B3-3 Cell and Nuclear Division

Question 1

Chromosome Number

Answer 1

The chromosome number is the number of chromosomes in each cell of an organism.
Different species of plants and animals have different chromosome number.
There is no relationship between the size of an organism and the number of chromosomes it possesses.

The chromosomes are sorted according to their sizes. There are
- two sets of chromosomes (maternal and paternal sets) chromosomes arranged in homologous pairs
- 22 pairs of autosomes and 1 pair of sex chromosomes
Genes on autosomes 1 to 22 are described as autosomal.
Genes on the sex chromosomes are described as sex-linked.

In human females, the two sex chromosomes are alike and known as X chromosomes. In males, there is only one X chromosome and one Y chromosome.

Question 2

Ploidy Level

Answer 2

Ploidy refers to the number of sets of chromosomes within the nucleus of a cell.

Haploid cells, such as gametes/sex cells, have only one set of chromosomes, abbreviated as n.

Diploid cells have two sets of chromosomes in the nucleus. Human beings, most animals, and many plants are diploid organisms since all the cells in the body are diploid except for their gametes.

In human, the common representation is: 2n = 46, where:
n is the number of chromosomes in a set (23 for humans)

The number written before n indicates the ploidy level i.e. the number of sets. For humans, there are 2 sets of chromosomes where one set comes from the me other set from the father.

Organisms with more than 2 sets of chromosomes are termed polyploid. For example, triploid = 3n and tetraploid = 4n.

Question 3

Human somatic cells vs gametes

• Generated by what process
• Each cell has how many sets of chromosomes
• Ploidy level
• Number of chromosomes in total

Answer 3

Human somatic cells

• All somatic cells are generated by mitosis.
• Each somatic cell has 2 sets of chromosomes, one set originating from the father and the other from the mother.
• Hence somatic cells are diploid (2n).
• Each set contains 23 chromosomes (n = 23).
• There will be 46 chromosomes (2n = 2x23 = 46) in total.
• Somatic cells will thus have 23 homologous pairs of chromosomes.

Human reproductive cells

• Gametes (such as spermatozoa or ova) are produced only by meiosis.
• Gametes have only 1 set of chromosomes.
• Hence are haploid (n).
• Each gamete carries 23 chromosomes (n= 23).
• This is to maintain a constant number of chromosomes and prevent chromosomal doubling in the next generation.

Question 4

Chromatin

Answer 4

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.

Question 5

Chromosome

Answer 5

The condensed form of chromatin. Additional proteins known as scaffolding proteins are associated with chromosomes and aid in their condensation.

Chromosomes are most visible during mitosis and meiosis.

Genes are hereditary units located at specific physical locations along each chromosome. This location is known as a gene locus.

Question 6

Homologous Chromosomes

Answer 6

In diploid organisms, homologous chromosomes are

(a) structurally similar: similar size, shape, centromere position and sequence of gene loci.
(b) not genetically identical: different alleles at the same gene loci

An allele is an alternative form of a gene.

Each chromosome of such a pair is called a homologue. They come from separate parents; one homologue comes from the mother and the other comes from the father.
Homologous chromosomes undergo synapsis and pair up during meiosis.

Question 7

Sister Chromatids

Answer 7

Sister chromatids are replicated forms of a single chromosome joined together by the centromere.
They are eventually separated during the anaphase of mitosis and anaphase II of meiosis II.

The DNA molecules of sister chromatids are identical due to semi-conservative DNA replication. Hence, they possesses the same alleles at each gene locus.

Sister chromatids are hence structurally identical: similar size, shape, centromere position, number of genes and sequence of gene loci.

Sister chromatids can be genetically identical or non-genetically identical as sister chromatids will no longer possess the same alleles at each gene locus after crossing over occurs in prophase l of meiosis l.

Question 8

Centromere

Answer 8

It is the specialised region of a chromosome where two sister chromatids are joined after semi-conservative DNA replication.

It consists of a short sequence of DNA found in heterochromatin, which are repeated several thousand times in tandem and never transcribed.

It is associated with protein structures known as kinetochores for the attachment of spindle fibers and is the last location to separate during cell division.

Question 9

Kinetochore

Answer 9

A structure formed by proteins associated with specific sections of the chromosome i.e. the centromere

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 kinetochores form part of the structure that plays an active part in the movement of chromosomes to the opposite poles of the cell.

