Miss Lee - Meosis

Question 1

What is meiosis

Answer 1

Meiosis is the process by which sex cells (gametes) are made in the reproductive organs. It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei.
The process involves 2 nuclear divisions.

Question 2

What is the Importance of meiosis

Answer 2

Takes place in sex organs
Gametes produced here
Important to have genetically different gametes
This promotes genetic variation and allows for Natural Selection to take place

Question 3

What is the overall basis of meiosis

Answer 3

Meiosis is preceded by interphase, during which DNA is replicated (in the S phase) to produce two genetically identical copies.
The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere.
The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I.

Question 4

How many pairs of chromosomes are there

Answer 4

A human somatic (body) cell contains 46 chromosomes. These consist of 23 pairs of homologous chromosomes.

Question 5

Where do the chromosomes come from

Answer 5

Each pair contains one chromosome from each parent. Other species have different numbers of these homologous pairs.

Question 6

What are sex cells / gametes

Answer 6

Sex cells, or gametes, have only one copy of each chromosome they are haploid.

Question 7

What is a somatic cell

Answer 7

A somatic cell, containing two of each, is called diploid

Question 8

What is meiosis (shorter answer)

Answer 8

Meiosis is the process of cell division underlying sexual reproduction. It is a two-stage process.

Question 9

What is the first stage in meiosis

Answer 9

• Meiosis I introduces genetic diversity by randomly dividing a cell’s genes in two. It results in two haploid cells.
• Miosis I is the first division, to diploid cells are created.

Question 10

What is the second stage of meiosis

Answer 10

• Meiosis II is similar to mitosis. It splits each chromosome into its two chromatids and places one in each daughter cell. It results in four haploid gametes
• Meiosis II is the second division with creates for haploid daughter cells

Question 11

What is the complete meiosis stage cycle

Answer 11

One diploid parent —> interphase (homologous of replicated chromosomes) —> meiosis 1 (homologous pair of replicated chromosomes separates) —> meiosis 2 (sister chromatids split and you have four haploid daughter cells)

Question 12

Why does genetic variation occur

Answer 12

Genetic variation occurs because it creates a diversity within a population this is vital for the species survival.

Question 13

How does genetic variation occur

Answer 13

Genetic variation occurs during meosis:

• Meiosis one, homologous pair of chromosomes swap parts of the genetic material. This is called crossing over.
• Then the chromosomes from each pair are randomly allotted to the daughter cells by independent assortment

Question 14

Prophase 1 in meiosis

Answer 14

The chromosomes condense. The nuclear envelope dissolves. The spindle fibres begin to form. Homologous chromosomes pair of falling bivalence. And crossing over the genetic information also occurs

Question 15

How does crossing over happening prophase 1

Answer 15

• In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad).
• The homologous chromosomes are held together at points called chiasmata (singular: chiasma).
• Crossing over of genetic material between non-sister chromatids can occur at these chiasmata.
• As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination).
• Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis.
• If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical).

Question 16

How does meiosis introduce variation into the haploid daughter cells in prophase 1 of meiosis 1

Answer 16

• During prophase 1 of meiosis 1 homologous chromosomes pair up as bivalents.
• Chiasmata (‘twisting around each other’) form between non-sister chromatids of different but homologous chromosomes.
The chromatids still have the same genes but now have a different combination of alleles.

Question 17

What happens in metaphase 1

Answer 17

• Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell.
• Homologous pairs of chromosomes assemble along the metaphase plate (instead of sister chromatids– as in mitosis).
• The orientation of each homologous pair is random and independent of any other homologous pair.
• This is called independent assortment.

Question 18

What happens in anaphase 1

Answer 18

• Spindle fibres contract and split the bivalent, homologous chromosomes separate.
• Each homologous chromosome (consisting of two chromatids) moves to opposite sides of the cell.
• The result is that 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole.

Question 19

How does meiosis introduce variation in the haploid daughter cells of anaphase 1

Answer 19

• When homologous pairs line up along the equator of the cell during metaphase 1 and get separated during anaphase 1 of meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell.
• This gives rise to new combinations of alleles.

Question 20

What is independent assortment

Answer 20

• During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
• Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
• This orientation of pairs of homologous chromosomes is random, as is the subsequent
assortment of chromosomes into gametes.
• The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I.
• As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs.
• Gamete combinations = 2n (where n represents the haploid number)

Question 21

What happens in meiosis in telophase 1

Answer 21

• Very similar to telophase in mitosis • Chromosomesassembleateach
pole and decondense.
• Nuclearenvelopemayreform.
• Cellundergoescytokinesistoform two haploid daughter cells

Question 22

What happens in the second division of prophase 2

Answer 22

• Chromosomes pair up and re-condense. • Nuclear envelop breaks down again.
• Spindle fibres reform.
• Centrioles move to opposite poles of the cell.

Question 23

What happens in the second division of metaphase 2

Answer 23

• Chromosomes (sister chromatids) are a lined on the equator by the spindle fibres.

Question 24

What happens in the second division of anaphase 2

Answer 24

• Centromere divides.
• Sister chromatids are separated
• Spindle fibres contract.
• Chromatids (now called chromosomes) move to the poles of the cells.
• This is the same as anaphase in mitosis.

Question 25

What happens in the second division of telophase 2

Answer 25

• Chromatids uncoil & decondense.
• Spindle fibres break down.
• Nuclear envelopes reform.
• The cell undergoes Cytokinesis
• 4 haploid daughter cells are produced

Question 26

What happens during the cytokinesis of meiosis

Answer 26

• Cytoplasm and surface membrane divide, creating four independent haploid daughter cells.