Download Advanced Higher Biology Unit 2 * Organisms and Evolution 2bii

Meiosis forms variable
gametes
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution
2bii – Meiosis forms variable gametes
Contents and number of slides for each topic
1.
2.
3.
4.
5.
Terminology (4)
Sexual life cycle of animals (1)
Meiosis – an overview (1)
Meiosis I (4)
Meiosis II (2)
6.
7.
8.
9.
10.
After meiosis (1)
Linkage maps (5)
Independent assortment (2)
Producing variation (1)
Other sexual life cycles (3)
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.
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.
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 length
– the same centromere position
– genes for the same characteristics
at the same loci (Latin: ‘places’).
Terminology
A homologous pair
A closer look at the genes on human chromosome 12
Potassium channel
Myosin
Centromere
One chain of collagen
Phenylalanine
hydroxylase
• Chromosome 12 has between
1200 and 1400 genes.
• Genes that are found on the
same chromosome are called
linked genes.
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
parent
Terminology
Sets of chromosomes
Haploid
female
gamete
(1 set)
Diploid cell
(2 sets)
Haploid
male
gamete
(1 set)
• 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.
YouTube: Diploid and haploid (1:55 min)
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.
Meiosis – an overview
• Meiosis reduces the number of chromosomes.
• Meiosis is not a cycle.
Diploid
gamete mother
cell
Chromosomes
duplicate
Pair of
homologous
chromosomes
Sister
chromatids
Meiosis I
Homologous
chromosomes
separate
Meiosis II
Sister
chromatids
separate
Haploid
cells
Meiosis I
Pairing of homologous chromosomes
Sister
chromatids
Bivalent
• 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.
Meiosis I
Crossing over
• 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.)
Chiasma
• Chiasmata never form between
sister chromatids.
• Chiasmata allow the shuffling of sections of DNA between
homologous chromosomes, a process called crossing over.
• Crossing over leads to the recombination of alleles, and so helps
to increase variation in the gametes.
YouTube: Recombination (3:40 min)
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.
Centrosome
Spindle fibres
• 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.
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.
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.
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.
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 birth
– only one of the cells develops further
– after an egg cell is released from the
ovary it will not undergo meiosis II
until a sperm nucleus has entered
– the nucleus of only one of the new
cells will fuse with the sperm nucleus.
Watch: animation of meiosis
• http://www.cellsalive.com/meiosis.htm
Linkage maps
Linked genes stay together
RRGG
rrgg
Gametes
Gametes
R G
r g
Offspring
R G
r g
• 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?
Linkage maps
Linked genes can recombine
Red eye
Grey body
• The offspring have red eyes and grey
bodies (see left).
• Crossing these flies with rrgg (see
right) produces four offspring
phenotypes (shown below).
Almost all the offspring
look like the parents.
Red eye
Grey body
Purple eye
Black body
Purple eye
Black body
A few of the offspring
show recombinant phenotypes.
Red eye
Black body
Purple eye
Grey body
Red eye
Grey body
Linkage maps
Recombinants are the result
of crossing over
R G
r g
Most
gametes
Gametes
R G
r g
R g
A few gametes
show
recombination
r G
r g
R G
r g
r g
r g
R g
r g
r G
r g
Phenotype
Number of
offspring
Red eye
Grey body
113
Purple eye
Black body
122
Purple eye
Black body
r g
r g
Red eye
Black body
9
Only 15 of the 250
offspring are
recombinants.
Purple eye
Grey body
6
So recombination
frequency is 6%.
Total = 250
Linkage maps
Data for other linked genes
• Repeating these types of crosses for other genes on
chromosome 2 gives different recombination frequencies.
Genes used in the crosses
Recombination frequency (%)
Purple eye v. Black body
6
Purple eye v. Lobe eye
17
Vestigial wing v. Lobe eye
5
Black body v. Lobe eye
23
• Chiasmata formation occurs at random positions along the
chromosomes.
• What does a small recombination frequency suggest about the
position of the two genes?
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.
Genes used in the crosses
Recombination frequency (%)
Purple eye v. Black body
6
Purple eye v. Lobe eye
17
Vestigial wing v. Lobe eye
5
Black body v. Lobe eye
23
Black
body
Purple
eye
6
Lobe
eye
Vestigial
wing
23
17
5
• What recombination frequency would be predicted for crosses
using purple eye v. vestigial wing?
Animation: Discovery of linkage maps
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.
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 23 = 8 combinations.
• In humans, with 23 pairs, there are 223 = 8 388 308 combinations … and crossing
over shuffles pieces between chromosomes!
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.
Allows shuffling of
sections of DNA
between homologous
chromosomes
Crossing over
Allows many
combinations of
chromosomes of
maternal and paternal
origin in the gametes
Independent
assortment
Brings genetic
information from two
different parents
together in one
organism
Sexual
reproduction
• 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!
Other sexual life cycles
Comparing the three types
Animals
Plants
Fungi & protists
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 / Wikimedia
Other sexual life cycles
Fungi and most protists
• Again, 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.