Cell and Nuclear Division Flashcards
(41 cards)
1
Q
What can we observe in a human somatic cell?
5 points
A
- There are two sets of chromosomes (maternal and paternal sets)
- chromosomes arranged in homologous pairs (23 of them)
- there are 22 pairs of autosomes and 1 pair of sex chromosomes.
- all somatic cells are diploid
- in females, the two sex chromosomes are alike and known as X chromosomes. In males, there is one X and one Y chromosomes.
2
Q
What does ploidy refer to
A
The number of sets of chromosomes within the nucleus of a cell.
3
Q
What does n indicate
A
- n is the number of chromosomes in a set
- the number written before n indicates the ploidy level.
4
Q
Features of human reproductive cells
A
- Gametes are only produced by meiosis
- Have only 1 set of chromosomes and thus are haploid
- Each gamete carries 23 chromosomes
- This is to maintain a constant number of chromosomes and prevent chromosomal doubling in the next generation
5
Q
What is chromatin?
A
The complex of nucleic acids (DNA) and associated histone and non-histone proteins
It is in a less condensed state present during interphase of the cell cycle, or in non-dividing cells.
6
Q
What are chromosomes? Homologous Chromosomes?
A
Chromosomes:
- The condensed form of chromatin. Additional proteins (scaffolding proteins) are associated with chromosomes and aid in their condensation
- They are most visible during mitosis and meiosis.
- (FYI??) Genes are hereditary units located at specific physical locations along each chromosome. This location is known as gene locus
Homologous chromosomes:
- Structurally similar (similar shape, size, centromere position and sequence gene loci)
- Not genetically identical: different alleles at the same gene loci
- Each chromosome of such a pair is called homologue. One homologue comes from the mother and the other comes from the father
- Homologous chromosomes undergo synapsis and pair up during meiosis.
7
Q
What are sister chromatids
A
- The replicated forms of a single chromosome joined together by the centromere
- Separated during anaphase of mitosis and anaphase 2 of meiosis 2 [written like (II)]
- DNA molecules of sister chromatids are identical and hence they posess the same alleles at each gene locus.
- This also makes them structurally identical. Same shape, size centromere position, number of genes and sequence of gene loci
- Sister chromatids can be genetically identical or non-genetically identical as sister chromatids no longer possess the same alleles at each gene locus after crossing over occurs in prophase 1 of meiosis 1.
8
Q
What is the centromere
A
- the specialised region of a chromosome where two sister chromatids are joined after semi-conservative DNA replication
- Consists of a short sequence of DNA found in heterochromatin, which are repeated several thousand times in tandem and are never transcribed.
- It is associated with kinetochore(protein structure) for the attachment of spindle fibres and is the last location to separate during cell division
9
Q
What are kinetochore
A
- A structure formed by proteins associated with specific sections of the chromosome (ie the centromere)
- The structure to which the microtubules of the spindle are attached during nuclear division
- The 2 kinetochores of a replicated chromosome face opposite poles of the cell, hence the kinetochore forms part of the structure that plays an active part in the movement of chromosomes to the opposite part of the cell
10
Q
What are centrioles
A
- A barrel-shaped organelle which is found only in animal cells
- Centrioles exists as a cylindrical pair in the cytoplasm. Each member of the pair is composed of nine triplets of microtubules arranged in a ring
- The members of the pair are perpendicular to each other
- They are located in a region of the cell known as the centrosome
- At the onset of mitosis, centrioles pairs are duplicated and each pair moves to the opposite poles of the cell, establishing the two poles of the cell.
11
Q
What is the centrosome
A
- A specialized region of the cell that includes a pair of centrioles and the surrounding cytoplasm, which contains proteins that aid in the assembly of spindle microtubules. (obv where the assembly begins as well)
- Also known as the microtubule organising centre (MTOC)
12
Q
What are microtubules
A
- the components of the cytoskeleton. They are rigid hollow rods approx 25nm in diameter.
