chapter 4: DNA structure, cell and nuclear division Flashcards
(40 cards)
1
Q
starting from page 19
what are the functions of centromeres?
A
- during cell division, centromeres are adhesion points for sister chromatids in a chromosome
- they are aslo the site of assembly of kinetochore proteins, which is where the spindle fibres attach to during cell division
- the attachement of spindle fibres to kinetochore enables:
- accurate alignment of chromosomes along the metaphase plate during metaphase of mitosis and meiosis
- accurate segragatio of homologous chromosomes during anaphase of meiosis I
- accurate SEGREGATION of sister chromatids to form daughter chromosomes during anaphase of mitosis and anaphase of meiosis II when centromere divides
- during anaphase of mitosis and meiosis
- chromosomes are pulled to opposite poles of the cell with the centromere leading
- enabling a complete set of chromosomes to be allocated to each daughter cell
2
Q
homologous chromosomes
what is polyploidy?
A
- in most organisms, chromosomes in cells can be divided into 2 sets
- however the nuclei of cells of some organisms may contain more than 2 sets of chromosomes
- this condition is known as polyploidy which is common in plants
- in cells hwich have 2 sets of chromosomes, the chromosomes occur in homologous pairs and are known as homologous chromosomes
- each member of the homologous chromosome is called homologue
3
Q
homologous chromosomes
what are the characteristics of homologous chromosomes (except sex chromosomes)
A
- homologous chromosomes are a pair of chromosomes which are similar in length, size, shape and centromere position
- homologous chromosomes carry genes which determine and control the same characteristics
- however, they are not necessarily genetically identical in the gene that they carry
- they may have different alleles
- they have identical sequences of gene loci
- ie. the same number of genes arranged in the same linear order
- each pair of homologous chromosome is made up of one maternal and one paternal chromosome
- they can form bivalents during meiosis and appear as double structures consisting of two genetically identical sister chromatids
4
Q
what are haploid cells?
A
- any cell with 1 complete set of chromosomes is called a haploid cell
- haploid cells contain one member of each homologous pair of chromosomes
- it has half the normal number of chromosomes as a diploid cell
- these cells have a haploid number of chromosomes
- the gametes of human beings are haploid cells
5
Q
what are diploid cells?
A
- any cell with 2 complete sets of chromosomes is called a diploid cell
- diploid cells contain chromosomes that are in pairs, with one from each parent
- these cells have a diploid number of chromosomes 2n
- the somatic cells are any cells other than gametes and those involved in gamete formation
6
Q
haploid and diploid cells in humans
A
- in each human somatic cell, there are 46 chromosomes
- the 46 chromosomes are 2 sets of 23 chromosomes- a maternal and paternal set
- the 2 sets of 23 chromosomes consist of 22 pairs of autosomes and a single pair of sex chroomosomes
- therefore, human somatic cells are diploid and the diploid number of human somatic cells is 46
7
Q
what are the functions of DNA?
A
- DNA molecules contain genes- heredity units that contain genetic information that determine an organism’s characteristics
- genes on DNA provide information for the synthesis of RNA via transcription and through RNA, controls protein synthesis (translation)
- proteins are the building blocks of cells and perform many important functions like enzymes, receptors, transport proteins
- the functions performed by these proteins determine an organism’s characteristics
-DNA undergoes replication prior to cell division so that genetic information can be transmitted to the next generation
8
Q
describe DNA replication before nuclear division
A
- a dividing cell replicates its DNA to produce two complete sets of genome which are then allocated to two daughter cells during cell and nuclear division
- DNA replication is necessary for accurate passing on of genetic information present in the parent cell to daughter cells after cell division
- this ensures all daughter cells are genetically identical
- ie, daughter cells receives a complete set of genome with the same number and type of chromosomes
- DNA replication must occur before nuclear division
- nuclear division occurs before cytokinesis to ensure that DNA is passed down from one generation of cells to the next with high fidelity
9
Q
what is the cell cycle?
