4.6.1.2 Meiosis

Question 1

What does meiosis do to the number of chromosomes?

Answer 1

• meiosis halves the number of chromosomes in gametes
• fertilisation restores the full number of chromosomes when the gametes fuse/join

Question 2

How do cells in reproductive organs divide and why do they divide like that?

Answer 2

cells in reproductive organs divide by meiosis to form gametes

Question 3

How many chromosomes do gametes contain?

Answer 3

gametes only have 1 copy of each chromosome - 23 chromosomes, 23 chromatids

Question 4

What happens when a cell divides to form gametes?

Answer 4

• the cell makes copies of its chromosomes so it has double the amount of genetic information
• the cell divides into 2 cells each with half the amount of chromosomes - 23 chromosomes, 46 chromatids
• the cell divides for the second time (so twice) to form 4 gametes each with a single set of chromosomes (quarter of the amount of chromosomes) - 23 chromosomes, 23 chromatids
• all gametes are genetically difference from each other as crossing over takes place in meiosis (where chromosomes exchange genetic material between each other) - results in random chromosomes ending up in each of the 4 cells (gametes)

Question 5

What happens when gametes join/fuse at fertilisation?

Answer 5

• gametes join at fertilisation to restore the normal number of chromosomes
• the new cell divides by mitosis to produce many copies
• more and more cells are produced so the number of cells increases - embryo forms
• as they embryo develops cells differentiate to gain different structures that allow them to carry out different functions

Question 6

Homologous chromosomes:

Answer 6

chromosomes which contain the same genes in the same order along their chromosomal arms

Question 7

Diagram of meiosis:

Answer 7

Question 8

Phases of meiosis:

Answer 8

• Prophase 1
• Metaphase 1
• Anaphase 1
• Telophase 1
• Prophase 2
• Metaphase 2
• Anaphase 2
• Telophase 2

Question 9

Phases of meiosis: Prophase 1

Answer 9

• the chromosomes in their replicated firm consist of 2 sister chromatids joined by a centromere (46 chromosomes - 92 chromatids)
• the chromosomes condense and become visible (have a shorter and thicker appearance)
• the nuclear envelope breaks down
• the centriole divides and the 2 centrioles move to opposite poles of the cell
• a spindle starts to form between the 2 centrioles - made up of protein called tubulin
• homologous chromosomes pair up and crossing over occurs where non-sister chromatids exchange genetic material (sections of DNA) with one another - helps increase genetic diversity

Question 10

Phases of meiosis: Metaphase 1

Answer 10

• the pairs of homologous chromosomes line up on the equator of the cell
• they attach to the spindle fibres by their centromeres
• the homologous chromosome pairs line up randomly on the equator this is known as independent assortment

Question 11

Phases of meiosis: Anaphase 1

Answer 11

• the individual chromosomes belonging to each homologous pair are pulled apart
• the centromere does not divide and so each chromosome still consists of 2 sister chromatids
• due to crossing over, sections of DNA have been exchanged between the homologous chromosomes
• proteins called motor proteins walk along the spindle fibres in opposite directions to pull the homologous chromosomes apart
• due to the random nature of how chromosomes lined up along the equator (in metaphase 1) they are now pulled to opposite sides of the cell randomly too - independent segregation

Question 12

Phases of meiosis: Telophase 1

Answer 12

• the separated chromosomes reach the opposite poles of the cell
• a nuclear envelope forms around each set of chromosomes that now contain half the normal number of chromosomes (23 chromosomes - 46 chromatids) - haploid
  
  • creates 2 separate nuclei in same cell
• the chromosomes still exist as 2 sister chromatids joined by a centromere - the second stage of meiosis will split the sister chromatids up

Question 13

Phases of meiosis: Prophase 2

Answer 13

• 2 nuclei - each nucleus contains 23 chromosomes, 46 chromatids
• the chromosomes consist of 2 sister chromatids joined by a centromere
• however, these may no longer be identical to one another due to crossing over (which took place in prophase 1)
• the chromosomes condense giving a shorter and thicker appearance
• the nuclear envelope also breaks down
• the centriole divides and the 2 centrioles move to opposite poles of the cell
• a spindle starts to form between the 2 centrioles

Question 14

Phases of meiosis: Metaphase 2

Answer 14

• the chromosomes line up on the equator of the cell attacher by the centromeres to the spindle
• the chromatids of each chromosome arrange themselves randomly along the equator independent assortment
• unlike in metaphase 1 it is the independent assortment of chromatids rather than chromosomes that occurs

Question 15

Phases of meiosis: Anaphase 2

Answer 15

• the chromosome splits so that the 2 joined chromatids are pulled apart
• motor proteins walk along the spindle fibres in opposite directions
• this causes the chromatids now called chromosomes to be pulled apart to opposite poles of the cell
• due to the random nature of how chromatids lined up along the equator in metaphase 2 they are know pulled to opposite poles of the cell randomly too - independent segregation

Question 16

Phases of meiosis: Telophase 2

Answer 16

• the separated chromosomes (each made up of 1 sister chromatid) reach the opposite poles of the cell
• a nuclear envelope forms around each of the 4 sets of chromosomes
• the 4 nuclei that form are all haploid but not genetically identical to each other
• each nuclei contains 23 chromosomes, 23 chromatids