to DNA in structure except:
1. Carbon #2 on the pentose is not "deoxygenated", it has a hydroxyl group attached
2. single stranded
3. RNA contains the pyrimidine uracil instead of thymine
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mRNA
- messenger RNA
| - Delivers the DNA code for amino acids to the cytosol where the proteins are manufactured
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rRNA
- ribosomal RNA
- combines with proteins to form ribosomes
- synthesized in the nucleolus
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Ribosomes
cellular complexes that direct the synthesis of proteins
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tRNA
- Transfer RNA
| - Collects amino acids in the cytosol and transfers them to the ribosomes for incorporation into a protein
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What does the similarity between uracil and thymine cause in DNA?
Common cause of mutations in DNA
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Differences between DNA and RNA
1. DNA made from deoxyribose; RNA made from ribose
2. DNA is double stranded; RNA is single stranded
3. DNA has thymine; RNA has uracil
4. DNA is produced by replication; RNA is produced by transcription
5. In animals, DNA is only in nucleus and mitochondria; RNA is also in the cytosol
6. One major type of DNA; 3 major types of RNA
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Transcription
- All RNA is made from a DNA template in this process
- It must take place in the nucleus or mitochondrial matrix b/c DNA can't leave there
- The steps are: Initiation, Elongation, and Termination
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Initiation of Transcription
- A group of proteins called initiation factors finds promotor on DNA strand, and assembles a Tx initiation complex, which includes RNA polymerase.
- After binding to promoter, RNA poly unzips DNA double helix creating a Tx bubble.
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RNA Polymerase
- Prokaryotes have one type
| - Eukaryotes have 3; one for each RNA.
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Promoter
- A sequence of DNA nucleotides that designates a beginning point for Tx.
- In prokaryotes, its located at the beginning of the gene
- The Tx start point is part of the promoter
- The 1st b.p. located at the Tx start point is designated +1
- B.p. located before the start point such as those in the promoter are designated by - #'s
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Consensus Sequence
- Most commonly found nucleotide sequence of a promotor recognized by the RNA poly of a given species
- Variation from it causes RNA poly to bond less tightly and less often to a given promoter, which leads to those genes being transcribed less frequently
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Elongation of Transcription
- RNA poly transcribes only 1 strand of the DNA nucleotide sequence into a complementary RNA nucleotide sequence
* Only 1 strand in a molecule of double stranded DNA is transcribed
* This strand is called the template strand or antisense strand
* The other strand, called the coding strand or sense strand, protects its partner against degradation.
- RNA poly moves along the DNA strand 3' to 5', building the new strand 5' to 3'
- Tx proceeds 10x more slowly than DNA replication
- RNA poly doesn't have proofreading & rate of error of Tx is higher than replication
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Termination of Transcription
- End of Tx
| - Requires special termination sequence and special proteins to dissociate RNA poly from DNA
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Genetic Regulation
- Replication makes no distinction between genes
| - Genes are activated/deactivated at the level of Tx via proteins called activators and repressors
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Why are genes activated/deactivated at the level of Tx?
1. mRNA has a short half life in cytosol, after Tx is over, it's degraded & protein is no longer translated
2. Many proteins can be transcribed from a single mRNA, so there is an amplifying effect
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Activators and Repressors
- Bind to DNA close to the promoter, and either activate or repress the activity of RNA poly
- Allosterically regulated by small molecules such as cAMP
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Primary Function of Gene Regulation In Prokaryotes
- To respond to the environmental changes
| - Changes in gene activity are a response to the concentration of specific nutrients in & around the cell
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Primary Function of Gene Regulation in Multicellular Organisms
- Lack of change or homeostasis is the hallmark of multicellular organisms
- To control the intracellular and extracellular environments of the body
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Prokaryotic vs. Eukaryotic mRNA
- Prok. mRNA includes several genes in a single transcript - polycistronic
- Euk. mRNA includes only one gene per transcript - monocistronic
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Operon
- Sequence of bacterial DNA
- The genetic unit, usually consisting of the operator, promoter, and genes that contribute to a single prokaryotic mRNA
- Ex: lac operon
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Lac Operon
- Codes for enzymes that allow E. coli to import and metabolize lactose when glucose isn't present in sufficient quantities
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How does the lac operon work?
