Topic 2.3 Eukaryotic cell cycle and division

Question 1

State what the cell cycle is and outline its stages

Answer 1

Regulated cycle of division with intermediate growth periods.

1. Interphase
2. Mitosis or meiosis (nuclear division)
3. Cytokinesis (cytoplasmic division)

Question 2

Outline what happens during interphase

Answer 2

G1(Gap 1): cell synthesises proteins for replication e.g. tubulin for spindle fibres and cell size doubles

S (Synthesis of DNA): DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere

G2(Gap 2): organelles divide

Question 3

What is the purpose of mitosis?

Answer 3

Produces 2 genetically identical diploid daughter cells for:
* Growth
* Cell replacement/ tissue repair
* Asexual reproduction

Question 4

What are diploid cells

Answer 4

Cells that have two complete sets of chromosomes

Question 5

Name the stages of mitosis

Answer 5

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Remember order through PMAT

Question 6

Outline what happens during prophase

Answer 6

1. Chromosome condense, becoming visible. (X-shaped: 2 sister chromatids joined at centromere)
2. Centrioles move to opposite poles of cell (animal cells) and mitotic spindle fibres form
3. Nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 7

Outline what happens during metaphase

Answer 7

Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres

Question 8

Outline what happens during anaphase

Answer 8

Requires energy from ATP hydrolysis

1. Spindle fibres contract = centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of cells. (looks like ‘V’ shapes facing each other)
3. Spindle fibres break down

Question 9

Outline what happens during telophase

Answer 9

1. Chromosomes decondense, becoming invisible again
2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome

Question 10

What happens during cytokinesis?

Answer 10

1. Cell membrane cleavage furrow forms
2. Contractile division of cytoplasm

Question 11

Explain how to prepare a temporary root tip squash

Answer 11

1. Place root in hydrochloric acid to halt cell division and hydrolyse middle lamella
2. Stain root tip with a dye that binds to chromosomes
3. Macerate tissue in water using mounted needle
4. Use mounted needle at 45^o to press down cover slip and obtain a single layer of cells. avoiding trapping air bubbles

Question 12

Explain the procedure for a root tip squash experiment

Answer 12

1. Prepare a temporary mount of root tissue
2. Focus an optical on the slide. Count total number of cells in the field of view and number of cells in a stage of mitosis
3. Calculate mitotic index (proportion of cells undergoing mitosis

Question 13

Name two dyes that bind to chromosomes

Answer 13

Toluidine blue (blue)

Acetic orcein (purple-red)

Question 14

Why is only the root tip used when calculating a mitotic index?

Answer 14

Meristematic cells at root tip are actively undergoing mitosis.

Cells further from root tip are elongating rather than dividing.

Question 15

What is meiosis?

Answer 15

A from of cell division that produces four genetically different haploid cells known as gametes

Question 16

What are haploid cells?

Answer 16

Cells with half the number of chromosomes found in the parent cell

Question 17

What happens during meiosis I?

Answer 17

1. Homologous chromosomes pair to form bivalents
2. Crossing over (exchange of sections of genetic material) occurs at chaismata
3. Cells divide into two. Homologous chromosomes separate randomly. Each cell contained either maternal or paternal copy

Question 18

What happens in meiosis II?

Answer 18

1. Independent segregation of sister chromatids
2. Each cell divides again, producing 4 haloid cells

Question 19

Draw diagrams to show cells after each stage of meiosis

Answer 19

Parent cell: XXXX

After meiosis I: 2 XX cells

After meiosis II: 4 II cells

Question 20

How does meiosis produce genetic variation?

Answer 20

• Crossing over during meiosis I
• Independent assortment (random segregation) of homologous chromosomes and sister chromatids

Results in new combinations of alleles.

Question 21

What is a chromosome mutation?

Name the types

Answer 21

Involves section of a chromosome rather than 1 base.

Types include: deletion, inversion, duplication, translocation.

Question 22

What is a translocation mutation?

Answer 22

A base sequence from one chromosome fuses with a non-homologous chromosome.

Question 23

What is a non-disjunction mutation?

Answer 23

Chromosomes fail to separate correctly in meiosis.

Gametes have at least one more (polysomy) or one less (monosomy) chromosomes than normal.

Question 24

Give examples of conditions caused by non-disjunction

Answer 24

trisomy-21 = Down’s syndrome

X-monosomey of sex chromosomes = Turner’s syndrome

Question 25

Give various methods of asexual reproduction, and the organisms that use that method

Answer 25

• Binary fission - simple microbes - a single microbe splits into two daughter cells
• Budding - other microbes e.g. hydra - two daughter cells form but they aren’t the same size
• Vegative reproduction - higher organisms (especially plants) e.g. strawberry runners

Asexual reproduction in animals is rare on higher animals, but is known in lizards. It’s more common in lower animals e.g. the green fly