Review #4 Flashcards
(24 cards)
Meiosis
produces 4, haploid (n = one copy of each chromosome) cells from 1, diploid (2n) cell
Haploid cells are gametes (sperm/ ova) – come together at fertilization to create a zygote
PRIOR to the beginning of meiosis, DNA replication occurs (copied chromosomes = sister chromatids - attach at centromeres)
MEIOSIS I
(genetic recombination and reduction division)
Prophase I
- Synapsis: homologous chromosomes (similar in shape/ size/ gene arrangement) line up next to each other (bivalents/ tetrads)
- Crossing over: Non-identical sister chromatids exchange DNA – cross over at places called chiasmata, chromosomes break in identical locations, pieces exchanged – creates NEW combinations of genes (recombinations)
Metaphase I
- Random orientation: homologous chromosomes line up randomly along middle of cell (2n possible orientations)
Anaphase I
- spindle fibers pull homologous chromosomes to opposite ends of the cell (independent assortment – genes on different chromosomes separate independently of each other)
Telophase I
- Reduction division (cytoplasm divides – each new cell now haploid)
MEIOSIS II
(resembles mitosis – separates sister chromatids)
Prophase II
New meiotic spindle forms (eggs in females arrested in this stage)
Metaphase II
Chromosomes (made of sister chromatids) line up along middle of cell
Anaphase II
Centromeres break, sister chromatids separate, one copy of each pulled to opposite ends of cell
Telophase II
Cytoplasm divides: 4 haploid cells that are genetically UNIQUE
Infinite genetic variation in meiosis:
Crossing over in prophase I and random orientation in metaphase I (random fertilization with another individual too – takes into account THEIR crossing over and random orientation!)
Problems in Meiosis
Non-disjunction
Failure of sister chromatids to separate (anaphase II)
Cells produced missing a chromosome (monosomy – ONE copy ONLY when fertilized) or have an extra chromosome (trisomy – three copies when fertilized)
Non-disjunction diagnosed using karyograms
Fetal cells obtained from amniotic fluid (amniocentesis) or chorionic villus (placenta)
Chromosomes arranged in pairs according to size/ structure
23rd pair used to diagnose gender (XX = female, XY = male)
DNA Profiling and Gel Electrophoresis
- Amplify (copy) DNA samples using PCR then cut with restriction enzymes/ endonucleases
- Run samples through gel electrophoresis and analyze banding patterns (shows fragment lengths/ SIZES)
Can also add probes (fluorescently labeled specific, complementary DNA sequences) to gels to identify genes of interest/ alleles that cause disease (sickle cell etc.)
Genes and Alleles
Genes located on specific places of chromosomes (locus) and come in different forms (alleles)
Alleles can be dominant (stronger – written with capital letter) or recessive (weaker – written with lowercase letter)
The physical expression of a trait (what you SEE) is a phenotype (example: green eyes) and the combination of alleles an organism has for a trait are its genotype (example: bb)
Genotypes are heterozygous (different alleles – Aa) or homozygous (same alleles – AA = homozygous dominant, aa = homozygous recessive)
Note: Heterozygous individuals (Nn) are carriers: they CARRY recessive alleles but do not show them in their phenotype due to the presence of a dominant allele (important in sex-linked traits and genetic diseases)
Genetics
science of heredity (passing on of genetic material from parent to offspring)
Prokaryotes: circular, naked (no proteins) chromosome – passed directly to offspring (asexual reproduction)
Eukaryotes: linear, with proteins (histones) – many pairs, passed to offspring through sexual reproduction
Pairs #1-22 in humans = autosomes, Pair #23 in humans = sex chromosomes (determine gender)
Genes carried on Chromosomes
genes are heritable factors (DNA) that determine specific traits (code for proteins)
The complete set of all DNA base sequences of an organism is its genome
#’s of genes/ chromosomes/ genome size UNIQUE to different species (more DNA does NOT always mean an organism is more complex/ advanced than another)
Some genes have codominant alleles (equal in strength – both SHOW if present)
Example: Blood groups (MUST use the I and i notations)
A (IA) and B (IB) are equally dominant and O (i) is recessive
If inherit IA and IB, blood type (phenotype) is AB (both alleles SHOW)
If inherit i and i, blood type is O (need BOTH recessives to have type O blood)
Punnett square (monohybrid – one trait – cross) shows possible genotypes of offspring of man with type O blood and woman with type AB blood
Genes carried on sex-chromosomes (sex-linked)
XX = female; XY = male (Note: males also inherit mitochondrial DNA from mothers/ from “egg”)
Some genes on larger X chromosome absent from shorter Y chromosome (X and Y NOT homologous)
Sex-linked traits more commonly seen in males: Only one X chromosome (whatever inherit will show – no other chromosome to overpower etc.)
Hemophilia and red-green color blindness on X-chromosome; Males inherit X from their mothers; IF recessive allele (for hemophilia or color blindness etc.) is on that X, the man WILL have the condition (Xn Y)
ONLY Females can be CARRIERS of sex-linked traits (two X chromosomes so can be heterozygous – a dominant allele on one X and a recessive allele on the other: XN Xn)
Some genes have multiple alleles (polygenic)
Polygenic traits show CONTINUOUS (bell-shaped curve) variation (not discrete variation)
Phenotypes do NOT fit into distinct categories; phenotypes are continuous because SO many alleles influence the expression of the gene: SKIN COLOR (melanin), HEIGHT, HAIR COLOR etc.
