7B Meiosis

Question 1

1. What is meiosis?

Answer 1

Meiosis is a specialised form of cell division that occurs in sexually reproducing organisms and is used to produce the gametes necessary for sexual reproduction. A germline cell duplicates its DNA and divides twice, creating 4 genetically unique daughter cells.

Question 2

2. In humans what are the gametes, what do they fuse together to create?

Answer 2

In humans, these gametes are sperm and egg (also known as ova) cells, which contain only one copy of each chromosome (n) and will fuse together during fertilisation to create two copies of each chromosome (2n) inside a developing zygote.

Question 3

3. In humans, what is the dividing cell called, where it is found?

Answer 3

In humans, the dividing cell is known as a germline cell and is found in the gonads (testes for males and ovaries in females).

Question 4

4. Explain the steps of meiosis 1 – prophase 1

Answer 4

• After DNA inside the diploid cell, that is a cell which has two complete sets of chromosomes, has replicated during interphase prior to meiosis, prophase 1 begins. In prophase 1, the DNA condenses into chromosomes with two identical sister chromatids.
• These chromosomes attach to the nuclear membrane and move around to find their homologue and when paired, they are called bivalents.
• The bivalents then undergo crossing over which is when equivalent sections of chromatids break and are recombined, which exchanges genes between the homologous chromosomes.
• The nuclear membrane then disappears, and the centrioles move to opposite sides of the cell with spindle fibres appearing

Question 5

5. Explain the steps of meiosis 1 –metaphase 1

Answer 5

• In metaphase 1, the spindle fibres extend towards the bivalents, and attach to the kinetochores on the centromeres of the chromosomes
• The bivalents are then lined up on the equator of the cell by the spindle fibres. When these line up, they go through a process called independent assortment. Independent assortment is the random orientation of bivalents along the metaphase plate which causes variation in the genome of the daughter cells.

Question 6

6. Explain the steps of meiosis 1 - anaphase 1

Answer 6

In anaphase 1, the spindle fibres shorten, pulling the bivalents apart and towards opposite poles of the cell.

Question 7

7. Explain the steps of meiosis 1 – telophase 1

Answer 7

In telophase 1, the chromosomes start to decondense, and the nuclear membrane starts to reform. A contractile ring then creates a cleavage furrow in the cell membrane which continues getting deeper.

Question 8

8. Explain the steps of meiosis 1 – end of meiosis 1 – interkinesis

Answer 8

At the end of meiosis 1, also known as interkinesis, the contractile ring fully constricts and the cell splits into two separate haploid cells

Question 9

9. Explain the steps of meiosis 2 – prophase 2

Answer 9

In prophase 2, the chromosomes begin to condense again, the nuclear membrane starts to disappear, and the centrioles move to opposite sides of the cell with spindle fibres appearing. The chromosomes still consist of two sister chromatids but are different due to crossing over.

Question 10

10. Explain the steps of meiosis 2 – metaphase 2

Answer 10

In metaphase 2, the spindle fibres extend from the centrioles towards the chromosomes. The spindle fibres then attach to the kinetochores on the centromeres of the chromosomes and the chromosomes are then lined up at the equator of the cell.

Question 11

11. Explain the steps of meiosis 2 – anaphase 2

Answer 11

In anaphase 2, the spindle fibres shorten, separating and pulling the sister chromatids to opposite poles of the cell.

Question 12

12. Explain the steps of meiosis 2 – telophase 2

Answer 12

In telophase 2, the chromosomes start to decondense, and the nuclear membrane starts to reform. A contractile ring then creates a cleavage furrow in the cell membrane which continues getting deeper.

Question 13

13. Explain the steps of meiosis 2 – end of meiosis 2

Answer 13

At the end of meiosis 2, the contractile ring pinches off the cell membrane splitting the cells into 4 non-identical haploid cells, with each having a single set of chromosomes and half the original amount of genetic information of the original cell.

Question 14

14. In terms of meiosis, what is it, where does it occur, how does it occur, and why does it occur?

Answer 14

• What: meiosis is a type of cell division which is essential to produce gametes for sexual reproduction
• Where: It occurs in plants - in the anthers and ovules - and in animals, in the testes (male) and ovaries (female)
• How: four daughter cells are produced each with half the number of chromosomes of the parent cell
• Why: So that genetic information is halved (from diploid  haploid). This is necessary to ensure that there is correct number of chromosomes at fertilisation