Question 10

Centrioles

Answer 10

A barrel-shaped organelle which is found only In animal cells.
Centrioles exist 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, centriole pairs are duplicated and each pair moves to the opposite poles of the cell, establishing the two poles of the cell.

Question 11

Centrosome

Answer 11

The centrosome is a specialised 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.

It is the region where the assembly of spindle microtubules begins.

It is also known as the microtubule organising centre (MTOC).

Question 12

Microtubules (also known as spindle fibre)

Answer 12

Microtubules are components of the cytoskeleton. They are rigid hollow rods approximately 25nm in diameter.

An organised system of microtubules that attaches to the centromeres of duplicated chromosomes and pulls them to opposite poles of the cell during eukaryotic cell division (during anaphase).

Question 13

Interphase

Answer 13

Interphase often accounts for about 90% of the length of the cell cycle. It is the longest phase of the cell cycle and hence most cells observed are in interphase (Fig. 14). Interphase is divided into three sub-phases, Gi, S and 62 and is a period of intense metabolic activity of the nucleus.

During all 3 sub-phrases, the cell grows by replicating DNA, producing proteins and cytoplasmic organelles such as mitochondria and ER. There is a clear purpose for each phase:

G1 Phase (Gap 1)
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

S Phase (Synthesis)
Each DNA molecule undergoes semi-conservative DNA replication, resulting in the production of two identical DNA molecules.
Histone proteins are synthesised and associate with each DNA molecule.
After the DNA has replicated in S phase, they remain fully extended and uncoiled.

G2 Phase (Gap 2)
Since the formation of new DNA is an energy-consuming process, the cell 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.

Question 14

Characteristics of cells in non-dividing and dividing forms

Answer 14

Non-dividing cells (Interphase)

DNA exists in the form of very loosely-coiled and decondensed threads called chromatin

Chromatin exists in two forms:
Euchromatin - Decondensed/less condensed chromatin, Transcriptionally active

Heterochromatin - Highly condensed chromatin, Transcriptionally inactive
Nuclear envelope still intact and nucleolus present
Cell is transcriptionally active; gene expression occurs

Dividing cells (Mitosis)

DNA exists in a highly condensed form.
Chromatin condenses with the aid of specific proteins into discrete chromosomes.

Nuclear envelope has disintegrated and chromosomes are released into the cytoplasm. The nucleolus no longer present.

The cell is now transcriptionally inactive. Protein synthesis is temporarily suspended as the cell prepares for nuclear division.

Question 15

Mitosis

Answer 15

Mitosis involves the nuclear division of one nucleus Into two genetically identical nuclei. Following cytokinesis, it produces two daughter cells that are genetically identical to each other and to the original parent cell.

PMAT

Mitosis occurs in somatic cells. It allows daughter nuclei to receive precisely the same number of chromosomes as the parental nucleus. Hence, the diploid (2n) condition is maintained from one generation of cells to the next.

Passing identical genetic material to daughter cells is a crucial function of cell division, Changes in the genetic material of the daughter cells, such as having a different number of chromosomes from their parent cell, may lead to cancerous cells.

Question 16

Mitosis - Prophase

Answer 16

This is the longest stage of mitosis. During prophase. changes occur in both the nucleus and cytoplasm.

In the nucleus

• Nuclear envelope disintegrates into small vesicles, which disperse.
• Nucleolus gradually disappears.
• Chromatin become more tightly coiled (ie shorten and thicken) and condense into discrete chromosomes observable under a light microscope.

In the cytoplasm

• In animal cells, centriole pairs migrate to opposite poles of the cell. (Note that plant cells do not possess centrioles but are able to assemble the spindle apparatus nonetheless.)
• The spindle fibre that began to form in G2 phase of interphase continues to develop.

Question 17

Mitosis - Metaphase

Answer 17

Centriole pairs are positioned at opposite poles of the cell.
Shortening and thickening of the chromosomes is at its maximum.

Two sister chromatids are joined at the centromeres of each chromosome.
Each of the two sister chromatids of a replicated chromosome has a kinetochore at the centromere.
Kinetochore microtubules attach to the kinetochores at the centromeres of chromosomes.