A
- the cell cycle can be defined as the period from the formation of a cell by division to the point that the cell itself divides into two genetically identical daughter cells
- most type of cells never divide again after they have grown and become specialised
the cell cycle consists of two phases:
interphase and the Mitotic (M) PHASE
interphase comprises 3 sub-phase
- G1 phase
- S phase (DNA replication occurs)
- G2 phase
**the mitotic (M) phase **
- mitosis (NOT MEIOSIS)
- cytokinesis
10
Q
what is cell cycle control?
A
- the order of sequential events that occur in the cell cycle must be strictly adhered to
- serious genetic damage can occur if cells progress to the next phase of the cell cycle before the previous phase is properly completed
- to do so, the cell has a control system comprising a group of signalling and regulatory proteins foudn in the cytoplasm
11
Q
what are the cell cycle control checkpoints for?
A
the cell cycle is controlled at three checkpoints
- at the G1 checkpoint
- at the G2 checkpoint
- at the metaphase (M) checkpoint
where are checkpoints needed?
- prevent premature progression of the cell cycle
- provides time for cell machinery to be repaired should there be any damage
12
Q
what is interphase?
A
- it is an important period in the cell cycle when the cell is not dividing
- during interphase, the cell prepares for cell division
- interphase is the longest phase which takes up about 90% of the cell cycle
- it consists of G1, S and G2 phases
13
Q
what happens during G1 phase and G1 Checkpoint?
A
G1 PHASE:
1. intensive cellular synthesis occurs
- synthesis of organelles
- synthesis of the following molecules:
- ATP, to build up a large energy store
- RNA
- proteins
- the cell increases in size
- DNA is in the uncondensed form as chromatin fibres
G1 CHECKPOINT:
1. checks that:
- cell size is adequate
- there is sufficient nutrients that are availible to support daughter cells
- growth factors (extracellular signal proteins that stimulate a cell to grow or divide) are present
- it decides whether the cell should continue in the G1 phase or initiate the molecular processes that signal the beginning of S phase
- if the cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2 and M phases and divide
- if not, it will exit the cycle and switch into a non dividing state called the G0 phase
14
Q
what is the G0 phase?
A
- Cells that pause in G₁ phase before S phase enter into a resting phase called Go phase
- They may remain in this phase for days to years before resuming cell division.
- At any given time, most of the cells in an animal’s body are in the Go phase.
- Some cells such as mature nerve cells and muscle cells remain there permanently
- Other cells such as liver cells can be “called back” from the Go phase to resume G1 phase in response to external cues such as growth factors which are released during injury.
15
Q
what are the three componenet sof a nucleotide monomer?
A
- a pentose sugar
- a phosphate group
- nitrogenous base
16
Q
what is the pentose in RNA and DNA called?
A
- the pentose in RNA is called ribose, where an OH group exists on carbon 2
- the pentose in DNA is called deoxyribose, where a H atom exists instead on carbon 2
17
Q
what are the structures of nitrogenous bases?
A
pyramidines:
- pyramidines have 1 ring in their structure
- cytosine, thymine and uracil
purines:
- purines have 2 rings in their structure
- adenine, guanine
each nucleic acid contains four different bases
- adenine, guanine and cytosine are found in both RNA and DNA
- thymine is found only in DNA
- uracil is found only in RNA
18
Q
what are the two types of nucleic acids?
A
- deoxyribonucleic acid (DNA)
- ribonucleic acid (RNA)
19
Q
what is the bond between the phosphate group and the pentose sugar?
A
a phosphoester bond
20
Q
formation of a polynucleotide:
- A polynucleotide chain is formed when nucleotides are joined together by ____ bonds/linkages/bridges during ____ reactions
- The phosphodiester bonds are formed between the
____ group on the 3’ carbon of one
nucleotide and the ____ group on the 5’ carbon of the next nucleotide - The repeating pattern of sugar-phosphate forms a
____ ____ - The 5’ end of a polynucleotide has a free phosphate group attached to 5’ carbon of a sugar
residue, while the 3’ end of a polynucleotide has a
free -OH group attached to 3’ carbon of a sugar
residue.
