Cell Division

Question 1

What happens during gap (G1 and G2) phase?

Answer 1

The cell:

• Builds up a large store of energy
• Manufactures proteins
• Synthesises organelles

-G1 phase occurs after cytokinesis, longest phase in the cell cycle

Question 2

Describe what happens during synthesis

Answer 2

• DNA replication occurs
• Amount of DNA content in the cell is doubled
• DNA cannot be seen clearly because DNA is still loosely packed in chromatin

Question 3

Describe what happens during early prophase.

Answer 3

• Chromatin threads become thicker and shorter (condenses) forming distinct chromosomes
• Nucleolus disappears
• In animal cells, the two pairs of centrioles move to opposite ends of the cell

Question 4

Describe what happens in the cell during metaphase.

Answer 4

• Mitotic spindle fully forms

- Chromosomes line up at the equator of the spindle.

Question 5

Describe what happens during late prophase.

Answer 5

• The nuclear envelope breaks down
• Microtubules develop from each pair of centrioles to form the spindle fibres
• The microtubules attach to the centromeres of the chromosomes and move the chromosome towards the centre of the cell.

Question 6

What happens during anaphase?

Answer 6

-The sister chromatids of each chromosome are pulled apart by
the spindle fibres / microtubules shortening
-The separated sister chromatids of each chromosome move to
opposite poles of the spindle

• Poles of cell pushed further
  apart. Cell elongates.
• Lengthening of microtubules to elongate the cell

Question 7

Describe what happens during telophase

Answer 7

• The chromatids reach their respective poles of the spindle:
• Telophase is the REVERSE of prophase:
• Nucleolus reappears
• Nuclear envelope reforms to form two daughter nuclei
• Chromosomes uncoil to become long & thin chromatin threads
• Spindle fires disintegrate

Question 8

Describe the process of cytokinesis.

Answer 8

-In animal cells, a cleavage furrow forms and pinches the cell in
two, producing two daughter cells
-In plant cells, a cell plate forms, growing outwards until it
touches the parental walls forming two separate cells

Question 9

What are homologous chromosomes?

Answer 9

-A diploid cell has two sets of chromosomes, one derived from each parent

-Any two chromosomes that
determine the same characteristics are called homologous chromosomes

-Although homologous chromosomes determine the same characteristics, they need not be identical.

• They can contain different alleles of the same gene
  e. g. gene for eye colour has alleles for blue and black iris

Question 10

Why is mitosis important?

Answer 10

-Production of genetically identical cells is required for:

• Growth of an organism
• Repair of worn-out parts of tissue, wound healing and replacement of dead/damaged cells
• Asexual reproduction

Question 11

Why must DNA replication and mitosis be precisely controlled?

Answer 11

• To ensure that all daughter cells are genetically stable

- Prevent gene mutation - may lead to uncontrolled division of cells - cancer

Question 12

What are the causes of cancer?

Answer 12

• Age - chances increase with age
• Chemicals - carcinogenic chemicals e.g. tar
• Radiation - e.g. too much exposure to UV radiation in sunlight causes skin cancer
• Viruses e.g. Cancer of the cervix caused by papilloma virus
• Genetic factor

Question 13

What happens during Meiosis I - Prophase I?

Answer 13

-Same things that happen during mitosis

-Unlike Mitosis:
-Synapsis occurs, where homologous
chromosomes pair up, forming a
bivalent

-Crossing over between chromatids
of homologous chromosome may
occur

-The point where they cross over is
called a chiasma (plural: chiasmata)
This results in new combination of
alleles along the chromosomes

Question 14

What happens during Metaphase I?

Answer 14

-Meiotic spindle is now fully formed
-Bivalents arrange themselves at the equator of the spindle, aided by the spindle fibres
-The arrangement of chromosomes of each bivalent is completely independent of the orientation of other bivalents
-Centromeres of chromosomes are attached to individual spindle
fibres

Question 15

What happens during Telophase I and cytokinesis I?

Answer 15

-Chromosomes reach opposite poles
-Spindle fibres disintegrate
-In animal cells, nuclear envelope and nucleolus reforms and cytokinesis occurs to form two haploid (n) daughter cells
-Most plant cells do not undergo Telophase I and move straight to
Metaphase Il

Question 16

What is the importance of Meiosis?

Answer 16

Produces haploid (n) gametes, which fuse together during
fertilisation to produce diploid (2n) cells
• This ensures that the number of chromosomes in an
organism is maintained through successive generations
fertilisation

-Meiosis results in genetic variation:
-Which is essential for evolution
-Genetic variation allows natural selection to occur and
individuals best adapted to the environment survive to
reproduce offspring
-This allows species to change
constantly and adapt when the
environment changes

Question 17

How does genetic variation come about during meiosis?

Answer 17

Crossing Over at Prophase I:
-crossing over of segments between non-sister chromatids of homologous
chromosomes
-New combination of alleles on the chromosomes
of the gametes

2. Independent arrangement and separation of chromosomes at the equator of the spindle during metaphase II, and the independent separation of chromatids of these chromosomes in anaphase II
3. Independent arrangement and separation of homologous chromosomes in metaphase I and anaphase I