Chromosomes migrate and align singly at the metaphase plate (also known as the equatorial 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.

Certain drugs like colchicine interfere with spindle function and can be used to arrest cells at metaphase.

Question 18

Mitosis - Anaphase

Answer 18

Anaphase 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 kinetochore microtubules are attached at the kinetochore/centromere, the chromosomes move with centromere leading towards the poles of the cell.

Separated daughter chromosomes are hence pulled along behind the centromeres, producing a characteristic ‘V’ shape pattern.

At the same time, the poles of the cell move farther apart as the polar microtubules slide past each other, hence 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 poles of the cell have equal and complete sets of chromosomes.

Question 19

Mitosis - Telophase

Answer 19

It begins when the daughter chromosomes reach their respective poles.

The chromosomes decondense into the chromatin form by uncoiling.

The nucleolus 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.

Mitosis, the equal division of one nucleus into two genetically identical nuclei, is now complete.

Question 20

To ensure integrity of genetic information in daughter cells

Answer 20

Interphase

Semi-conservative DNA replication during S phase of interphase.

Semi-conservative replication of DNA requires the parental DNA molecule to serve as the template for making genetically identical copies of daughter DNA molecule.
Semi-conservative DNA replication occurs before the disintegration of the protective nuclear membrane.
Semi-conservative DNA replication occurs prior to the distribution of genetic material to the two daughter cells.

Question 21

To ensure equal distribution of nuclear DNA to daughter cells

Answer 21

Prophase - Coiling of long, thin chromatin into condensed, thick and discrete chromosomes.
To prevent entanglement of chromatin and DNA breakage during the separation of genetic material.

To ensure that each daughter cell will have the complete diploid set of DNA:

Metaphase
Chromosomes align singly at the metaphase plate/equatorial plate of cell. No pairing of homologous chromosomes.

Anaphase
Separation of sister chromatids towards opposite poles of the cell by shortening of kinetochore microtubules.

Telophase
Daughter chromosomes reach the opposite poles of the cell before cytokinesis.

Question 22

Cytokinesis

Answer 22

Cytokinesis is the division of the cytoplasm to produce two daughter cells.

In preparation for cytoplasmic division, the cell organelles become evenly distributed towards the two poles of the parent cell, along with the chromosomes, during telophase.
This allows for the equal allocation of the cell organelles e.g. Golgi apparatus, mitochondria and cytoplasm to each daughter cell.

Two smaller, genetically identical cells result. These cells may then grow and develop into different forms via differentiation and developmental processes.

It generally begins simultaneously with telophase. Note that cytokinesis in animal cells differs from that in plant cells.

Question 23

Cytokinesis in Animals

Answer 23

Cytokinesis 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 cytoplasmic 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.

Question 24

Cytokinesis in Plants

Answer 24

No cleavage furrow is formed. The process occurs by the growth of a cell plate 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 a cell plate.

The 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 by the fusion of the vesicle membranes, fuses with the plasma membrane along the perimeter of the parent cell.

This results in the formation of two daughter cells, each with its own plasma membrane. Cellulose is laid down between the two membranes of the cell plate to form the cell wall.

Question 25

Significance of Mitosis - Genetic Stability

Answer 25

Mitosis produces two daughter cells which have the same number of chromosomes. Semi-conservative DNA replication during S phase of interphase gives rise to genetically identical daughter DNA molecules. Daughter chromosomes are subsequently distributed equally to the daughter cells. There is no variation in genetic information during mitosis. Hence daughter cells are genetically identical to the parent cell. The production of genetically identical cells ensures the preservation of genetic stability across generations of cells and hence, in the organism.

Question 26

Significance of Mitosis - Growth, repair, and regeneration

Answer 26

Growth If a tissue is to grow, it is important that the new cells formed are genetically identical to the existing cells in order to carry out the same functions. Repair Damaged cells are replaced by exact copies of the original, thus allowing for a tissue to be restored to its former condition. Regeneration Mitosis also allows for the regeneration of missing parts, such as legs in crustacean, and cell replacement occurs, to varying degree, in multicellular organisms.