A
- A polynucleotide chain is formed when nucleotides are joined together by **phosphodiester ** bonds/linkages/bridges during condensation reactions
- The phosphodiester bonds are formed between the
-OH group on the 3’ carbon of one
nucleotide and the phosphate group on the 5’ carbon of the next nucleotide
- The repeating pattern of sugar-phosphate forms a
sugar-phosphate backbone - The 5’ end of a polynucleotide has a free phosphate group attached to 5’ carbon of a sugar
residue, while the 3’ end of a polynucleotide has a
free -OH group attached to 3’ carbon of a sugar
residue.
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Q
structure of DNA
- DNA is a polymer of ____ linked by ____ bonds.
- a DNA molecule consists of two____ ____ that are
coiled around each other to form a ____ ____ - The two chains run in ____ directions (antiparallel)
- One strand runs from the ____ to ____ direction while the other runs from the 3’ to 5’ direction
- Each chain has a sugar-phosphate backbone on the ____ of the helix, and nitrogenous bases project into the ____ of the helix
- The two polynucleotide chains/strands are held together by ____ bonds formed between the paired bases
A
structure of DNA
- DNA is a polymer of deoxyribonucleotides linked by phosphodiester bonds.
- a DNA molecule consists of two polynucleotide strands that are coiled around each other to form a double helix
- The two chains run in opposite directions (antiparallel)
- One strand runs from the 5’ to 3’ direction while the other runs from the 3’ to 5’ direction
- Each chain has a sugar-phosphate backbone on the exterior of the helix, and nitrogenous bases project into the interior of the helix
- The two polynucleotide chains/strands are held together by hydrogen bonds formed between the paired bases
22
Q
what are the complementrary bases and how many hydrogen bonds are formed?
A
- Adenine (A) always pairs with thymine (T) via 2 hydrogen bonds
- Cytosine (C) always pairs with guanine (G) via 3 hydrogen bonds.
23
Q
what are the differences in organisation of genetic material in non-dividing cells and dividing cells?
A
in non-dividing cells,
- When the cell is not dividing, even as it replicates its DNA in preparation for cell division during S
phase of interphase, each DNA molecule exists in the form of a **long thin chromatin fibre **which is
not visible under a light microscope
in dividing cells,
- During cell division, DNA must be evenly distributed to daughter cells, such that each daughter cell
ends up with a complete set of genome.
- The replication and distribution of DNA is manageable because the DNA molecules are packaged into chromosomes.
- At the beginning of nuclear division, each chromatin fibre starts to condense and become densely
coiled and folded, forming chromosomes which are much shorter and so thick that they can be seen
under a light microscope
24
Q
the structure of a chromosome:
- During the period between nuclear divisions (when the cell is not dividing), each chromatin fibre consists of a ____ ____ DNA molecule that is wound around protein molecules known as ____
- Before the nucleus divides, the DNA molecule is replicated during the S phase of ____
- As a result, each chromosome becomes a ____ structure containing two identical DNA molecules by the time the cell is ready for nuclear division
- These double structures are called ____ chromatids
- Each sister chromatid contains one linear DNA molecule.
- Since they are exact copies, the sister chromatids have the same DNA sequence and same alleles.
- The two sister chromatids are most closely joined at a region called the centromere
A
- During the period between nuclear divisions (when the cell is not dividing), each chromatin fibre consists of a single, linear DNA molecule that is wound around protein molecules known as histones
- Before the nucleus divides, the DNA molecule is replicated during the S phase of interphase
- As a result, each chromosome becomes a double structure containing two identical DNA molecules by the time the cell is ready for nuclear division
- These double structures are called **sister ** chromatids
- Each sister chromatid contains one linear DNA molecule.