- dynamic structures made of globular proteins known as tubulin
- An organised system of microtubules attaches to the centromeres of duplicated chromosomes and pulls them to opposite poles of the cell during eukaryotic cell division.
13
Q
Name the 3 types of microtubules that can be distinguished in mitotic animal cells and describe their features
A
- Astral microtubules/asters
- Radiate from the centriole towards the peripheral regions of the cell
- only present in cells that contain centrioles
- serve as a support for the functioning of the microtubules - Kinetochore microtubules
- Fibres attached to the kinetochore, which forms on the centromere of each chromatid.
- Serve to attach the chromosomes to the spindle fiber
- Pull the sister chromatids towards the opposite poles of the cell during anaphase. - Polar microtubules
- Fibres running from pole to pole overlapping at the equator of the spindle
- responsible for elongating the whole cell along the polar axis during the anaphase
14
Q
The cell cyle consists of:
A
- Interphase which is divided into G1, S and G2
- Mitosis (subdivided into prophase, metaphase anaphase and telophase)
- Cytokinesis which is the division of the cytoplasm to form two daughter cells, which usually occurs simultaneously with telophase
15
Q
Describe the purpose of the G1 phase
A
- begins after cytokinesis of the previous cell division - cells are thus small in size and low in ATP
- hence, cells increase in size and acquire ATP during this phase
- intensive cellular gene expression and synthesis of appropriate organelles and proteins
16
Q
Describe the purpose of the S phase
A
- Each DNA molecule undergoes semi-conservative DNA replication, resulting in the production of two identical DNA molecules
- Histone proteins are synthesized and associate with each DNA molecule
- After the DNA have replicated in S phase, they remain fully extended and uncoiled
17
Q
Describe the purpose of the G2 phase
A
- Since the formation of new DNA is an energy-consuming process, the cells undergoes a second growth and energy acquisition stage
- Cells increase in size and acquire ATP during this phase
- Further synthesis of appropriate organelles and proteins occurs
- Centrioles replicate and the mitotic spindle begins to form
18
Q
Describe prophase
A
Longest stage of mitosis. Changes occur in both the nucleus and the cytoplasm
In the nuclues
- Nuclear envelope disintegrates into small vesicles, which disperse
- Nucleolus gradually disappears
- Chromatin become more tightly coiled and condense into discrete chromosomes
In the cytoplasm
- Centriole pars migrate to opposite poles of the cell (only in animal cells obv)
- The spindle fibre that began to form in G2 phase of the interphase continues to develop
19
Q
Describe metaphase
A
- Centriole pairs are positioned at opposite pookle of the cell
- shortening and thickening of the chromosomes is at its maximum
- 2 sister chromatids joined at the centromeres of each chromosome has a kinetochore at the centromere
- kinetochore microtubules attach to the kinetochores at the centromere of chromosome
- chromosomes migrate and align singly at the metaphase plate which is the plane equidistant from the spindle poles. They are pulled to the metaphase plate by the action of kinetochore microtubules
- there is no pairing of homologous chromosomes at the metaphase plate during mitosis
20
Q
describe anaphase
A
- it is the shortest stage of mitosis
- Centromeres divide and sister chromatids are separated
- Once the centromeres of the sister chromatids are separated, the chromatids are known as daughter chromosomes
- daughter chromosomes are pulled to the opposite poles of the cell as their kinetochore microtubules shorten
- as these kinetochores microtubules are attached at the centromere, the chromosomes move with the centromere leading towards the poles of the cell
- at the same time, the poles of the cell move farther apart as the polar microtubules slide past each other, elongating the cell
- special motor proteins are involved in the rapid and abrupt movement of chromosomes towards the poles of the cell during anaphase
- At the end of anaphase, the two pole of the cell have equal and complete sets of chromosomes
21
Q
Describe telophase
A
Begins when the daughter chromosomes reach their respectives poles
- the chromosomes decondense into the chromatins form by uncoiling
- the nucleus and the nuclear envelope re-form
- this results in the two nuclei taking on the granular appearance of interphase
- microtubules disassemble. A pair of centrioles ends up in each daughter cell
22
Q
Give an overview of cytokinesis
A
- the division of the cytoplasm to produce 2 daughter cells.
