2. Sexual Reproduction & Meiosis

Question 1

What is a gamete

Answer 1

Sex cells (e.g. sperm cells and egg cells) that fuse together during sexual reproduction

Question 2

What happens in sexual reproduction

Answer 2

In sexual reproduction 2 gametes (an egg and a sperm) join together at fertilisation to form a zygote. The zygote then divides & develops into a new organism.

Question 3

What is Meiosis

Answer 3

• Meiosis is a type of cell division that happens in the reproductive organs to produce gametes.
• Meiosis involves a reduction division. Cells that divide by meiosis have the full no. of chromosomes to start with, but the cells that are formed from meiosis have half the number - haploid cells.
• Cells formed by Meiosis are all genetically different bc each new cell ends up w a different combination of chromosomes.

Question 4

How many chromosomes do human body cells have

Answer 4

Human somatic cells (body cells) have 23 pairs of chromosomes. (DO NOT SAY 46)
This is called the diploid number (2n).

Question 5

How many chromosomes do human gametes have

Answer 5

Human gametes (sex cells) have 23 chromosomes (no pairs).
This is called the haploid number (n).

Question 6

What does Meiosis consist of

Answer 6

Meiosis involves 2 division:
- meiosis 1
- meiosis 2

• Meiosis 1 = reduction division (it halves the chromosome number).
• Like mitosis, meiosis 1 & 2 are each split into prophase, metaphase, anaphase & telophase stages.

Question 7

What happens during Interphase in Meiosis

Answer 7

• The whole of meiosis begins with INTERPHASE.
• During Interphase, the DNA unravels & replicates to produce sister chromatids.

Question 8

What happens in Prophase I

Answer 8

• The chromosomes condense & arrange themselves into homologous pairs & crossing-over occurs.
• Centrioles move to opposite poles of cell, forming spindle fibres.
• Nuclear envelope breaks down

Question 9

What happens in Metaphase I

Answer 9

• The homologous pairs attach to the spindle fibres by their centromeres & line up along equator of cell.

Question 10

What happens in Anaphase I

Answer 10

• Spindle contract, separating the homologous pairs - one chromosome goes to each pole of the cell.

Question 11

What happens in Telophase I

Answer 11

• A nuclear envelope forms around each group of chromosomes
• CYTOKINESIS (division of the cytoplasm) occurs & two haploid daughter cells are produced.

Question 12

See pg 62 for diagram of Meiosis I

Question 13

What happens is Meiosis II

Answer 13

The two haploid daughter cells undergo Prophase II, Metaphase II, Anaphase II, Telophase II & Cytokinesis - which are a lot like stages in mitosis.

Question 14

What happens in Anaphase II

Answer 14

• The pairs of sister chromatids are separated - each new daughter cell inherits one chromatid from each chromosome.
• Four (genetically different) haploid daughter cells are produced - gametes.

Question 15

prophase 2, metaphase 2, telophase 2???

Answer 15

ask mw

Question 16

What happens during crossing-over in Prophase I

Answer 16

• During Prophase I of meiosis I, homologous pairs of chromosomes come tg & pair up.
• The chromatids twist around eachother & chromatids swap over.
• The chromatids still contain the same genes but now have a different combination of alleles

Question 17

See pg63 for steps of crossing-over diagram

Question 18

2 main events during Meiosis that lead to genetic variation

Answer 18

1. Crossing-over of chromatids
2. Independent assortment of chromosomes

Question 19

Lead to genetic variation: Crossing-over of chromatids

Answer 19

• Crossing-over of chromatids in Meiosis I means each of the four daughter cells formed from meiosis contains chromatids w different alleles:
  1. Chromosomes of homologous pairs come tg
  2. Prophase I - chromatids cross over
  3. One chromosome from each homologous pair ends up in each cell (by end of Meiosis I).
  4. Each cell has a different chromatid - so different set of alleles, increasing genetic variation in potential offspring (by end of Meiosis II).

Question 20

Lead to genetic variation: Independent assortment of chromosomes

Answer 20

1. Each homologous pair of chromosomes in your cells is made up of one chromosome from mother (maternal) & one chromosome from father (paternal).
2. When the homologous pairs line up in Metaphase I & are separated in Anaphase I, it’s completely RANDOM which chromosome from each pair ends up in which daughter cell.
3. So the four daughter cells produced by meiosis have completely different combinations of those maternal & paternal chromosomes.
4. This is called Independent assortment (separation) of the chromosomes.
5. This leads to genetic variation in potential offspring

Question 21

See pg63 for diagrams for genetic variation diagrams 1 & 2

Question 22

What are homologous pairs

Answer 22

• Humans have 46 chromosomes total - 23 pairs. One chromosome in each pair from mum & one from dad.
• The chromosomes that make up each pair are the same size & have the same gene, although they could have different alleles.
• These pairs of chromosomes are called HOMOLOGOUS PAIRS.