U4 - Meiosis

What is meiosis?

A form of nuclear division (other form is mitosis):

• However, in meiosis the result is four, genetically different, haploid cells, rather than two genetically identical diploid cells

What can meiosis also be called?

A reduction division - as we reduce the number of chromosomes

What is the purpose of meiosis?

To produce gametes

What are chromosomes in normal body cells like?

• 2 copies of each chromosome

• One from the mother, one from the father

• Each of these copies will have the same genes but not necessarily the same alleles

• These are called homologous chromosomes

What happens to the chromosomes before meiosis starts?

• Every chromosome duplicates

• But, the 2 copies stay attached to each other

• They are still referred to as one chromosome (may be calld bivalent)

What are the 4 stages of meiosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

These all happen twice

What is the result of meiosis?

• Four daughter cells

  • Will be haploid and genetically different from each other

What happens when one of these haploid cells fuses with another in the process of fertilisation?

We produce a normal, diploid cell called a zygote (this will then divide by mitosis to give a new organism)

What happens in prophase I of meiosis?

• Chromosomes condense

• Spindle forms

• Nuclear envelope breaks down

• Crossing over happens

What happens in metaphase I of meiosis?

• Homologous chromosomes pair up and line up in pairs along the equator of the spindle

  • INDEPENDENT ASSORTMENT

What happens in anaphase I of meiosis?

• Homologous chromosomes are pulled apart by the spindle fibres

What happens in telophase I and cytokinesis of meiosis?

• Chromosomes decondense

• Spindle breaks down

• Two nuclear envelopes form

• Cell membrane and cytoplasm splits

What happens in prophase II of meiosis?

• Chromosomes condense

• Spindle forms

• Nuclear envelope breaks down

What happens in metaphase II of meiosis?

• Chromosomes line up on equator

What happens in anaphase II of meiosis?

• Sister chromatids pulled apart to opposite poles of the cell

What happens in telophase II and cytokinesis of meiosis?

• Chromosomes decondense

• Two nuclear envelopes form in each cell

• Spindle fibres break down

• Cell membrane and cytoplasm split

• 4 genetically different, haploid gametes

What is crossing over?

During prophase I, homologous chromosomes often exchange sections of DNA by forming chiasmata.

This means that entirely new combinations of alleles can be formed

What is independent assortment?

The order in which chromosomes randomly line up during metaphase I

What are the similarities between meiosis and mitosis?

• Both have PMAT

• Both types of eukaryotic cell division

• Both need DNA replication before they can happen

What is the difference in the number of cell divisions between meiosis and mitosis?

• Meiosis = 2 divisions

• Mitosis = 1 division

What is the difference in the products between meiosis and mitosis?

• Meiosis = four, genetically different, haploid gametes

• Mitosis = two, genetically identical, diploid cells

What is the difference in the uses between meiosis and mitosis?

• Meiosis = Only for making gametes (sexual reproduction)

• Mitosis = growth, repair, asexual reproduction

What is the difference in the arrangement of chromosomes in metaphase I between meiosis and mitosis?

• Meiosis = homologous chromosomes pair up

• Mitosis = one long line of chromosomes

What is the difference in the crossing between meiosis and mitosis?

• Meiosis = has crossing over

• Mitosis = no crossing over

What are the 3 sources of genetic diversity from meiosis?

1. Allows sexual reproduction

2. Independent assortment

3. Crossing over

How does sexual reproduction create genetic diversity?

Allows new combinations of alleles in offspring

How does independent assortment create genetic diversity?

Gives new combinations of chromosomes in each gamete

How does crossing over create genetic diversity?

Gives new combinations of alleles on each chromosome

What is non-disjunction?

The failure of homologous chromosomes or sister chromatids to split evenly in meiosis

What is non-disjunction the cause of?

• Chromosome mutations

• Gametes with different numbers of chromosomes (some have one extra, some have one less)

What happens if a non-disjunct gamete is fertilised?

We get a zygote with an unsusual number of chromosomes

• This is almost always total to the zygote as the amount of polypeptides made will be massively different

What is polyploidy?

A numerical change in a whole set of chromosomes

What is a gene mutation?

Change in DNA base sequence (only effects one gene)

What is a chromosome mutation?

Change in chromosome number (affects many genes)