Cell Division

Question 1

importance of mitosis

Answer 1

• enables growth of an organism
• needed for repair of worn-out parts of the body
• allows asexual reproduction to occur: eg, mitosis cause shoots & roots to develop in storage organs, these shoots & roots can then grow into new daughter plants that are identical to their parent plants

Question 2

explain the need for the production of genetically identical cells

Answer 2

• DNA replication copies all info stored within chromosomes
• ensures that daughter cells produced by mitosis contain all sections of DNA needed for subsequent cell division & differentiation
• resulting daughter cells are genetically identical

Question 3

main stages of mitosis

Answer 3

Prophase:
- Chromatin threads condense, coil and shorten to become chromosomes (seen as X-shaped structures)
- In an animal cells, asters, made of microtubules, form around the centrioles.
- Nucleolus and nuclear envelope disappear.
- A spindle forms with spindle fibers extending from one pole of the cell to the other.

Metaphase:
- Chromosomes line up along the equatorial plane of the spindle.
- Centromere of each chromosome is attached on both sides to a spindle fibre.

Anaphase:
- Each centromere divides.
- Spindle fibres shorten and pull the chromatids apart to opposite poles of the cell.
- Once the chromatids are separated, they are called daughter chromosomes

Telophase:
- Spindle fibres break down.
- Nuclear envelope forms around the chromosomes at each pole of the cell.
- Nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become thin chromatin threads.

Question 4

homologous pairs of chromosomes

Answer 4

• homologous chromosomes have the same shape, same genes & same length
• have the same order of sequence of gene loci but alleles in those gene loci may be the same

Question 5

main stages of meiosis

Answer 5

Prophase I:
- Chromatin threads condense, coil and shorten to become chromosomes.
- Homologous chromosomes pair along their whole length (synapsis)
- Chromatids of homologous chromosomes may cross each other at the chiasma.
- Nuclear envelope and nucleolus disappear. Spindle fibres form.

Metaphase I:
- Pairs of HC arrange themselves along the equatorial plane
- Each chromosome is attached to a spindle fibre.

Anaphase I:
- HC separate and are pulled to opposite poles of the cell as the
spindle fibres shorten.

Telophase I:
- A nuclear envelope forms around the chromosomes at each pole.
- Division of cytoplasm occurs
- Cytoplasm cleaves into two, producing two daughter cells, each with haploid number of chromosomes.
- Centrioles divide.

Prophase II:
- Centrioles move to opposite poles of the cell.
- Nuclear envelope disappears.
- Spindle fibres appear.

Metaphase II:
- Chromosomes arrange themselves along the equatorial plane of the spindle.

Anaphase II:
- Centromeres divide.
- Sister chromatids separate to become daughter chromosomes, which are pulled towards opposite poles of the cell.

Telophase II:
- Spindle fibres disappear.
- Nuclear envelopes form around the two daughter chromosomes at each pole.
- Nucleolus reforms.
- Cleavage of cytoplasm results in four daughter cells being produced, each with half the
number of chromosomes as the parent cell.

Question 6

haploid vs diploid

Answer 6

Haploid cells contain only one set of chromosomes.
Diploid cells have two sets of chromosomes.

Question 7

explain the need for a reduction division process prior to
fertilisation in sexual reproduction

Answer 7

• when nucleus of male gamete fuses with nucleus of female gamete, the diploid number of chromosomes is restored in zygote
• maintains the normal diploid number of chromosomes in the species

Question 8

how meiosis and fertilisation can lead to variation

Answer 8

• meiosis results in variation in the gametes produced: occur due to crossing over & independent assortment of chromosomes
• independent assortment of chromosomes means one chromosome from each pair can combine with either chromosome of the other pair, which results in 4 different gametes being produced from 2 chromosomes
• since fertilisation is random (any sperm can fuse with any egg), such variations in gametes produce variations in offspring
• variations increase the chances of survival of the species during changes in the environment