Meiosis forms variable gametes - PowerPoint Presentation

Slide1

Meiosis forms variable gametes

CfE Advanced Higher Biology

Unit 2: Organisms and EvolutionSlide2

2bii

– Meiosis forms variable gametes

Terminology (4)Sexual life cycle of animals (1)Meiosis – an overview (1)Meiosis I (4)Meiosis II (2)After meiosis (1)Linkage maps (5)Independent assortment (2)Producing variation (1)Other sexual life cycles (3)

Contents and number of slides for each topicSlide3

SQA mandatory key information

Increased variation through the production of haploid gametes by meiosis

– meiosis I, meiosis II, gamete mother cell, chromosome, chromatid, homologous pairs, crossing over, chiasmata, independent assortment, linked genes and frequency of recombination. In many organisms, gametes are formed directly from the cells produced by meiosis. In other groups, mitosis may occur after meiosis to form a haploid organism; gametes form later by differentiation. Slide4

Key concepts

Homologous chromosomes are pairs of chromosomes of the same size, same centromere position and with the same genes at the same loci. Each homologous chromosome is inherited from a different parent; therefore the alleles of the genes of homologous chromosomes may be different.

Crossing over occurs at chiasmata during meiosis I. This process shuffles sections of DNA between the homologous pairs allowing the recombination of alleles to occur. Genes on the same chromosome are said to be linked. Correlation of the distance between linked genes and their frequency of recombination. Independent assortment occurs as a result of meiosis I with homologous chromosomes being separated irrespective of their maternal and paternal origin. Slide5

Terminology

Homologous chromosomes

A human body cell has 46 chromosomes in its nucleus.There are two copies of each of 23 types of chromosomes.The two chromosomes in a pair are called homologous chromosomes and they have:the same lengththe same centromere positiongenes for the same characteristics at the same loci (Latin: ‘places’).Slide6

Terminology

A homologous pair

A closer look at the genes on human chromosome 12

Phenylalanine hydroxylase

One chain of collagen

Myosin

Potassium channel

Centromere

Chromosome 12 has between 1200 and 1400 genes.

Genes that are found on the same chromosome are called

linked genes

.Slide7

Terminology

Homologous pairs can be different

Homologous pairs are found because one copy of each chromosome has come from the female parent and the other copy has come from the male parent.Because they are inherited from different parents, the alleles of the genes on each homologous chromosome may be different.

From female parent

From male parentSlide8

Terminology

Sets of chromosomes

A single set of chromosomes comes from the female parent in her gametes.Another single set of chromosomes comes from the male parent in his gametes.Each gamete cell has a single set of chromosomes and is haploid.A cell with a full double set of homologous chromosomes is diploid.

Haploid female gamete

(1 set)

Haploid male gamete

(1 set)

Diploid cell

(2 sets)

YouTube: Diploid and haploid (1:55 min)Slide9

Sexual life cycle of animals

Most of the life cycle is spent as a diploid multicellular organism.

Meiosis produces genetically variable haploid cells which develop into gametes.During fertilisation, gametes fuse their haploid nuclei to produce a diploid cell.

Mitosis

then produces genetically identical diploid cells to make a multicellular organism.Slide10

Meiosis – an overview

Meiosis reduces the number of chromosomes.

Meiosis is not a cycle.

Diploid

gamete mother cell

Chromosomes duplicate

Meiosis I

Homologous chromosomes separate

Meiosis II

Sister chromatids separate

Haploid cells

Sister chromatids

Pair of

homologous chromosomesSlide11

Sister chromatids

Bivalent

M

eiosis I

Pairing of homologous chromosomes

During interphase, the homologous chromosomes duplicate so each is now made up of two

sister chromatids

.

This is still a

diploid

cell, with two sets of homologous chromosomes (though it has four sets of genetic information).

At the start of meiosis I, homologous chromosomes pair up so that they are aligned gene by gene.

Protein strands form a complex to link the sister chromatids and the homologous pairs to form a

bivalent

.Slide12

Chiasma

Meiosis I

Crossing over

C

hiasmata

allow the shuffling of sections of DNA between homologous chromosomes, a process called

c

rossing over

.

C

rossing over leads to the

recombination of alleles

, and so helps to increase variation in the gametes.

A

chiasma

(Greek: ‘cross mark

’)

forms at a random position between the homologous pairs

.

Human

chromosomes usually have

two

or

three chiasmata.(Chiasma = singular. Chiasmata = plural.)Chiasmata never form between sister chromatids.YouTube: Recombination (3:40 min) Slide13

Centrosome

Spindle fibres

Meiosis I

Alignment on the metaphase plate

The protein

complex between

all the chromatids breaks down.

The centromeres still hold the sister chromatids together.

Chiasmata

still hold the homologous pair together so they can be aligned.

The nuclear membrane breaks down.

Centrosomes

send out microtubules to connect with

kinetochores

which lie beside each centromere.

The microtubules form

spindle fibres

linking across the cell.

The homologous chromosomes align in the centre of the cell.Slide14

Meiosis I

Separating homologous chromosomes

The microtubules of the spindle fibres begin to shorten. The microtubules pull on the kinetochores so the homologous chromosomes separate to opposite ends of the cell.The chromosomes group in each end of the cell and a nuclear membrane forms around them.Cytokinesis separates the two cells.The sister chromatids are no longer identical due to the crossing over.Slide15

Meiosis II

Alignment on the metaphase plate

Each cell is

haploid

,

with

one copy of each homologous chromosome (though

it has

two sets

of genetic information).

The nuclear membrane breaks down again.

Centrosomes again send out microtubules and bind to the kinetochores of each sister chromatid.

