Title: Significance of meiosis
13th January 2014
Learning question: Why is meiosis important?
Homework:
LP5 by Thursday 16th January
Starter List three difference between meiosis and mitosis
Meiosis
• Is the special form of cell division used to
produce gametes
• It has two functions:
– To form haploid cells with half the
chromosome number
– To re-arrange the chromosomes with a novel
combination of genes
Meiosis
• Meiosis comprises of two successive
divisions without DNA replication in
between
• The stages of these divisions are on your
worksheet…
– Either draw or cut and stick the corresponding
picture in the boxes
Meiosis
• Rearrange the following steps of meiosis
– Separation of chromatids 4
– Formation of bivalents 2
– Pairing by homologous chromosomes 1
– Production of haploid cells 5
– Chiasma formation and exchange between
chromatids 3
Genetic Variation
• Meiosis and sexual reproduction allows for
genetic variation
• This allows species to adapt to their
environments through evolution
• There are three sources of genetic
variation in sexual reproduction
• Can you name them?
– Independent Assortment
– Crossing Over
– Random Fertilisation
Independent Assortment
• Happens in metaphase I, when the
bivalent line up on the equator
• Each bivalent is made up of two
chromosomes
• The chromosomes can line up in any order
on the equator resulting in different
possible combinations
Independent Assortment
• In the sample below, n=2 and there are 4
possible gametes
• In humans n=23 so there are over 8 million
possible different gametes!
Crossing Over
• This occurs in prophase I
• While the two homologous chromosomes
are joined in a bivalent, bits of one
chromosome are swapped (crossed over)
with the corresponding bits of the other
chromosome
Crossing Over
• There are always equal amounts crossed
over so chromosomes stay the same
length
• Crossing over means that the maternal
and paternal alleles can be mixed
Random Fertilisation
• This takes place when two gametes fuse
to form a zygote
• Each gamete has a unique combination of
genes and any of the numerous male
gametes can fertilise the egg
Gene Mutation
• DNA is very stable but bases can change
during DNA replication
• Most mutation have no phenotypic effect,
these are called silent mutations
Variation
• Below is an outline of causes of variation
• As well as genetic causes, individuals are
influenced by the environment
Genotype + environment = phenotype
Discontinuous Variation
• Are distinct, separate peaks/bars
• Are distinct categories to place individuals
• Tends to have no overlap between
categories
• Are controlled by a small number of genes
• Are largely unaffected by the environment
Continuous Variation
• Is a continuous range of values so get a smooth
curve
•
•
•
•
Are no distinct categories to place individuals
Tends to have overlap between categories
Are controlled by a large number of genes
Are significantly affected by the environment
Sex Determination
• Females produce eggs which carry the X
chromosome.
• Sperm carry either X or Y
• Males therefore determine the sex of the
child
Females = XX
Males = XY
Sex Determination
• It is though that Y chromosomes dislike acid
conditions within the female reproductive
systems
• Y chromosomes are smaller and will therefore
float in a sample of semen if centrifuged
Clinics can use this information to select sex