Cell Division

Question 1

The cell cycle

Answer 1

1. Interphase
   This is the period between divisions. The majority of the life of the cell is spent in INTERPHASE G1,

G1 phase- Cell increases in size and new biomass is made
S phase- DNA replicates by semi-conservative DNA replication
G2 phase- Cell gets ready for division, synthesis & stores of ATP and new organelles synthesised.

2. Nuclear Division (Mitosis or Meiosis)
   Period when the nucleus divides into two, or 4.
3. Cell Division (Cytokinesis)
   Division of the cytoplasm to produce two new cells or four new cells.

Question 2

Gene definition

Answer 2

section of DNA that codes for one polypeptide

Question 3

Chromosome definition

Answer 3

independent DNA molecule which has been supercoiled into a condensed form

Question 4

Chromosome structure

Answer 4

During semi conservative DNA replication, 2 genetically identical copies of the DNA molecule are made.

They remain attached at a central point called the centromere.

Each of the DNA copies now supercoils using histones

While they are still joined, the two condensed DNA molecules are called identical sister chromatids.

The whole structure is the chromosome.

Question 5

Homologous chromosomes definition

Answer 5

have the same genes in the same gene loci but may have different alleles of the genes.

Question 6

Calculating diploid and haploid cells

Answer 6

diploid = 2n
haploid = n
n = number of chromosomes

Question 7

What is mitosis used for?

Answer 7

• Increasing cell numbers and growth of an organism
• Repair of damaged tissues (not cells)
• Replacement of worn out/ dead cells

Question 8

What is mitosis?

Answer 8

DNA molecules are replicated, the identical copies separated and are passed on to the two daughter cells.
The 2 daughter cells are genetically identical.
They have the same number of chromosomes as the parent cell.
They are genetic clones of each other.
It is also used by some organisms for asexual reproduction

Question 9

Prophase

Answer 9

1) The nuclear membrane starts to break down.

2) The centrioles start to move to the poles of the cell and make spindle fibres

3) The chromosomes supercoil and condense

4) Each chromosome appear as 2 identical sister chromatids joined at the centromere

Question 10

Metaphase

Answer 10

1)The centrioles complete the production of spindle fibres

2)The chromosomes are attached to the spindle fibres by their centromere

3) the chromosomes align down the equator of the cell.

Question 11

Anaphase

Answer 11

1) The spindle fibres contract/shorten.

2)The centromere splits

3)The identical sister chromatids are pulled to
opposite poles

4) making a “V” shape

Question 12

Telophase

Answer 12

1)A nuclear membrane starts to reform around each set of chromosomes

2)The chromatids unwind and become invisible

Question 13

Unregulated Mitosis

Answer 13

• Tumour suppressor genes code for proteins that slow down the cell cycle.
• Proto-onco genes code for proteins that speed up the cell cycle

Mutation= uncontrolled cell division= tumour develops

Question 14

Treating cancer

Answer 14

One method is to give drugs that inhibit cell division

These drugs may stop:
DNA replication, spindle formation, Cytokinesis or other processes linked to mitosis

Question 15

Binary Fission

Answer 15

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

Question 16

Virus Replication

Answer 16

Being non-living, viruses do not undergo cell division.

Following injection of their nucleic acid, the infected host cell replicates the virus particles.

Viruses can only replicate inside host cells.
use the host cell enzymes to replicate and transcribe viral DNA and translate viral proteins, making more virus particles, which then burst out of the cell.

Question 17

Preparation of plant roots

Answer 17

50 °C Hydrochloric acid:
To break down the cellulose cell walls, ensuring the stain can enter and the tissue and the root tissue can be squashed into a layer one cell thick so that light passes through the thin sample of tissue

Ethano-orcein stain:
To stain the chromosomes so they can be viewed under the microscope.

Question 18

Mitotic Index

Answer 18

Number of cells with condensed chromosomes / total number of cells

Question 19

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.

Answer 19

1. Examine many cells;
2. To ensure representative sample;

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

Question 20

Serial Dilution

Answer 20

M1 X V1 = M2 X V2

M1 = Desired diluted concentration (mol dm-3)
V1 = Desired Volume (cm3)
M2 = Original concentration (mol dm-3)
V2 = Unknown Volume of stock solution (cm3)

Question 21

Meiosis

Answer 21

used by diploid organisms to produce haploid gametes for sexual reproduction.
increases variation within the population.
Meiosis results in four genetically different daughter cells.
halves the chromosome number before fertilisation to restore the full chromosome number at fertilisation 2n.

Question 22

Meiosis process

Answer 22

DNA is replicated in S phase (interphase) of the cell cycle. The chromosomes then go through two nuclear divisions.
Each division has a prophase, metaphase, anaphase and telophase.
In the first meiotic division, the homologous chromosomes pair up and are separated
In the second division, the chromatids are separated.

Question 23

Causes of Variation

Answer 23

mutations
random fertilisation
crossing over (occurs in meiosis 1)
independent segregation (occurs in meiosis 1)

Question 24

Crossing over

Answer 24

1. The homologous chromosomes associate. Bivalent is formed
2. Chiasmata form
3. Equal lengths of alleles are exchanged.
4. Producing new combinations of alleles.

(during prophase or metaphase)

Question 25

INDEPENDENT SEGREGATION

Answer 25

homologous chromosomes attach to the spindle fibres and they pair up side by side and can be reshuffled in any combination.
- maternal and paternal chromosomes are arranged in any order

Question 26

Give two differences between mitosis and meiosis.

Answer 26

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;