Question 27

Asexual Reproduction

Answer 27

Many animal and plant series are propagated by asexual reproduction involving the mitotic division of cells. The offspring is identical to i.e. a clone, of their parents. Unicellular eukaryotic organism e.g. amoeba divides and forms duplicate offspring, the division of one cell reproduces an entire organism. This form of asexual reproduction involves mitotic cell division. The offspring cells are generally identical to the original parent organism- thus ensuring the preservation of favourable traits from generation to generation. Another example of asexual reproduction is vegetative propagation of plants.

Question 28

Meiosis + Interphase

Answer 28

Meiosis occurs only in specialised cells within the gonads of sex organs of sexually reproducing organisms. During meiosis, specialised reproductive or sex cells called gametes are produced. Gametes are haploid cells (n) and cannot be generated from diploid (2n) precursor cells by mitosis. Meiosis is preceded by an interphase where semi-conservative DNA replication occurs. This process of replication is similar to the semi-conservative DNA replication preceding mitosis. For each chromosome, the result is two genetically identical sister chromatids, which remain attached at their centromeres. The chromosomes only replicate once. The pair of centrioles also replicates during interphase. Meiosis involves two successive nuclear divisions, named meiosis I and meiosis II. The two nuclear divisions result in the production of four haploid daughter cells. This is to ensure that at the end of meiosis, each daughter cell contains only half the original complement of chromosomes of the original parent cell.

Question 29

Meiosis I - Prophase I

Answer 29

The nucleolus disappears and the nuclear membrane disintegrates. The chromatin condenses (shortens and thickens) until the chromosomes become discrete. 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 (unique to prophase I of meiosis), where the homologous are bridged by the synaptonemal complex consisting of proteins and RNA. The process is precise and brings the genes on each chromosome into precise alignment. The pairing of homologous chromosomes is essential for the exchange of alleles during crossing over in prophase I of meiosis. Crossing over occurs during prophase I of meiosis, where non-sister chromatids of homologous chromosomes undergo exchange of alleles. As a result, sister chromatids are now genetically non-identical and are known as recombinant chromatids Chiasma (plural: chiasmata) 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 may be formed at one or more points between non-sister chromatids of homologous chromosomes. Chiasmata become visible after synapsis ends, with the two homologues remaining associated due to sister chromatid cohesion.

Question 30

Meiosis I - Metaphase I

Answer 30

Kinetochore microtubules attach to the kinetochore at the centromeres of 1 chromosome (homologue) of each bivalent, while kinetochore microtubules from the opposite pole attach to the other homologue. Homologous chromosomes or bivalents randomly align at the metaphase plate or equatorial plate. Independent assortment of homologous chromosomes occurs at this stage. It refers to the fact that when a bivalent lines up on the metaphase plate, the orientation of homologues towards the poles in any one bivalent is random, and is independent of that of any other bivalent. Gives rise to genetic variation due to random distribution of paternal and maternal chromosomes into each gamete .

Question 31

Meiosis I - Anaphase I

Answer 31

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 'V’ pattern (Fig. 33). The centromeres in anaphase I of meiosis remain intact and the sister chromatids remain attached to each other. Physical segregation is referred to as disjunction, meaning the separation of chromosomes from one another. Note that non-disjunction, which is the failure of chromosome to separate, results in mutation.

Question 32

Meiosis I - Telophase I

Answer 32

Chromosomes arrive at opposite poles of the cell. Microtubules usually disassemble. 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. (In that case the chromatids are known as recombinant chromatids)

Question 33

Meiosis I - Cytokinesis I

Answer 33

Cytokinesis I occurs simultaneously with telophase I of meiosis, forming two haploid (n) daughter cells. Note that cytokinesis I does not occur in all species.

Question 34

Meiosis I (Change in number of chromosomes, amount of DNA, Events unique to meiosis I)

Answer 34

Meiosis I is also known as reduction division. The homologous chromosomes separate into two cells (the sister chromatids do not separate) Each daughter cell receives only one homologue of each pair homologous 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 the parent cell prior to replication. The events unique to meiosis occur in meiosis 1. These include: (a) synapsis and crossing over of homologous chromosomes (b) their subsequent segregation to different daughter cells

Question 35

Meiosis II - Interphase, Prophase II

Answer 35

The second meiotic division usually follows almost immediately after meiosis I. Depending on the species, the interphase between the 2 divisions is either very short or non-existent. Even if interphase is present, note that no further semi-conservative DNA replication occurs during the interphase. Prophase II Nucleoli disperse and nuclear envelopes disintegrate (if formed during telophase I of meiosis). Chromatin undergoes condensation (i.e. shorten and thicken) 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 arranged at right angles to the spindle of meiosis I.