- low glucose levels lead to high cAMP levels --> cAMP binds & activates CAP --> CAP binds to site adjacent to promoter on lac operon --> promoter is activated & allows Tx & Ts of 3 proteins --> operator serves as binding site for lac repressor --> lac repressor is inactivated by presence of lactose in cell --> lac repressor will bind to operator unless lactose binds to lac repressor
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Location CAP site & Operator of Lac Operon
- CAP site is located upstream to promoter
| - Operator on lac operon is located downstream to promoter
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Genes of Operon
- Transcribed on one mRNA
| - Genes outside operon may code for activators and repressors
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Enhancers
- Commonly used by eukaryotes b/c their gene regulation is more complicated, involving interaction of many genes
- Their function is similar to activators/repressors, but they act at a much greater distance from the promoter
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Post-Transcriptional Processing of RNA in Eukaryotes
- Each type of RNA undergoes it
| - Allows for additional gene regulation
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Post-Transcriptional Processing in Prokaryotes
- rRNA & tRNA go through it, but mRNA is directly translated to protein
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Primary Transcript
- The initial mRNA nucleotide sequence arrived at through transcription
- AKA pre-mRNA or hnRNA
- longer than mRNA that will be translated into a protein
- Before leaving nucleus, it's cleaved into introns and exons
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How is the primary transcript processed?
3 ways:
1. addition of nucleotides
2. deletion of nucleotides
3. modification of nitrogenous bases
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5' Cap
- Even before eukaryotic mRNA is completely transcribed, its 5' end is capped using GTP
- 5' cap serves as attachment site in protein synthesis & as protection against degradation by exonucleases
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Polyadenylation of 3' End
3' end is polyadenylated with a poly A tail to protect it from exonucleases
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snRNPs
- Enzyme-RNA complexes
- Recognize nucleotide sequences at the ends of the introns
- Several associate with proteins to form a complex called a splicesome
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Splicesome
- Inside, the introns are looped bringing the exons together
| - The exons are spliced together to form the single mRNA strand that ultimately codes for a polypeptide
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Introns
- Introns don't code for proteins and are degraded within the nucleus
- Intron sequences are much longer than exon sequences
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Exons
- Exons of some genes may be spliced in different order allowing them to code for different polypeptides
- Average # of exons per gene is 7
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Denaturation of DNA
- Separation of the 2 strands of the double helix
| - caused by heat, high [ ] salt solution, or high pH solution
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Melting Temperature (Tm)
- Temp needed to separate DNA strands
- G & C base pairs have a greater Tm (b/c have 3 H-bonds)
- Heating to 95 degrees celsius is generally enough to denature any DNA sequence (just below boiling point of water)
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DNA vs. Denatured DNA
- Denatured DNA is less viscous, denser, and more able to absorb UV light
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Nucleic Acid Hybridization
- When denatured, DNA prefers to be double stranded and will look for a complementary partner
- Double stranded combinations can be formed through nucleic acid hybridization: DNA-DNA, DNA-RNA, & RNA-RNA
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Hybridization Techniques
Allows scientists to identify nucleotide sequences by binding a known sequence with an unknown sequence
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Restriction Enzymes
- AKA restriction endonucleases
- Digest or cut nucleic acid only at certain nucleotide sequence along the chain called a restriction site or recognition sequence.
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How do bacteria use restriction enzymes?
- Bacteria cut the viral DNA into fragments with restriction enzymes to defend themselves from viruses.
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How do bacteria protect their own DNA from restriction enzymes?