- Since they are exact copies, the sister chromatids have the same DNA sequence and same alleles.
- The two sister chromatids are most closely joined at a region called the centromere
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what is the structure of centromeres?
1. The centromere is a constricted region on chromosomes that holds sister chromatids together /
where sister chromatids are most closely attached.
2. Centromeres allow for the attachment of spindle fibres / kinetochore microtubules during nuclear
division
3. Centromeres consist of non-coding repetitive DNA sequences made up of tandem repeats.
4. Centromeric sequences are never transcribed and so are found as heterochromatin when the cell
is not dividing.
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what happens during the S phase?
(synthesis phase)
**- DNA replication and histone synthesis occurs**
- Before a cell can divide to form daughter cells, all of its DNA must be replicated and then the two copies are separated evenly so that each daughter cell ends up with a complete set of chromosome
- During S phase, DNA replicates → the amount of DNA in the nucleus doubles.
- histones are synthesised
**- DNA exists in an uncondensed form as chromatin fibre**
- A loosely coiled chromatin fibre now consists of two identical DNA molecules after DNA replication in S phase.
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what happens during the G2 phase?
**- intensive cellular synthesis occurs:**
- synthesis of orgauellas continues.
synthesis of the following molecules:
- ATP
- spindle proteins (for formation of spindle fibres later on)
Etc.
- The cell continues to increase in size.
**- chromatin fibres begin to condense**
- DNA replicated during the S phase start to condense and when they first become visible under the light microscope, the first stage of mitosis,
prophase begins.
**- Replication of centrioles completed.**
- Centrioles have replicated and two centrosomes are observed in the cell.
- In animal cells, each centrosome consists of two centrioles.
**- spindle fibres begin to form**
28
what occurs during the G2 checkpoint?
- this checkpoint occurs at the end of G2 phase
1. Checks that:
- Cell size is adequate.
- DNA replication is complete and successful.
- there is no DNA damage.
2. It decides whether the cell should continue in the G2 phase or initiate the molecular processes that signal the beginning of mitosis
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**what is mitosis?**
(what comes after mitosis)
(what are the four main stages of mitosis)
- process by which a nucleus divides to produce TWO daughter nuclei, which contains sets of chromosomes which are genetically identical to the original nucleus.
- Mitosis followed by cytokinesis results in an increase in the number of genetically identical cell.
- It is the method by which growth, replacement and repair of cells occurs in multicellular eukaryotes.
- In unicellular eukaryotes, mitosis results in asexual reproduction leading to an increase in population number.
Mitosis can be subdivided into four main stages: **prophase, metaphase, anaphase and telophase.**
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MITOSIS:
**what happens during prophase?**
(centrioles)
(chromatin fibres)
(spindle fibres)
(nucleolus and nuclear envelope)
**centrioles**:
- In animal cells, the two centriole pairs migrate to opposite poles of the cell in early prophase.
**chromatin fibres:**
- Chromatin fibres coil and condense to become discrete chromosomes.
- Each chromosome consists of two genetically identical sister cuvomatiols joined at the centromere.
**spindle fibres**: (kinetochore and non-kinetochore microtubules)
- Formation of mitotic spindle.
- From each pair of centrioles, short microtubules develop and radiate to form an aster
- Longer microtubules called spindle fibres extend from the centrioles and begin to connect to the
kinetochore
- a specialised protein structure located at the centromere region on each chromosome.
- Spindle fibres that attach to kinetochore are also known as kinetochore microtubules
- Other spindle fibres (non-kinetochore
microtubules) which do not attach to
kinetochore extend across the cell, and
begin to interact and overlap with spindle fibres from opposite poles of the cell
nucleolus and nuclear envelope:
- at the end of prophase, the nucleolus has disappeared and nuclear envelope has disintegrated
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MITOSIS:
**what happens during metaphase?**
(spindle fibres)
(chromosomes)
**spindle fibres:**
- Kinetochore microtubules are already attached to the kinetochore at the centromere of chromosomes
- Non-kinetochore microtubules interact with those from the opposite poles of the cell
**chromosomes**:
- metaphase begins when chromosomes align single on the metaphase plate (or equator)
33
MITOSIS:
what happens during the **Metaphase (M) checkpoint?**
- This checkpoint occurs during metaphase of mitosis.
