Cell cycle & Cell division - key points

Question 1

Describe the features of prophase

Answer 1

Nuclear membrane begins to breakdown
Centrioles move to the poles of the cells
Chromatin supercoils and condenses into chromosomes

Question 2

Describe the features of Metaphase

Answer 2

Spindle fibres form
Spindle fibres attach
To the centromere of chromosomes
Chromosomes align at the equator

Question 3

Describe the features of Anaphase

Answer 3

Spindle fibres shorten
Centromere splits
Sister chromatids are separated
Chromatids pulled to opposite poles of the cells

Question 4

Describe the features of Telophase

Answer 4

Nuclear membrane begins to reform;
Chromosomes unwind;

Question 5

What is a homologous pair of chromosomes?

Answer 5

two chromosomes that carry the same genes in the same loci

Question 6

Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate (2)

Answer 6

Description; Explanation;
E.g, 1. Examine large number of fields of view / many cells;
2. To ensure representative sample;

OR
3. Repeat count;
4. To ensure figures are correct;

OR

5. Method to deal with part cells shown at edge /count only whole cells;
6. To standardise counting;

Question 7

Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. Explain how (7)

Answer 7

1. Homologous chromosomes pair up;
2. maternal and paternal chromosomes are arranged in any order;
3. Independent segregation;
4. Crossing over;
5. (Equal) Portions of chromatids are swapped between chromosomes;
6. Produces new combination of alleles;
7. Chromatids separated at meiosis II/ later;

Question 8

Describe the process of crossing over and explain how it increases genetic diversity (4)

Answer 8

1. Homologous pairs of chromosomes form a bivalent;
2. Chiasmata form;
3. Equal lengths of chromatids / alleles are exchanged;
4. Producing new combinations of alleles;

Question 9

Give two differences between mitosis and meiosis (7)

Answer 9

Mitosis given first
1. One division, two divisions in meiosis;
2. (Daughter) cells genetically identical, daughter cells genetically different in meiosis;
3. Two cells produced, (usually) four cells produced in meiosis;
4. Diploid to diploid/haploid to haploid, diploid to haploid in meiosis;
5. Separation of homologous chromosomes only in meiosis;
6. Crossing over only in meiosis;
7. Independent segregation only in meiosis:

Question 10

Describe binary fission in bacteria.

Answer 10

1. Replication of (circular) DNA;
2. Replication of plasmids;
3. Division of cytoplasm (to produce daughter cells);

Question 11

Describe the appearance and behaviour of chromosomes during mitosis (5 marks) (8)

Answer 11

1. During prophase, chromosomes supercoil and condense to become visible
2. Chromosomes appear as 2 identical sister chromatids joined by centromere
3. During metaphase, chromosomes line up on the equator of the cell
4. Chromosomes attach to the spindle fibres
5. By their centromeres
6. During anaphase, the centromere splits
7. Sister chromatids are pulled to opposite poles of the cell making a v-shape
8. During telophase, chromatids uncoil and become thinner

Question 12

Describe and explain the processes that occur during meiosis that increase genetic variation (5 marks) (5)

Answer 12

1. homologous chromosomes pair up / bivalents form
2. Independent segregation occurs
3. Maternal and paternal chromosomes are reshuffled in any combination
4. crossing over leads to exchange of parts of chromatids
5. creates new combinations of alleles