- By methylation (adding -CH3)
| - Methylation is usually associated with inactivated genes
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Palindromic Sequence
- A restriction site is typically a palindromic sequence 4-6 nucleotides long
- Reads the same backwards as forwards
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Sticky Ends
- Most restriction enzymes cleave DNA strand unevenly, leaving complementary single stranded ends, these ends are called sticky ends
- These ends can reconnect through hybridization
- Once paired, the phosphodiester bonds of the fragments can be joined by DNA ligase
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Recombinant DNA
- Two DNA fragments cleaved by the same endonucleases that can be joined together regardless of the origin of the DNA
- Artificially recombined
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Recombinant DNA & Bacteria
- Recombinant DNA can be made long enough for bacteria to replicate & then placed within the bacteria using a vector
- The bacteria can then be grown in large quantities forming a clone of cells containing the vector with the recombinant DNA fragment
- The clones can be saved separately forming a clone library
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Vector
A plasmid or an infective virus
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Clone Library
- Not all bacteria take up the vector & not all vectors take up the DNA fragment
- By including in the original vector a gene for resistance to a certain antibiotic and the lacZ gene (enables bacteria to metabolize the sugar X-gal), libraries can be screened.
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Screening Clone Libraries
- When an antibiotic is added to a library, clones w/o resistance will be eliminated. (screens for clones that didn't take up vector)
- Clones w/ an active lacZ gene turn blue in the presence of X-gal, clones w/ the cleaved form of the gene don't turn blue, clones w/ the DNA fragment will not turn blue when placed on medium w/ X-gal. (an endonuclease is also inserted to inactivate the lacZ gene)
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Probe
The radioactively labeled complementary sequence of the desired DNA fragment
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Complementary DNA
- AKA cDNA
- Eukaryotic DNA contains introns and bacteria have no mechanism for removing introns
- Therefore its useful to clone DNA w/ no introns
- In order to do this, the mRNA produced by the DNA is reversed transcribed using reverse transcriptase -> The DNA product is cDNA
- Adding DNA polymerase to cDNA produces a double strand of the desired DNA fragment
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Polymerase Chain Reaction (PCR)
- Fast way to clone DNA
| - By heating to denature then cooling
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Southern Blotting
- Technique used to identify target fragments of known DNA sequence in a large population of DNA
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Process of Southern Blotting
- The DNA is cleaved into restriction fragments
- Fragments are separated according to size by gel electrophoresis (larger move slower)
- Gel is made alkaline to denature DNA
- Membrane is used to blot gel which transfers the separated single stranded DNA fragments
- A radio-labeled probe w/ a complementary nucleotide sequence is added to membrane
- The probe hybridizes w/ & marks the target fragment, and then visualized w/ radiographic film
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Northern Blot
- Same as southern blot but w/ RNA
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Western Blot
Detects a protein with antibodies
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Restriction Fragment Length Polymorphisms (RFLP)
- Identifies individuals as opposed to specific genes
| - The DNA fingerprints used to identify criminals in court cases
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Single Nucleotide Polymorphisms (SNPs)
- Genome of one human differs from genome of another at about one nucleotide in every 1000
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Genetic Code
mRNA nucleotides are strung together to form a genetic code which translates the DNA nucleotide sequence into an A.A. sequence, and ultimately into a protein
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Degenerative
More than one codon can code for an A.A.
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Unambiguous
Any single series of 3 nucleotides will code for only 1 A.A.
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Codon
3 consecutive nucleotides on a strand of mRNA
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Stop Codons
- don't code for A.A.
- UAA, UGA, & UAG
- Signal an end to protein synthesis
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Start Codons
- AUG
| - Also acts as codon for methionine
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How are sequences of RNA nucleotides written?
5' to 3'
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Figuring Out Probability of Possible A.A. Sequences
```
Base = how many different ones there are
Exponent = how many there are
```
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Translation
- Process of protein synthesis directed by mRNA
| - Each of the 3 RNA's play a role in Ts
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Role of mRNA
The template which carries the genetic code from the nucleus to the cytosol in the form of codons
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Role of tRNA
- Contains a set of nucleotides that is complementary to the codon, called the anticodon
- sequesters the A.A. that corresponds to its anticodon
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Role of rRNA
- w/ protein makes up the ribosome, which provides the site for Ts to take place
- Actively participates in Ts process
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Ribosome
- composed of small & large subunit made from rRNA & many separate proteins
- Prokaryotic ribosomes are smaller than eukaryotic ribosomes
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What are ribosomes and their subunits measured in?