Checks that:
- Chromosomes are under bipolar tension (in other words, properly attached to kinetochore microtubules originating from the two different poles of the cell).
- Chromosomes are aligned at the metaphase plate.
- It decides whether the cell should continue in metaphase or trigger anaphase, telophase (exit from mitosis) and cytokinesis.
- Dysregulation leads to non-disjunction and aneuploidy/ polyploidy
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MITOSIS:
** what occurs during anaphase? **
(sister chromatids)
(spindle fibres)
**sister chromatids**:
- centromere divide
- two sister chromatids separate and each becomes a full-fledged daughter chromosome
**spindle fibres:**
- Chromosomes are then pulled towards opposite poles of the cells as the kinetochore microtubules shorten.
- The shortening of the kinetochore microtubules happens when tubulin subunits (globular proteins that make up microtubules) are removed from the kinetochore ends of the microtubules.
- As the microtubules are attached at the centromere region, the chromosome move with centromere first.
- Non-kinetochore microtubules lengthen and this causes the cell to elongate.
35
MITOSIS
**what happens during telophase?**
(spindle fibres)
(chromosomes)
(nucleolus and nuclear envelope)
**spindle fibres**:
- telophase begines when spindle fibres disintegrate
**chromosomes:**
- chromsomes reach poles of cell, uncoil and decondense
- DNA now exist in the form of chromatin fibres
**nucleolus and nuclear envelope:**
- the nuclear envelope reforms around each set of chromosomes at each pole
- the nucleoulus reappears
36
what is citokinesis?
- the division of the cytoplasm to form two separate daughter cells after nuclear division
- (mitosis, meiosis I and meiosis II)
- in cytokinesis, the cytoplasm and cell organelles are distributed evenly between the daughter cells
37
what happens during cytokinesis in animal cells?
- In preparation for cytokinesis, the cell organelles become evenly distributed towards the two poles of a cell at telophase along with the chromosomes.
- The cell surface membrane begins to invaginate towards the region previously occupied by the spindle equator resulting in formation of a
cleavage furrow.
- The cleavage furrow deepens until the parent cell is pinched into two, producing two completely separated daughter cells
- each with its own nucleus and share of cytosol and organelles
38
what happens during cytokinesis in plant cells?
- Due to the presence of the cell wall, there is no
formation of cleavage furrow.
- Small fluid-filled vesicles produced from the Golgì
apparatus move to the centre of the cell, guided by
microtubules.
- The vesicles fuse together to form a cell plate across the equatorial plane.
- cell wall materials carried in the vescicles are collected in the cell plate as it grows
- The cell plate enlarges until its surrounding
membrane fuses with the plasma membrane along
the perimeter of the cell.
- Meanwhile, a new cell wall arising from the contents of the cell plate has formed between the daughter cells.
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**IMPORTANCE OF MITOSIS:**
- The significance of mitosis is its ability to maintain ____ ____ as the 2 daughter cells that are formed are genetically ____
- Nuclei of the 2 daughter cells receive same ____ and ____ of chromosomes as the original ____ nucleus.
- The daughter nuclei are genetically identical to the parental nucleus because their ____ were derived from parental chromosomes by the exact replication of their DNA during the ____ ____ of interphase
- The significance of mitosis is its ability to maintain **genetic stability** as the 2 daughter cells that are formed are genetically **identical**
- Nuclei of the 2 daughter cells receive same **number** and **type** of chromosomes as the original **parent** nucleus.
- The daughter nuclei are genetically identical to the parental nucleus because their **chromosomes** were derived from parental chromosomes by the exact replication of their DNA during the **S phase** of interphase
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