Topic1:8 - meiosis

Question 1

What are somatic cells?

Answer 1

body cells that are diploid and contain a full set of chromosomes

Question 2

The number of chromosomes in a diploid cell is referred to as:

Answer 2

the diploid number.

Question 3

describe how all chromosomes in a somatic cell occur in homologous pairs

Answer 3

All chromosomes are in homologous pairs with one copy from a paternal source and one from a maternal source.

The chromosomes of a homologous pair contain the same number and type of genes in the same location. (they code for the same trait e.g eye colour)

Question 4

Somatic cells are formed by:

Answer 4

New somatic cells are formed by mitosis and cytokinesis and are genetically identical.

Question 5

What are gametes?

Answer 5

sperm or egg cells which form from diploid germ-line cells by meiosis.

They:

• are involved in reproduction.
• are haploid cells, so they contain half the normal number of chromosomes (haploid number).

-contain only one copy from each homologous pair.

Question 6

stages of meiosis

Answer 6

there are 2 main stages:

-meiosis 1
-meiosis 2

Question 7

briefly summarise meiosis 1

Answer 7

-first division separates homologous pairs and produces haploid cells.

-begins after DNA replication.

-consists of Prophase1, Metaphase 1, Anaphase 1, Telophase 1
followed by cytokinesis.

Question 8

briefly summarise meiosis 2

Answer 8

-second division separates the sister chromatids in the haploid product of meiosis 1 and results in four genetically different haploid gametes.

-begins immediately after Meiosis 1.

-consists of Prophase 2, Metaphase 2, Anaphase 2, Telophase 2
followed by cytokinesis.

Question 9

meiosis 1 - explain what occurs in prophase 1:

Answer 9

early in prophase 1:
-replicated DNA condenses and becomes visible and the nuclear membrane breaks down.

Late in prophase 1:
-homologous chromosomes pair up
-they are placed so closely to each other that genetic material can cross over between non-sister chromatids that are part of the same homologous pair/tetrad.
-Enzymes cut and re-join the DNA of each non-sister chromatid.
-Crossing over results in the exchange of maternal and paternal gene variants and so the four chromatids are each genetically unique.

Question 10

meiosis 1 - explain what occurs in metaphase 1:

Answer 10

-Homologous chromosomes pair up and position either side of the metaphase plate.
-Both chromosomes in a pair are attached to the same spindle fibre.
-The placement of the paternal and maternal chromosomes to each side is random and called independent assortment.
-Independent assortment is the second mechanism that can introduce genetic variation in the gametes.

Question 11

meiosis 1 - explain what occurs in anaphase 1:

Answer 11

Spindle fibres shorten and pull apart the homologous chromosomes (each still consisting of 2 sister chromatids) to opposite poles.

<-X X->

Question 12

meiosis 1 - explain what occurs in telophase 1:

Answer 12

new nuclear membranes form around each set of chromosomes to form 2 haploid nucleii.

Question 13

meiosis 1 - explain what occurs in cytokinesis 1:

Answer 13

separates the cytoplasm and forms two haploid cells.
Each cell contains one set of chromosomes/one copy of each pair.

Question 14

meiosis 2 - explain what occurs in prophase 2:

Answer 14

-Chromosomes condense and become visible.
-The nuclear membrane breaks down.
-A new spindle forms.

Question 15

meiosis 2 - explain what occurs in metaphase 2:

Answer 15

-Each chromosomes randomly aligns on the equator so can lead to genetic variation (independent assortment).
-One chromosome attaches to each spindle fibre.

<-X-> /

Question 16

meiosis 2 - explain what occurs in anaphase 2:

Answer 16

spindle fibres shorten and separate the non-identical sister chromatids to opposite poles of the cell.

Question 17

meiosis 2 - explain what occurs in telophase 2:

Answer 17

nuclear membranes form around each set of chromosomes.
Chromosomes decondense and are no longer visible.

Question 18

meiosis 2 - explain what occurs in cytokinesis 2:

Answer 18

the cytoplasm divides as new membranes form and 4 new haploid cells (gametes) are the final result of meiosis.

Question 19

list 4 sources of genetic variation in sexual reproduction?

Answer 19

1. mutations
2. crossing over
3. independent assortment
4. fertilisation

Question 20

briefly describe how mutations cause genetic variation in sexual reproduction

Answer 20

Mutations (spontaneous and induced) change the base sequence in DNA, making the

Question 21

briefly describe how crossing over cause genetic variation in sexual reproduction

Answer 21

Crossing over during late prophase 1 of meiosis changes the DNA base sequence in the homologous chromosomes so that each sister chromatid is genetically unique.

Question 22

briefly describe how crossing over cause independent assortment in sexual reproduction

Answer 22

Independent assortment during metaphase 1 of meiosis randomly affects how many maternal or paternal genes will end up in each gamete’s set of genetic material.

Question 23

briefly describe how crossing over cause fertilisation in sexual reproduction

Answer 23

It is a random event, as to which ovum is released, and which sperm will reach the ovum first, the resulting combinations of chromosomes provides a source of genetic variation.

Question 24

What does fertilisation result in?

Answer 24

Fusion of nuclei of haploid male and female gametes results in a new diploid cell called a zygote.

Question 25

what is fertilisation?

Answer 25

Fertilisation restores the diploid number as the human zygote contains 46 chromosomes in 23 homologous pairs. (Other species have different diploid numbers) As it is a random event, as to which ovum is released, and which sperm will reach the ovum first, the resulting combinations of chromosomes provides a source of genetic variation.

Question 26

What is genetic variation in asexual reproduction

Answer 26

In asexual reproduction a single parent organism divides, by either binary fission or mitosis, and produces offspring that are genetically identical to the parent organism.

Question 27

what is budding?

Answer 27

Some asexually reproducing organisms reproduce by a process called budding where the adult produces a miniature offspring that separates from the parent and then grows bigger.

Question 28

what is the source of genetic variation in asexual reproduction?

Answer 28

Mutation is the only source of genetic variation.