Mitosis + Meiosis

Question 1

What cells can divide?

Answer 1

• Within multicellular organisms, not all cells retain the ability to divide
• Eukaryotic cells which can divide have a cell cycle
• These are stem cells

Question 2

Interphase and mitosis

Answer 2

• Interphase - DNA replications occurs and proteins are synthesised
• Mitosis - when the eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA produced by the parent cell during DNA replication

Question 3

Stages of the cell cycle (whole thing)

Answer 3

• Interphase - the whole stage before mitosis
  1. G1 - growth of cytoplasm and organelles
  2. S - DNA replication
  3. G2 - protein synthesis
  4. Mitosis - PMAT
  5. Cytokinesis- when they divide into two cells

Question 4

Cell cycle in detail

Answer 4

1. Interphase - occurs before mitosis. Cell actively synthesises proteins. DNA is replicated.
   Mitosis
2. Prophase - DNA condenses and so the chromosomes become visible. The nuclear envelope breaks down and the nucleolus disappears.
3. Metaphase - Chromosomes lines up along the equator of the cell. Centrioles move to the poles and form spindle fibres which attach to the centromeres of the chromosomes.
4. Anaphase - spindle fibres contract and pull apart the sister chromatids to opposite poles of the cell
5. Telophase - the cell membrane pinches and begins to divide the cell in two
   Mitosis
6. Cytokinesis - this occurs after mitosis. The cell membrane and cytoplasm divide. Two genetically identical daughter cells are produced.

Question 5

Cancer - normal cells

Answer 5

• stay in G1 stage until given a specific signal to enter the S phase
• are mortal - so divide about 50 times, lose the ability to divide and eventually die
• those that suffer significant chromosome damage destroy themselves

Question 6

Cancer - normal cells

Answer 6

• stay in G1 stage until given a specific signal to enter the S phase
• are mortal - so divide about 50 times, lose the ability to divide and eventually die
• those that suffer significant chromosome damage destroy themselves

Question 7

Cancer - cancer cells

Answer 7

• cancer cells enter S phase without a signal
• cancer cells are immortal and divide endlessly
• cancer cells do not destroy themselves
• grow irregularly and usually have different shapes to healthy cells
• able to move around due to loss of molecules on the surface that usually hold the cells in place with their neighbours

Question 8

Cancer - metastasis

Answer 8

• malignant tumours being able to break free and move through the blood or lymph to form secondary tumours

Question 9

Cancer - tumours

Answer 9

• if a tumour has a bloody supply it is called a large mass
• benign tumours are held in a membrane so do not spread
• malignant tumours can do metastasis

Question 10

Binary fission

Answer 10

• occurs in prokaryotic cells

1. Replication of circular DNA and plasmids
2. Division of cytoplasm to produce two daughter cells, each with a single copy of circular DNA and a variable number of plasmids

Question 11

Meiosis - the process

Answer 11

1. Before meiosis starts, the cell is in interphase - synthesising proteins and replicating its DNA
   Prophase I
2. DNA condenses so that the chromosome becomes visible.
3. The nuclear envelope breaks down and the nucleolus disappears.
   Metaphase I
4. Chromosomes line up along the equator in their homologous pairs
5. Centrioles move to the poles of the cell and form spindle fibres.
6. The spindle fibres attach to the centromeres of the chromosome
   Anaphase I
7. The spindle fibres contract and pull the chromosomes apart to opposite poles of the cell
   Telophase I
8. The cell membrane pinches and begins to divide the cell in two.
9. The nuclear envelope reforms.
   Cytokinesis I
10. Two haploid cells are formed. They have half the genetic info but still have sister chromatids so have twice the amount of DNA needed - this is known as reduction division
    Prophase II
11. The DNA slightly uncoiled during cytokinesis. It condenses again to make chromosomes visible.
    Metaphase II
12. Similar to earlier, the chromosomes line up along the equator of the cell but not in their homologous pairs this time.
13. The centrioles move to the poles of the cell and form spindle fibres, which attach to the centromeres of chromosomes.
    Anaphase II
14. Spindle fibres contract, pulling the the sister chromatids apart to opposite poles of the cell.
    Telophase II
15. The cell membranes pinch and begin to divide the cells in two.
16. The nuclear envelopes reform.
    Cytokinesis II
17. Four genetically different, haploid daughter cells are produced.

Question 12

Crossing over

Answer 12

• one of the events that creates more variation in gametes produced by meiosis

1. The individual chromosomes of each homologous pair come into close contact with one another and cross over at the chiasma.
2. They twist and look tangled.
3. During the twisting process, tensions are created and portions of the chromatids break off.
4. These broken portions may then rejoin with the chromatids of its homologous pair.
5. Typically, it is the equivalent portions of homologous chromosomes that are exchanged.
6. Through this process, new genetic combinations of maternal and paternal alleles are produced

Question 13

Independent segregation

Answer 13

• happens during the lining up of homologous pairs of chromosomes during the first meiotic division (metaphase I)
• the homologous pairs line up randomly
• therefore when they are pulled apart in anaphase I, the combination of chromosomes going into the daughter cells is random (a mixture of the mother’s and father’s chromosomes)

Question 14

Importance of genetic variation

Answer 14

• natural selection - adaptations that allow the organism to survive better in their environment
• those that survive will pass on beneficial alleles to their offspring
• allows for species to adapt to changing environments

Question 15

Random fertilisation

Answer 15

• when fertilisation occurs, the egg and sperm that fuse together are random
• this further increases genetic variation within a species

Question 16

Explain genetic mutations

Answer 16

• gene mutations involve a change in the base sequence (primary structure/amino acid sequence) of chromosomes
• they can arise spontaneously during DNA replication
• includes base deletion and substitution
• due to the degenerate nature (where in multiple codons can code for the same amino acid) of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids
• mutagenic agents can increase the rate of gene mutation
• mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction (the failure of sister chromatids or homologous chromosomes to separate) during meiosis

Question 17

PMAT - Prophase

Answer 17

DNA condense and becomes visible.
The nuclear envelope breaks downs.
The nucleolus disappears.

Question 18

PMAT - Metaphase

Answer 18

Chromosomes line up along the equator of the cell.
Centrioles move to the poles and form spindle fibres.
These attach to the centromeres of the chromosomes.

Question 19

PMAT - Anaphase

Answer 19

Spindle fibres contract and pull apart the sister chromatids to opposite poles of the cell.

Question 20

PMAT - Telophase

Answer 20

The cell membrane pinches and begins to divide the cell in two.
The nuclear envelopes reform around the genetic materials.