Terms of sedimentation coefficients given in svedberg units (s)
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Eukaryotic Ribosome
- 40s & 60s subunits, combined 80s
| - manufactured in the nucleolus & exported separately to cytoplasm
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Prokaryotic Ribosome
- 30s & 50s, combined 70s
| - Don't have nucleolus but synthesis is similar to eukaryotes
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Initiation of Translation
- After posttranscriptional processing in a eukaryote, mRNA leaves the nucleus & enters cytosol
- The 5' end attaches to the small subunit w/ the help of initiation factors (proteins)
- A tRNA possessing the 5'-CAU-3' anticodon sequesters the A.A. methionine & settles in at the P site (peptidyl site)
- This is the signal for the large subunit to join & form the initiation complex
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Elongation of Translation
- A tRNA w/ its corresponding A.A. attaches to the A site (aminoacyl site) at the expense of 2 GTPs.
- The C-terminus (carboxyl end) of methionine attaches to the N-terminus (amine end) of the A.A. @ the A site in a dehydration rxn catalyzed by peptidyl transferase
- Then translocation occurs which requires 1 GTP
- The tRNA that carried methionine moves to the E site where it can exit the ribosome
- The tRNA carrying the newly formed dipeptide moves to the P site, clearing the A site for the next tRNA
- Elongation repeats until stop codon reaches P site
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Translocation
The ribosome shifts 3 nucleotides along the mRNA toward the 3' end
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Termination of Translation
- When a stop (or nonsense) codon reaches the A site, proteins known as release factors bind to the A site, allowing a water molecule to add to the end of the polypeptide chain
- The polypeptide is freed from the tRNA & ribosome, & the ribosome is broken up into its subunits to be used again for another round of protein synthesis
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Folding
- As the polypeptide is being Ts, it begins folding
- The A.A. sequence determines the folding conformation
- The folding process is assisted by proteins called chaperones
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Post Translational Modification
- Sugars, lipids, or phosphate groups may be added to an A.A.
- Polypeptide may be cleaved in 1 or more places
- Separate polypeptides may join to form quaternary structure of a protein
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Polypeptides Injected Into Lumen
- Ts beings on a free floating ribosome
- A signal peptide @ the beginning of the translated polypeptide may direct the ribosome to attach to the ER, in which case the polypep is injected into the lumen.
- Polypeps injected into the lumen may be secreted from the cell via the golgi or may remain partially attached to the membrane
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Signal-Recognition Particle (SRP)
Carries entire ribosome complex to a receptor protein on the ER
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Mutation
- Any alteration in genome that isn't genetic recombination
| - May occur @ chromosomal or nucleotide level
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Gene Mutation
Alteration in the sequence of DNA nucleotides in a single gene
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Chromosomal Mutation
When structure of chromosome is changed
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Somatic vs. Germ Cell Mutations
Mutations in somatic cells are not passed to offspring, mutations in germ cells are
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Spontaneous Mutation
- Due to random errors in the natural process of replication & genetic recombination
- Have same effect on cell as an induced mutation
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Induced Mutations
- Due to physical or chemical agents called mutagens
| - Have same effect on cell as spontaneous mutation
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Mutagen
Physical or chemical agent that increases the frequency of mutation above the frequency of spontaneous mutations
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Point Mutation
A mutation that changes a single base-pair of nucleotides in a double strand of DNA
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Example of a Point Mutation
- Base-Pair substitution mutation: One b.p. is replaced by another
- can be a transition mutation: A-T --> G-C
- can be a transversion mutation: A-T --> T-A
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Missense Mutation
- Base-pair mutation that occurs in the A.A. coding sequence of a protein, & an alteration of a single A.A. may or may not have effect on the function of a protein
- Ex: Sickle Cell Anemia
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Frameshift Mutation
- Insertion or deletion may result in this
- Results when deletions or insertions occur in multiples of 3
- Often result in a completely nonfunctional protein
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Nonsense Mutation
- If a base-pair substitution or an insertion or deletion mutation creates a stop codon
- Very serious -> stops Ts of a functional protein entirely
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Chromosomal Deletions
Portion of chromosome breaks off, or is lost during homologous recombination and/or crossing over events
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Duplications