Environment can also influence these traits (UV light, diet/ nutrition etc.)
Mutations
Changes in genetic material (DNA): rare
Base substitution (one base in DNA changed; causes wrong codon in mRNA; causes wrong amino acid in translation)
Example: Sickle cell anemia (in DNA: GAG changed to GTG; in mRNA: wrong codon codes for valine instead of glutamic acid, in polypeptide chain): Hemoglobin misshapen (sickle shaped) – cannot carry oxygen as well
WHY does the sickle cell allele (and other disease-causing alleles) persist in populations if so detrimental? Provides SOME advantage (malaria resistance
with sickle cell) – only beneficial if you LIVE in an environment where malaria is present though. The ENVIRONMENT determines if the allele is good or bad!
Dihybrid
Dihybrid (two trait) crosses (unlinked genes)
Heterozygous cross shows phenotypic ratio of 9:3:3:1 (TtRr x TtRr) – numbers in ratios are out of 16
To set up punnett square (4x4), NUMBER the alleles in the genotypes (1,2,3,4) and place the following combinations over/ next to each box for each parent (1,3), (1,4), (2,3), (2,4)
Dihybrid crosses
(LINKED genes/ linkage group) – Thomas Hunt Morgan and fruit flies
Linked genes do NOT follow law of independent assortment: inherited together because on SAME chromosome
Do NOT show typical ratios (9:3:3:1 or 1:2:1 etc) – VARY significantly (Chi-square test, comparing observed and expected, shows significant difference between observed and expected phenotype ratios in offspring)
Genotypes written as VERTICAL pairs with TWO horizontal lines in between them
ONLY way for recombination in linked genes is crossing over (prophase I): unlinked genes follow independent assortment to create new combinations (of chromosomes)
MOST offspring will show parental phenotypes because genes inherited TOGETHER on same chromosome (only a small percentage show NEW phenotypes, not present in parents – from crossing over- these are recombinants)
Huntington’s disease
Some genetic disorders caused by dominant alleles/ lethal dominants
To make multiple copies of a gene:
PCR (polymerase chain reaction) or gene cloning
PCR (polymerase chain reaction)
makes MANY copies of SMALL amount of DNA (“amplifies” it) using a thermocycler
Gene Cloning
(clone = genetically identical copy) using recombinant DNA
Produce recombinant DNA (DNA from two or more different sources/ organisms)
Cut vector (plasmid) and gene of interest with restriction enzyme (endonuclease)
Combine DNA fragments (will base pair at sticky ends)
Add DNA ligase (to seal fragments together)
Insert recombinant DNA back into host (bacteria, yeast, sheep etc)
Able to do because DNA/ genetic code UNIVERSAL!
Allow cells to reproduce gene (and make protein)
Ex: Insulin for diabetics , Factor IX for hemophiliacs
Gene transfer
Recombinant DNA made (donor + host) and placed into host organism
Host organism now transgenic (GMO = genetically modified organism): has had an artificial genetic change to its genome
Genes transferred to treat disease (gene therapy), for medical treatments (insulin), and for commercial use (crops/ livestock)
. Reproductive cloning
exact genetic copy of entire organism) Using adult, differentiated cells (Dolly the sheep!)
Take nucleus of differentiated cell and place in egg (remove egg’s nucleus first) – somatic nuclear transfer
“Zap” with electricity (to trick it into thinking it’s fertilized)
Mitotic divisions in “embryo”
Place “embryo” in surrogate mother and allow to develop into baby (CLONE – exact genetic copy)
Therapeutic cloning
Use embryonic stem cells (undifferentiated) to produce new tissues for transplantation
Arguments for: can be screened for genetic abnormalities; natural process (identical twins); increased chance of offspring (for infertile couples); helps burn victims/ paralysis/ leukemia patients etc.; reduced risk of rejection (genetically identical)
Arguments against: destroys embryo (when does life begin?); higher rates of miscarriage/ developmental disorders; long term health effects unknown; suppression of patient’s immune system risky; human clones?
Human Genome Project
ALL base sequences (including mutations) sequenced for humans (as a species) – Sanger technique with dideoxynucleotides (ddNT’s)/ computers
Mapping outcome: know number, location, and basic sequence of all human genes
Screening outcome: Specific gene probes to detect genetic disorders/ carriers
Medical outcome: Specific genes targeted to produce specific proteins for those who cannot
Ancestry outcome: Improved insight into human origins/ evolution/ migration patterns etc.
Gel Electrophoresis
DNA sample (from crime scene, bones, father/ baby) amplified (many copies) using PCR
Cut DNA with restriction enzymes then run through gel using electric current (separates based on SIZE and charge – fragment lengths UNIQUE to each individual due to unique sequences of DNA)
Produces banding pattern in gel (bands represent sizes of fragments – smaller fragments travel faster/ farther)
Used in DNA Profiling (typically use highly repetitive/ satellite DNA because unique to every individual):
1. Forensic Investigations (identifies crime scene suspects/ victims)
2. Paternity testing (half of baby’s bands from mom, other half MUST come from dad)