- Cell organelles become evenly distributed towards the two poles of the parent cell (for equal allocation), along with chromosomes during telophase.
- 2 smaller, genetically identical cells result. These cells may then grow and develop into different forms via differentiation and the developemental processes.
- Begins simultaneously with telophase
23
Q
Describe cytokinesis in ANIMALS
A
- Involves the formation of a cleavage furrow, which pinches the cell in two
- It first forms as a shallow groove in the cell surface near the metaphase plate.
- On the cytplasmic side of the furrow is a contractile ring of microfilaments As the ring of microfilaments contracts, the cleavage furrow deepens until the parent cell pinches into two daughter cells, each with a complete nucleus and share of cytosol, organelles and other subcellular structures.
24
Q
Describe cytokinesis in PLANTS
A
- No cleavage furrow is formed due to the presence of cell wall
- Cell plate grows during telophase across the metaphase plate of the plant cell. Vesicles derived from the Golgi apparatus move to the middle of the cell where they fuse, producing the cell plate.
- Vesicles contain materials to construct both a primary cell wall for each daughter cell and a middle lamella that cements the primary cell walls of adjacent cells together. These materials carried in the vesicles collect in the cell plate as it grows
- The cell plate enlarges until its surrounding membrane, formed bu the fusion of the vesicle membranes, fuses with the plasma memberane along the perimeter of the parent cell.
- Results in the formation of 2 daughter cells, each with its own plasma membrane
- Cellulose is laid down between the 2 membranes of the cell plate to form the cell wall.
25
Explain why genetic stability makes mitosis significant
- Semi-conservatice DNA replication during S phase of interphase gives rise to genetically identical daughter DNA molecules.
- Subsequently daughter chromosomes are distributed equally to daughter cells, giving rise to the daughter cells being genetically identical to the parent cells.
- The production of genetically identical cells ensures the preservation of genetic stability across generations of cells and hence, in the organism (IMPT POINT)
26
Explain why GRR is a significance of mitosis.
Growth: For a tissue to grow, it is important that the new cells formed are genetically identical to carry out the same functions
Repair: Damaged cells have to be replaced by the exact copies of the original, allowing for a tissue to be restored to its former condition.
Regeneration: Allows for the regeneration of missing parts and cell replacement
27
Explain how mitosis's significance extends to asexual reproduction
The offspring cells/organism are genetically identical to the original parent, thus ensuring the preservation of favouritable traits
28
List the procceses of Prophase I
- the nucleolus disappears and the nuclear membrane disintegrates
- the chromatin condenses until the chromosomes become discrete
The following are unique events to prophase 1 of meiosis
- Homologous chromosomes pair up to form a bivalent. The four chromatids in each bivalent are collectively known as a tetrad.
- This physical pairing process is known as synapsis, where the homologues are bridged by synaptonemal complex consisting of proteins and RNA. The process is precise and brings the genes on each chromosome into precise alignment
- Thiis pairing is essential for the exchange of alleles during crossing over in prophase I of meiosis
- Non-sister chromatids of homologous chromosomes then undergo a exchange of alleles known as crossing over. As a result, sister chromatids are now genetically non-identical and are known as recombinant chromatids
29
What is chiasma
- It refers to the X-shaped microscopic visible region where crossing over has occurred earlier in prophase I of meiosis between non-sister chromatids of homologous chromosomes
- Chiasmata (plural form) may be formed at one or more points between non-sister chromatids of homologous chromosomes
- Chiasmata becomes visible after synapsis ends, with the two homologues remaining associated due to sister chromatid cohesion.