When a DNA fragment breaks free of 1 chromosome & incorporates into a homologous chromosome
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Aneuploidy
- When deletion/duplication occurs with entire chromosome
| - Example: Down syndrome -> 3 copies of chromosome 21
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Polyploidy
When deletion/duplication occurs with sets of chromosomes
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Translocation Mutation
- When a segment of DNA from 1 chromosome is inserted into another chromosome
- can be caused by transposition
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Inversion Mutation
- The orientation of a section of DNA is reversed on a chromosome
- can be caused by transposition
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Transposition
- takes place in prokaryotes and eukaryotes
| - one mechanism by which a somatic cell of a multicellular organism can alter its genetic make up without meiosis
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Transposable Elements/Transposons
- DNA fragments
- can excise themselves from a chromosome & reinsert themselves at another location
- can contain one gene, several genes, or just a control element
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Forward & Backward Mutations
Refer to an already mutated organism that is mutated again
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Forward Mutation
Changes the mutated organism even more from its original state
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Backward Mutation
Tending to revert the mutated organism back to its original state
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Wild Type
The original state
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MUTATION FLOW CHART
---
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Cancer
Unrestrained & uncontrolled growth of cells
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Tumor
Mass of cancer cells
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Proto-onco-genes
Certain genes that stimulate normal growth in human cells
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Oncogenes
- Proto-onco-genes can be converted to oncogenes
| - genes that cause cancer by mutagens such as UV radiation, chemicals, or random mutations
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Carcinogens
Mutagens that can cause cancer
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How long is all the DNA in a single cell when it is stretched out?
5 ft
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Histones
- Globular proteins that sections of DNA that aren't in use are wrapped tightly around
- basicity of them gies them a net positive charge at normal pH of the cell
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Nucleosome
- 8 histones wrapped in DNA
| - Wrap into coils called solenoids, which wrap into supercoils
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Chromatin
- The entire DNA/protein complex (including a very small amount of RNA)
- 1/3 DNA by mass, 2/3 protein, small amount of RNA
- when transcribed, it must be uncoiled
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Heterochromatin
Condensed chromatin
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Constitutive Heterochromatin
Permanently coiled chromatin
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Euchromatin
- Chromatin that can be uncoiled & transcribed
| - only coiled during nuclear division
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Order From Chromsome to Histone
Chromsome -> supercoil w/in chromosome -> coiling w/in supercoil -> chromatin fiber -> nucleosome w/in chromatin -> Histone & DNA w/in nucleosome
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Chromsomes
- In nucleus of human somatic cell, there are 46 double stranded DNA molecules
- The chromatin associated w/ each one of these molecules is called a chromosome
- Each chromosome contains hundreds or thousands of genes
- Before & after replication = 46 chromosomes in nucleus of human cells
- The duplicates can be referred to separately as sister chromatids.
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Homologues
- In human cells, each chromosome possesses a partner that codes for the same traits as itself, they are called homologues
- Traits are the same, but actual genes may be different
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Diploid
Any cell that contains homologous pairs
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Haploid
Any cell that doesn't contain homologues
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Cell Life Cycle (Somatic Cell of Multicellular Organism)
- Divided into 4 stages:
1. The first growth phase (G1)
2. Synthesis (S)
3. Second growth phase (G2)
4. Mitosis or meiosis (m)
5. Cytokinesis (c)
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Interphase
Collectively G1, S, & G2
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G1
- Cell splits & begins to grow producing new organelles & proteins
- Heterochromatin unwound & decondensed into euchromatin
- RNA & protein synthesis are very active
- Checkpoint @ end to see if conditions favorable for division
- Normally the longest stage
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If the conditions are favorable in G1 phase, what phase does it enter?
S Phase
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If conditions are not favorable in G1 phase, what phase does it enter?
G0 phase
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What is the main factor triggering beginning of S phase from G1?