The chromosomes align in the centre of the cells.Slide16

Meiosis II

Separating sister chromatids

The protein complex

between the centromeres breaks

down.

The

microtubules of the spindle fibres begin to shorten.

The microtubules pull on the kinetochores so the

sister chromatids

separate to opposite ends of the cell

.

After being separated, sister chromatids are called chromosomes.

The

new chromosomes

group in each end of the cell and a nuclear membrane forms around them.

Cytokinesis separates the two cells.Slide17

After meiosis

Haploid cells become gametes

Meiosis produces four genetically different haploid cells.Each cell has one copy of every homologous chromosome.In human males, each cell develops to form a sperm cell.In human females, it is more complex:meiosis I occurs in the last 3 months before birthonly one of the cells develops furtherafter an egg cell is released from the ovary it will not undergo meiosis II until a sperm nucleus has enteredthe nucleus of only one of the new cells will fuse with the sperm nucleus.Slide18

http://www.cellsalive.com/meiosis.htm

Watch: animation of meiosisSlide19

Gametes

R G

Offspring

R

G

r g

Gametes

r g

Linkage maps

Linked genes stay together

Purple eye

(r)

and

black body (g)

are two

alleles

found on chromosome 2 of

Drosophila melanogaster.Red eyes (R) and grey body (G) are the dominant alleles

.Crossing RRGG with rrgg.

The genes are linked so all the offspring inherit one chromosome with R G and the other with r g .

What is the phenotype of the offspring?

RRGG

rrggSlide20

Linkage maps

L

inked genes can recombine

The offspring have red eyes and grey bodies (see left).

Crossing these flies

with

rrgg

(see right)

produces

four offspring phenotypes (shown below).

Red eye

G

rey body

Purple eye

Black body

Purple eye

Black body

Red eye

G

rey body

Purple eye

Grey body

Red eye

Black body

Almost all the offspring

look like the parents.

A few of the offspring

s

how

recombinant phenotypes

.Slide21

r

G

r g

R g

r g

R G

r g

r

g

r g

r g

r g

R G

r g

Linkage maps

Recombinants are the result

of crossing over

Gametes

r g

Phenotype

Number of offspring

R G

Red eye

Grey body

113

r g

Purple eye

Black body

122

R g

Red eye

Black body

9

r G

Purple eye

Grey body

6

Red eye

G

rey body

Purple eye

Black body

A few gametes show recombination

Most gametes

Only 15 of the 250

offspring are

r

ecombinants.

So recombination frequency is 6%.

Total = 250 Slide22

Linkage maps

Data for other linked genes

Chiasmata formation occurs at random positions along the chromosomes.What does a small recombination frequency suggest about the position of the two genes?Repeating these types of crosses for other genes on chromosome 2 gives different recombination frequencies.Genes used in the crossesRecombination frequency (%)Purple eye v. Black body6

Purple eye v. Lobe eye

17

Vestigial wing v. Lobe eye

5

Black body

v. Lobe eye

23Slide23

Linkage maps

How to map the genes

The recombination frequency for linked genes correlates with the distance between the loci of the genes on the chromosome.What recombination frequency would be predicted for crosses using purple eye v. vestigial wing?Genes used in the crossesRecombination frequency (%)Purple eye v. Black body6

Purple eye v. Lobe eye

17

Vestigial wing v. Lobe eye

5

Black body

v. Lobe eye

23

Purple eye

Black body

Vestigial wing

Lobe eye

17

23

6

5Animation: Discovery of linkage mapsSlide24

Independent assortment

Alignment during Meiosis I

All diploid organisms have more than one homologous pair of chromosomes.Homologous pairs align in the centre of the cell.The orientation of the homologous chromosomes is irrespective of their maternal or paternal origin.Even with just three pairs of homologous chromosomes, there are four possible alignments.Slide25

Independent assortment

Chromosome combinations in gametes

At meiosis I, homologous pairs are separated irrespective

of the maternal or paternal origin of the chromosome

.

This leads to variation in the combinations of chromosomes found in the haploid cells at

the end of meiosis

II.

With three pairs

of chromosomes, there are

2

3

= 8 combinations.

In humans, with 23 pairs, there are 2

23

= 8 388 308 combinations … and crossing over shuffles pieces between chromosomes!Slide26

Producing variation

Meiosis and sexual reproduction

Meiosis produces haploid cells that are genetically variable.Sexual reproduction uses two haploid gamete cells to make a new diploid organism.Two human parents can produce offspring with more than 70 million million combinations of chromosomes … without considering the effects of crossing over. We are all unique!

Allows shuffling of sections of DNA between homologous chromosomes

Crossing over

Brings genetic information from two different parents

together in

one organism

Sexual reproduction

Allows many combinations of chromosomes of maternal and paternal origin in the gametes

Independent assortmentSlide27

Other sexual life cycles

Comparing the three types

AnimalsPlantsFungi & protistsSlide28

Other sexual life cycles

Plants

In mosses and ferns, mitosis occurs after meiosis and so produces a large multicellular haploid organism with differentiated cells.Gametes are formed later by differentiation of haploid cells.In higher plants, a tiny male haploid organism is held within a pollen grain and a tiny female haploid organism is in the ovule.Fern image: Olegivvit / WikimediaSlide29

Other sexual life cycles

Fungi and most

protistsAgain, mitosis occurs after meiosis and produces a unicellular or multicellular haploid organism.Gametes form later by the differentiation of the haploid cells.The diploid zygote goes straight into meiosis to form gametes.The malarial parasite (Plasmodium spp.) has this type of lifecycle.