Cell Cycle, Mitosis & Meiosis

Question 1

Cell Cycle: Stages
1) Interphase - Period between divisions

2) Nuclear division (Mitosis/Meiosis) - Period when the nucleus divides into two (mitosis), or 4 (meiosis)

3) Cell division (Cytokinesis) - Division of the cytoplasm which follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis)

Question 2

Interphase: Stages
1) G1 Phase - Cell increases in size and new biomass is made
2) S Phase - DNA replicates by semi-conservative replication
3) G2 Phase - Cell prepares for division, synthesis, and stores of ATP and new organelles synthesised

Question 3

A gene is a section of DNA that codes for one specific polypeptide (protein)
The base sequence of DNA on the DNA codes for the sequence of amino acids in a protein.

Question 4

A chromosome is an independent DNA molecule which has been supercoiled into a condensed form

Question 5

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 histone proteins to help them coil in eukaryotes). While they are still joined, the two condensed DNA molecules are called identical sister chromatids. The whole structure is the chromosome.

Question 6

Homologous chromosomes:
Two chromosomes that carry the same genes in the same loci / location

Question 7

Somatic cells such as skin cells and liver cells are diploid (2n), whereas gametes such as sperm cells are haploid (n)

Question 8

Mitosis is used for:

· Increasing cell numbers and growth of an organism

· Repair of damaged tissues (not cells)

· Replacement of worn out/ dead cells

Question 9

Mitosis: Stages
1) Prophase - chromosomes supercoil and become visible

2) Metaphase - chromosomes line up on equator of cell

3) Anaphase - spindles get shorter, centromere splits identical sister chromatids separate

4) Telophase - two nuclei formed

Question 10

Prophase:
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 / shorten / thicken and become visible.
4) Each chromosome appear as 2 identical sister chromatids joined at the centromere

Question 11

Metaphase:
1) Spindle fibres form
2) The chromosomes are attached to the spindle fibres by their centromere
3) the chromosomes align down the equator of the cell.

Question 12

Anaphase:
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 13

Telophase:
1) Nuclear membrane starts to reform
2)The chromatids /chromosomes unwind / uncoil / become longer / thinner and become invisible

Question 14

Describe the appearance and behaviour of chromosomes during mitosis: [5 marks]

Answer 14

1. During prophase, chromosomes supercoil and condense to become visible;
2. Chromosomes appear as 2 identical sister chromatids joined by a 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 15

Tumour:
1) Mass of cells/tissue
2) Uncontrolled mitosis/cell division

Question 16

Tumour suppressor genes code for proteins that slow down the cell cycle.

Proto-onco genes code for proteins that speed up the cell cycle.

Question 17

If a mutation occurs in one of these control genes, then cell division continues, uncontrolled.
It is often rapid and the cells produced are abnormal. A tumour develops.
If these cells start to spread into the tissue/body, the tumour is regarded as a cancer.

Question 18

One method to treat cancer is to give drugs that inhibit cell division (mitosis)

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

Question 19

Binary fission in bacteria:

-replication of the circular DNA (not associated with histones) and of plasmids
di-vision of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids.

Answer 19

The cell elongates from the middle, separating the two DNA molecules, which are attached to different parts of the cell membrane. A new cell wall (murein), is formed down the middle of the elongated cell which eventually meets, dividing the cell in two.

This is NOT MITOSIS
There is no nucleus, no chromatids, no centrioles and no spindle fibres.

Binary fission is very fast and bacteria can double every 20 min under optimum conditions, though the doubling time is usually slower.

Question 20

Virus Replication:
1) The protein moleucles on the capsid attached to the cell surface membrane of the hosts cells.

2) The virus genetic material enters the cells cytoplasm.

3) The viruses nucleic acid (DNA/RNA) is inserted into the host cells own nuclear DNA.

4) The host cell is now infected.

5) The viral DNA is transcribed and translated into proteins (protein coats etc)

6) The cell replicates the viral DNA.

7) The genetic material and proteins coats are assembled into virions

8) Eventually these virions burst out of the cell, often destroying the host cell

Question 21

Mitotic Index = (Number of cells in PMAT) ÷ (Total number of visible cells)

Question 22

Calculating the length of each stage of mitosis:

(Total number of cells in mitosis) ÷ (Total number of cells)
𝑥 Time of 1 cell cycle in minutes

Question 23

Serial Dilution:
If you are given a stock solution and want to make a series of dilutions of a known volume, you can use the following formula:

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 24

Meiosis:

-Used by diploid (2n) organisms to produce haploid gametes (n) for sexual reproduction
-Results in four genetically different daughter cells
-Halves the chromosome number (ensuring each egg or sperm only has one copy of each chromosome n) before fertilisation to restore the full chromosome number at fertilisation 2n

Question 25

Process of Meiosis

In a meiotic division, the 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
(INDEPENDENT SEGREGATION 2n to n).
In the second division, the chromatids are separated

Question 26

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

Answer 26

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 27

Meiosis increases genetic variation between the daughter cells via two processes during Meiosis 1 :

-Crossing over between non - sister chromatids of homologous chromosomes
-Independent segregation of homologous chromosomes during Meiosis

-Random fertilisation of haploid gametes produced by meiosis also causes variation within a population.

Question 28

Crossing Over:

1. The homologous chromosomes associate (Bivalent is formed)
2. Chiasmata forms
   (Chromosomes entangle / twist)
3. Equal lengths of (non-sister) chromatids / alleles are exchanged.
4. Producing new combinations of alleles

Question 29

Independent Segregation:

1) Homologous chromosomes attach to the spindle fibres and pair up side by side randomly

2) The maternal and paternal chromosomes are reshuffled in any combination on the equator of the cell

Question 30

Calculating the possible number of different combinations of chromosomes:

2^n

n= the number of chromosomes

Question 31

Describe and explain the processes that occur during meiosis that increase genetic variation. [5 marks]

Answer 31

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 (non-sister) chromatids/alleles between homologous chromosomes
5. (both) create new combinations of alleles.

Question 32

Give two differences
between mitosis and
meiosis

Answer 32

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 33

Describe how the process of meiosis results in haploid cells. Do not include descriptions of how genetic variation is produced in meiosis

Answer 33

1. DNA replication (during late interphase);
2. Two divisions;
3. Separation of homologous chromosomes (in first division); 4. Separation of (sister) chromatids (in second division); 5. Produces 4 (haploid) cells/nuclei

Question 34

Describe binary fission in
bacteria

Answer 34

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

Question 35

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 35

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 36

Describe how you would
determine a reliable mitotic
index (MI) from tissue
observed with an optical
microscope.

Answer 36

1. Count cells in mitosis in field of view;
2. Divide this by total number of cells in field of view;
3. Repeat many/at least 5 times