30
Think down the processes of metaphase I
- Kinetochore microtubules attach to the kinetochore at the centromere of one chromosome of each bivalent, while kinetochore microtubules form the opposite pole attach to the other homologue
- Homologuous chromosomes or bivalents randomly algin at the metopahse plate or equatorial plate
- independent assortment of homologous chromosomes occurs at this stage.
- this refers to the fact that when a bivalent lines up on the metaphase plate, the orientation of homologues towards the poles in any one event is random, and is independent of that of any other bivalent.
31
Think down the processes of anaphase I
- the two homologous chromosomes of each bivalent separate and move towards the opposite poles of the cell.
- homologous chromosomes are pulled to opposite poles with centromeres leading, producing a characteristic āVā shaped pattern
- the centromeres in anaphase I of meiosis remain intact and the sister chromatids remain attached to each other.
- This physical segregation is referred to as disjunction, meaning the separation of chromosomes from one another.
- NOTE: non-disjunction is the failure of chromosomes to separate results in mutation.
32
Think down the processes of telophase I
- chromosomes arrive at opposite poles of the cell
- microtubules usually disable
- in animals and some plants, chromatids usually decondense and a nuclear envelope reforms around each set of chromosomes.
- nuclei formed are haploid (n) because the chromosome number and ploidy level have been halved. Each of the chromosomes still exists as two chromatids joined at the centromeres which may not be genetically identical due to crossing over.
33
Cytokinesis I
- Occurs simultaneously with telophase I of meiosis, forming 2 haploid daughter cells
- Note that it does not occur in all species
34
Meiosis I key concepts summary
Meiosis 1 is also known as reduction division
- The homologous chromosomes separate into 2 cells (sister chromatids do not separate)
- Each daughter cell receives only one homologue of each homologous pair of chromosomes
- Chromosome number and ploidy level is reduced by half. Daughter cells formed are haploid (n)
- The amount of DNA is the same as that of parent cell prior to replication
Unique events of meiosis that occur in meiosis I
a) Synapsis and crossing over of homologous chromosomes
b) their subsequent segregation to different daughter cells.
35
Is there further semi-conservative DNA replication during the interphase between meiosis I and II
No
36
Prophase II
- Nucleoli disperse and nuclear envelopes disintegrate ( if formed during telophase I)
- Chromatin undergoes condensation to re-form discrete chromosomes.
- In animal cells, centrioles move to opposite poles of the cells at the end of prophase II of meiosis.
- New spindle fibres appear and are arrangeed at right angles to the spindle of meiosis I
37
Metaphase II
- Kinetochore microtubules attach to the kinetochores at the centromeres of the chromosomes
- Chromosomes migrate and align singly at the metaphase plate of the cell
- Metaphase plate of meiosis II is perpendicular to that of meiosis I
38
Anaphase II
- Centromeres divide and the two chromatids separate and are called daughter chromosomes
- Daughter chromosomes are pulled to opposite poles of the cell, centromeres first/leading as their kinetochore microtubules shorten
- The poles of the cell move further apart as the polar microtubules slide past each other, hence elongating the cell
39
Telophase II
- The chromosomes decondense into the chromatin form by uncoiling
- Microtubules disassemble and a pair of centrioles end up in each daughter cell
- Nuclear envelope reforms around each nucleus
40
Cytokinesis II
This produces four haploid genetically non-identical daughter cells from the original single diploid parent cell, each with half the chromosome number and ploidy level of a normal somatic cell
41
Meiosis II key concepts summary
Due to the crossing over that may have occurred during prophase I of meiosis, the sister chromatids in meiosis II may not be genetically identical to each other.
Meiosis 2 is known as equational division (equal)
- Chromosome number does not change in this nuclear division
- However, as a consequence of reduction division in meiosis I, haploid gamete cells have half the chromosome number and ploidy level compared to the parent cell
- The amount of DNA in the daughter cell is half of that of the parent cell before replication
This ensures that subsequent fertilisation results in a diploid zygote with a constant number of chromosomes characteristic of every somatic cell in the sexually reproducing species.
