Meiosis

What do diploid (normal) cells have two of each?

two chromosomes of each type - one inherited from each parent

How many times does the nucleus divide during mitosis?

once

What fuses to produce a fertilised egg in sexual reproduction?

two sex cells (gametes) - one from each parent

What is the fertilised egg (zygote) the origin of?

all the cells that the organism develops

Therefore, how any chromosomes must a gamete contain?

half the standard (diploid) number of chromosomes

Why must gametes contain half the standard (diploid) number of chromosomes in a cell?

so the chromosome number of an organism doesn't double with every round of reproduction

What type of cell is one that contains half the chromosome number of the parent cell?

haploid cell

What cell division forms gametes?

meiosis

How many times does the nucleus divide during meiosis?

divides twice

What does meiosis produce?

4 non-genetically identical, haploid daughter cells

What is meiosis known as (due to each gamete it produces containing half the chromosome number of the parent cell)?

reduction division

What is each characteristic of an organism coded for by?

coded for by 2 copies of each gene - one from each parent

Why does each nucleus of the organism's cells contain two full sets of genes?

a pair of genes for each characteristic

Homologous chromosomes?

matching sets of chromosomes (in each nucleus)

What type of cells have homologous chromosomes?

diploid cells

What does each chromosome in a homologous pair have the same (2)?

each chromosome has the same genes and the same loci

What leads to differences in a particular characteristic (e.g. blue and brown eyes)?

genes for that particular characteristic may vary

Why are the genes for blue or brown eyes still the same type?

they both code for eye colour (but the specific colour is different)

Why are they different versions of the same gene though?

code for different colours (blue or brown)

Term for different versions of the same gene?

alleles (or gene variants)

What do all the different alleles of a gene have the same?

the same locus

Locus?

the specific, fixed physical location of a gene or DNA sequence on a chromosome

Because homologous chromosomes have the same genes in the same positions what will they look like when they're visible in prophase?

homologous chromosomes will have the same length and size

What will also be the same between homologous chromosomes?

the centromeres will be in the same positions

The two main stages of mieosis?

Meiosis I and Meiosis II

What division is the reduction division and why?

Meiosis I - the pairs of homologous chromosomes are separated into 2 cells

What type of cells does Meiosis I create?

haploid

Why are the cells created by Meiosis I haploid?

each cell only contains one full set of genes instead of two

What is meiosis II similar to?

mitosis

What is separated to form two more cells during meiosis II?

the pairs of chromatids present in each daughter cell

Prophase 1: similarities to mitosis - what condenses?

chromosomes

Prophase 1: Similarities to mitosis - what disintegrates?

the nuclear envelope

Prophase 1: Similarities to mitosis - what disappears?

the nucleolus

Prophase 1: Similarities to mitosis - what begins its formation?

spindle formation begins

Prophase 1: differences to mitosis - what pairs up?

homologous chromosomes

What do homologous chromosomes pairing up form?

bivalents

What do chromosomes have to move through to pair up?

move through liquid cytoplasm to pair up/to be brought together

Crossing over?

chromosomes are large molecules of DNA so them moving through the liquid cytoplasm to be brought together results in the chromosomes entangling

Metaphase 1: the only difference between mitosis metaphase and metaphase 1 of meiosis?

the homologous pairs of chromosomes assemble along the metaphase plate (instead of the individual chromosomes)

Metaphase 1: Independent assortment?

the orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair - the maternal or paternal chromosomes can end up facing either pole

Metaphase 1: what can independent assortment result in many different combinations of?

many different combinations of alleles facing the poles

Why is Anaphase 1 of meiosis different from anaphase during mitosis?

homologous chromosomes pulled to the opposite poles and the chromatids stay joined to each other

Anaphase 1: what happens to sections of DNA on 'sister' chromatids that became entangled during crossing over?

they now break off and re-join - sometimes resulting in an exchange of DNA

Chiasmata?

the points at which the chromatids break and rejoin

Anaphase 1: when this exchange of DNA occurs, what does it form?

recombinant chromatids (DNA) - with genes being exchanged between chromatids

Anaphase 1: What is the combination of alleles on the recombinant chromatids different from?

the combination of alleles on either original chromatids - the genes being exchanged to form the recombinant chromatids may be different alleles of the same gene

Anaphase 1: what arises from this new combination of alleles?

genetic variation - the sister chromatids are no longer identical

Telophase 1: what is it essentially the same of?

telophase in mitosis

Telophase 1: (similarities to mitosis) what assemble at each pole?

chromosomes

Telophase 1: (similarities to mitosis) what reforms?

nuclear membrane

Telophase 1: (similarities to mitosis) what uncoils?

chromosomes

Telophase 1: (similarities to mitosis) what does the cell undergo that splits it into two cells?

cytokinesis

What is complete at the end of Telophase 1?

the reduction of chromosome number from diploid to haploid

Prophase 2: what do the chromosomes still consist of?

two chromatids

Prophase 2: what happens to the chromosomes?

condense and become visible again

Prophase 2: what breaks down?

the nuclear envelope

Prophase 2: what begins?

spindle formation

Metaphase 2: how does it differ from metaphase 1?

the individual chromosomes assemble on the metaphase plate (like during metaphase in mitosis)

Metaphase 2: what causes more genetic variation to be produced?

independent assortment again

Metaphase 2: why is there independent assortment?

due to crossing, over the chromatids are no longer identical

Anaphase 2: how does it differ from anaphase 1?

the individual chromatids of the individual chromosomes are pulled apart to opposite poles after division of centromeres (same as anaphase in mitosis)

Telophase 2: (similarities to telophase in mitosis) What assembles at the poles?

chromatids

Telophase 2: (similarities to telophase in mitosis) what to the chromosomes uncoil to form again?

uncoil to form chromatin again

Telophase 2: (similarities to telophase in mitosis) what reforms?

nuclear envelope

Telophase 2: (similarities to telophase in mitosis) what becomes visible?

nucleolus

What does cytokinesis lead to the formation of?

4 daughter cells in total

Why will the new cells be haploid?

due to the reduction division

Why will the all the new cells be genetically different from each other and the parent cell?

due to the processes of crossing over and independent assortment