Cell Cycle and Cell Division

Question 1

What is a chromosome? (1)

Answer 1

Independent DNA molecule in a condensed form which contains many genes (using histones)

Question 2

What is a gene? (1)

Answer 2

Section of DNA coding for a specific polypeptide

Question 3

What is the cell cycle? (2)

Answer 3

The regular sequence of growth and division that cells undergo

Question 4

What are the stages of the cell cycle? (3)

Answer 4

Interphase
Mitosis or meiosis
Cytokinesis

Question 5

What happens during interphase? (3)

Answer 5

G1: cell increases in size and new proteins are made
S: DNA replicates by semi-conservative replication
G2: Cell gets ready for division, synthesises and stores of ATP and new organelles synthesised

Question 6

What is the purpose of mitosis? (5)

Answer 6

Produces 2 identical daughter cells for:
Repair of damaged tissues
Replacement of worn out/dead cells
Increasing cell number and growth of an organism
Asexual reproduction

Question 7

What are the stages of mitosis? (4)

Answer 7

P M A T
P - prophase
M- metaphase
A - anaphase
T - telophase

Question 8

What happens during prophase? (4)

Answer 8

Chromosomes supercoil and condense to become visible
Nuclear membrane starts to break down (chromosome free in cytoplasm)
Centrioles move to opposite poles and make mitotic spindle fibres
Each chromosome appears as 2 identical sister chromatids joined at the centromere (X-shaped)

Question 9

What happens during metaphase? (2)

Answer 9

Sister chromatids line up along the equator
And attached to spindle fibres by their centromere

Question 10

What happens during anaphase? (3)

Answer 10

Spindle fibres contract
Centromere splits
Identical sister chromatids pulled to opposite poles and appear as 2 distinct chromosomes (‘V’ shape)

Question 11

What happens during telophase? (2)

Answer 11

Nuclear membrane starts to reform around each set of chromosomes
Chromosomes unwind and become thinner to become invisible again

Question 12

What happens during cytokinesis? (2)

Answer 12

Cytoplasm splits
2 identical daughter cells formed

Question 13

What are homologous chromosomes? (2)

Answer 13

Chromosomes that have the same genes in the same gene loci but may have different alleles of the genes
One chromosome is maternal and the other is paternal

Question 14

What is meiosis? (2)

Answer 14

Type of cell division that results in four genetically different daughter cells each with half the number of chromosomes of the parent cell

Question 15

What happens during the first stage of meiosis? (3)

Answer 15

Homologous chromosomes pair to form a bivalent
Crossing over takes place at the chiasmata
Cell divides into two and homologous chromosomes separate randomly

Question 16

What happens during the second stage of meiosis? (2)

Answer 16

Independent segregation
Each cell divides to produce 4 genetically different daughter cells

Question 17

How does meiosis create genetic variation? (2)

Answer 17

Crossing over and independent segregation
Both result in new combination of alleles

Question 18

What is crossing over? (4)

Answer 18

Homologous chromosomes associate (bivalent formed)
Chiasmata forms
Equal lengths of alleles exchanged between non-sister chromatids at the chiasmata
New combination of alleles

Question 19

What is independent segregation? (2)

Answer 19

Maternal and paternal chromosomes are re-shuffled in any combination on the equator of the cell
Creates new combinations of alleles therefore increasing genetic variation

Question 20

What is binary fission? (1)

Answer 20

A type cell division used by prokaryotic cells to reproduce & split into two

Question 21

What is the process of binary fission? (4)

Answer 21

Replication of circular DNA and of plasmids
Cell elongates from the middle and separates the two DNA molecules, which are attached to different parts of the cell membrane
New cell wall (murein) forms down the middle of the elongated cell which eventually meets
Cell divides into 2

Question 22

Why does mitosis not take place in prokaryotes? (4)

Answer 22

There is no nucleus, no chromatids, no centrioles and no spindle fibres

Question 23

Explain the procedure of a root tip squash experiment (3)

Answer 23

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

Question 24

Explain how to prepare a temporary mount (7)

Answer 24

Place end section of root in acid to halt cell division
Wash end section of root in water
Cut tip from root section
Thin section
Put on slide in water/solution/stain
Break up the tissue with a mounted needle
Add cover slip and squash

Question 25

Why is only the tip of the root used (2)

Answer 25

Where mitosis occurs
Cells further from the root tip are elongating rather than dividing

Question 26

Why are viruses classified as non-living? (3)

Answer 26

No cytoplasm
No metabolism
Cannot self-replicate

Question 27

Outline how viruses replicate (4)

Answer 27

Attachment proteins attach to receptor on host cell membrane
Enveloped viruses fuse with cell membrane OR release DNA/RNA into cytoplasm OR viruses inject DNA/RNA
Host cells use viral genetic information to synthesise new viral proteins

Question 28

How do new viral particles leave the host cell (2)

Answer 28

Bud off and use cell membrane to form envelope
Cause lysis of host cell

Question 29

Why is it difficult to develop effective treatments against viruses (2)

Answer 29

Replicate inside living cells so difficult to kill them without killing host cells