Cell size based upon ratio of cytoplasm to DNA
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G0
- nongrowing state distinct from interphase
- allows for the differences in length of the cell cycle
- mature neurons & muscle cells remain in G0 permanently
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S Phase
- energy devoted to replicating DNA
- organelles & proteins produced more slowly
- Each chromosome is exactly duplicated - by convention cell still considered to have same # of chromosomes, only now, each chromosome is made of 2 identical sister chromatids
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G2
- Cell prepares to divide
- organelles continue to duplicate
- RNA & protein (esp. tubulin for microtubules) are actively synthesized
- 10-20% of the cell life cycle
- G2 checkpoint checks for mitosis promoting factor (MPF)
- When level of MPF is high enough, mitosis is triggered
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Mitosis
- Nuclear division w/o genetic change
- 4 stages: Prophase, metaphase, anaphase, & telophase (PMAT)
- Varies among eukaryotes
- Results in genetically identical daughter cells
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Prophase
- Characterized by condensation of chromatin into chromosomes
- centrioles move to opposite ends of cell
- First the nucleolus then the nucleus disappear
- The spindle apparatus begins to form
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Spindle Apparatus
- Aster: microtubules radiating from centrioles
- Kinetochore microtubules growing from centromeres
- Spindle microtubules connecting the 2 centrioles
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Centromeres
Group of proteins located toward center of chromosome
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Kinetochore
Structure of protein & DNA located at centromere of the joined chromatids of each chromosome
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Metaphase
Chromosomes align along equator of the cell
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Anaphase
- Begins when sister chromatids split at their attaching centromeres & move toward opposite ends of cell
- this split is termed disjunction
- cytokinesis may commence toward the end of this phase
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Telophase
- The nuclear membrane reforms followed by the reformation of the nucleolus
- Chromosomes decondense
- Cytokinesis continues
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Cytokinesis
Actual separation of cellular cytoplasm due to constriction of microfilaments about the center of the cell
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Meiosis
- Double nuclear division that produces four haploid gametes or germ cells
- In humans, only spermatogonium & oogonium undergo meiosis
- All other cells are somatic & undergo mitosis only
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Primary Spermatocyte/Oocyte
After replication occurs in S phase of interphase, this is what cell is called
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Rounds of Meiosis
- Meiosis I : Reduction Division
| - Meiosis II
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Differences in Meiosis vs. Mitosis : Prophase I
- Homologous chromosomes line up along side each other, matching their genes exactly
- crossing over may occur
- Each duplicated chromosome appears as an 'x', the side by side homologues exhibit a total of 4 chromatids & are called tetrads
- If crossing over doesn't occur, the 2 chromosomes are zipped along each other where nucleotides are exchanged, and form the synaptonemal complex
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Crossing Over
- Exchange sequences of DNA nucleotides
| - Genetic recombination may occur during crossing over
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Chiasma
'x' shape
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Linked
Genes located close together on a chromosome are more likely to cross over together
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Differences in Meiosis vs Mitosis: Metaphase I
- Homologues remain attached and move to metaphase plate
| - Rather than single chromosomes aligned along the plate as in mitosis, tetrads align
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Differences in Meiosis vs. Mitosis: Anaphase I
Separates homologues from their partners
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Differences in Meiosis vs. Mitosis: Telophase I
- Nuclear membrane may or may not reform & cytokinesis may or may not occur
- If cytokinesis occurs, the new cells are haploid with 23 replicated chromosomes, and are called secondary spermatocytes or oocytes
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Polar Body
- In case of female, one of the oocytes, called the 1st polar body is much smaller & degenerates
- occurs to conserve cytoplasm
- 1st polar body may or may not undergo meiosis II, producing 2 polar bodies
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Meiosis II
- Proceeds w/ prophase II, metaphase II, anaphase II, and telophase II
- Final products: Haploid gametes each w/ 23 chromosomes
- In case of oocyte, a single ovum is formed
- In case of spermatocyte, four sperm cells are formed
- In females, telophase II produces 1 gamete & a 2nd polar body
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Nondisjunction
- If during anaphase I or II the centromere of any chromosome doesn't split
- Primary nondisjunction = in anaphase I
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Primary Nondisjunction
- Results in one of the cells having 2 extra chromatids ( a complete extra chromosome), and the other will be missing a chromosome
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Secondary Nondisjunction
Results in one cell having 1 extra chromatid and one cell lacking 1 chromatid
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Nondisjunction in Mitosis
Nondisjunction can occur in mitosis but it is less severe b/c genetic info in new cells isn't passed on to every cell in body
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DIAGRAM OF MEIOSIS
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Under the light microscope, metaphase in mitosis would appear like what in meiosis?
Metaphase II