Question 36

Meiosis II - Anaphase II

Answer 36

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.

Question 37

Meiosis II - Telophase II

Answer 37

The chromosomes decondense into the chromatin form by uncoiling. Microtubules disassemble and a pair of centrioles end up in each daughter cell. Nuclear envelopes reform around each nucleus.

Question 38

Meiosis II - Cytokinesis II

Answer 38

Cytokinesis 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.

Question 39

Meiosis II (Change in number of chromosomes, amount of DNA, Events unique to meiosis II)

Answer 39

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 II is known as equational division (equal in chromosomal number). The chromosome number does not change in this nuclear division. However, as a consequence of reduction division in meiosis l, 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.

Question 40

Significance of Meiosis - To increase genetic variation in gametes

Answer 40

Prophase I - Formation of bivalents to allow for crossing over to occur between the non-sister chromatids of homologous chromosomes - Gives rise to new combinations of paternal and maternal alleles in each chromatid. Homologous chromosome pairs are segregated into different daughter cells. Metaphase I - Independent assortment of paired homologous chromosomes at the metaphase plate - Random distribution of maternal and paternal chromosomes in each gamete

Question 41

Significance of Meiosis - To distribute half the nuclear DNA to each gamete

Answer 41

Interphase I - DNA replicates only once - Semi-conservative DNA replication is immediately followed by 2 successive nuclear divisions to produce haploid cells Metaphase I - Alignment of paired homologous chromosomes at the metaphase plate - To ensure equal distribution of chromosomes to each daughter cell in subsequent Anaphase I of meiosis Anaphase I - Segregation of homologous chromosomes - Equal distribution of chromosomes to each daughter cell. Chromosome number in each of the 2 resultant daughter cells is halved. Ploidy level in each of the 2 resultant daughter cells is haploid. Metaphase II - Chromosomes align singly at the metaphase plate - To ensure equal distribution of chromosomes to each gamete. Ploidy level of each gamete is haploid.

Question 42

Significance of Meiosis - Sexual Reproduction

Answer 42

Sexual reproduction (production of gametes) Meiosis results in the production of gametes with half the number of chromosomes in the parent cell in all organisms carrying out sexual reproduction If this does not occur, the fusion of gametes during subsequent fertilisation will result in a doubling of chromosome number for each successive generation. Meiosis, therefore, stabilises and maintains a constant chromosome number in every generation of a species.

Question 43

Significance of Meiosis - Genetic Variation

Answer 43

Meiosis also allows for new combinations of alleles in gametes. This leads to genetic variation in the genotype and phenotype of the offspring produced by the fusion of gametes. (a) Crossing over at prophase I of meiosis During prophase I of meiosis I, synapsis occurs and homologues pairs up along their lengths to form bivalents. This facilitates chiasmata formation and crossing over between non-sister chromatids of homologous chromosomes, resulting in recombination of segments of non-sister chromatids between homologous chromosomes. This leads to the formation of new combinations of alleles on the chromosomes of gametes. The resultant recombinant sister chromatids will then carry alleles different from those carried on the non-recombinant sister chromatids. Gamete cells that contain the recombinant chromosomes are called recombinant gametes, while those that contain the non-recombinant chromosomes are called parental gametes. (b) Independent assortment of homologous chromosomes at metaphase I of meiosis and metaphase II of meiosis II At metaphase I of meiosis, each pair of homologous chromosomes aligns at the metaphase plate. The orientation of the bivalents with respect to the poles is random. There is hence a 50% chance that a daughter cell gets a paternal chromosome or a maternal chromosome. As the orientation of one bivalent is independent of another, the first meiosis division results in random assortment of paternal and maternal chromosomes daughter cells between nuclei of daughter cells. This is termed independent assortment. Likewise, at metaphase II of meiosis. chromosomes randomly align singly at the metaphase plate. The orientation of one non-sister chromatid (due to crossing over) is independent of the other.

Question 44

Significance of Meiosis - Random fertilisation

Answer 44

Random fertilisation of gametes carrying different combinations of chromosomes adds to genetic variation of the zygote formed.