Higher Human Biology
Unit 1: Cell Function and
Inheritance
Chapter 10 Meiosis
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Learning Intentions
•
•
•
•
To revise chromosomes
To revise mitosis
To examine meiosis
To look at effect of chromosomes crossing
over during meiosis
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Mitosis - Revision
• Mother cell divides into 2 identical (daughter) body
cells.
• Each is a exact copy of the original.
• Double set of chromosomes in each cell (diploid).
• 1 nuclear division.
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Homologous chromosomes
• In pairs.
• Not identical, but the
genes “describe” the
same characteristics.
• e.g. Eye colour can
have blue or brown
alleles.
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Meiosis: Need for reduction
division.
• Sexual reproduction
provides the opportunity
for genetic material form
one individual to meet
that of another at
fertilisation and form a
diploid zygote.
• For this process to be possible, the sex cells
must be haploid.
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Meiosis: Need for reduction
division con’t.
• The formation of the
haploid cells requires a
haploid mother cell to
undergo a reduction
division.
• The division of 4 cells
accompanied by a
reduction in chromosome
number.
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Meiosis: Summary
• Mother cell divides to produce 4 gametes
(sex cells).
• Gametes contain half the genetic
material of the parent cell (haploid).
• 2 nuclear divisions.
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Sites of meiosis
Site of
meiosis
Testes
(seminiferous
tubule)
Ovary
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Diploid gamete
mother cell
Sperm mother
cell
Egg mother cell
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Haploid
gametes
formed
Sperm
Eggs (ova)
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Process of meiosis
• Involves 2 consecutive nuclear divisions
(followed by cell divisions)
• The gamete mother cell (4n) divides into
two cells and these then divide again.
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1st Meiotic Division
Each
chromosome
replicates
forming 2
identical
chromatids
Interphase
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Nuclear material
becomes visible,
each
chromosome has
two chromotids
attached to a
centromere,
Homologous
Members of
chromosomes
each
pair up and come homologous pair
to lie alongside
begin to repel
one another so one another and
that their
move apart at
centromeres and
exact points
genes match
called
exactly
CHIASMATA –
Crossing over
occurs.
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The nuclear
membrane
disappears,
spindle fibres
form, one
chromosome
of each pair
moves to one
pole and its
partner moves
to the opposite
pole.
Nuclear
membrane
forms round
each group of
chromosomes
and then the by
division of the
cytoplasm
resulting in the
formation of two
haploid cells.
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2nd Meiotic Division
Each haploid cell
now undergoes a
2nd meiotic
division
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Single
chromosomes
(each made of 2
chromatids) line up
at each equator
On separation from
its partner each
chromatid is
regarded as a
chromosome.
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Each of the four
gametes formed
contains half the number
of chromosomes present
in the original gamete
mother cell.
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Meiosis Summary
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Meiosis
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Chromosome Complement
shows the number of sets of chromosomes, where n=1 set
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Task: TYK Qu’s Torrance pg 76
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Variation
Much of the VARIATION that exists amongst
the members of the species is inherited and
determined by alleles of genes.
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Variation
Caused by:
• Idependent assortment of chromosomes
• Chiasmata (crossing over of genetic
information)
Results in variation between organisms
of same species
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Independent assortment
• When homologous pairs of
chromosomes line up at the first
meiotic division, the final position of
any one pair is RANDOM relative to
the other pair.
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The potential for infinite
variation
• In the next diagram this is shown, here it gives
rise to 22 (i.e. 4) different combinations in the
gametes. This may lead to the formations of new
phenotypes.
• The larger the number of chromosomes present
the greater number of possible combinations.
• A human egg mother cell with 23 chromosomes
has the potential to produce 223 (i.e. 8388608)
different combinations.
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Independent Assortment
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Crossing Over.
• This is is the
exchange of genetic
material by two
chromosomes
twisting around one
another and
‘swapping positions’.
• This leads to genetic
variation.
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Crossing over con’t
• Two genes situated on the same
chromosome are said to be LINKED
• Involves chromatids being broken and the
broken end of one joining with that of
another.
• This way alleles of linked genes can
become sepaated and lead to new
combinations of alleles.
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Crossing over -Chiasmata
• The diagram shows crossover in a
homologous pair of chromosomes,
which leads to 4 variations of
gamete.
• Consider a human gamete mother
cell with 23 homologous pairs all
capable of undergoing chiasmata.
• The potential for increased variation
by this method is enormous.
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Sexual reproduction
• During sexual reproduction, two
haploid gametes (each carrying
a unique version of the species’
genetic blueprint) meet at
fertilisation.
• This mixing of part of one
parents genotype with that of
another produces a new
individual who is genetically
different from both parents and
from all other members of the
species
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Importance of genetic variation
• Such variation is of great importance
because it helps the species adapt to a
changing environment.
• E.g. Adapting or becoming resistant to a
new disease. Those of the species who
can adapt will live to pass on these
beneficial genes.
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Genetic fingerprinting
• Some of the DNA in the human genome
contains short, no-coding sequences of
bases that are repeated many times.
• These may vary in size and length form
person to person.
• DNA can be extracted and separated to
show this pattern = GENETIC
FINGERPRINT, it is unique to every
person.
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DNA fingerprint uses.
• A person inherits 50% of their
DNA from each parent.
• Every band in the genetic
fingerprint must match one in
theirs fathers print and one in
their mothers.
– Used to settle paternity disputes
– Forensics DNA in cells, e.g.
Blood, mucous, sperm, can be
tested against the subject.
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Task: TYK Qu’s Torrance pg 78
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Task: Applying YK Qu’s
Torrance pg 78-79
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Essay Question 2004 (2A)
• Describe the events in meiosis which give
rise to variation in gametes (10).
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Marking instructions
1 Meiosis as nuclear/cell/reduction division to produce sex cells/sperm and eggs
(ie definition of meiosis which includes knowledge of what a gamete is)
2 Pairing of homologous chromosomes
3 Crossing over occurs
4 Chromatids break and rejoin/genetic exchange (labelled diagram accepted)
5 These points are called chiasmata
6 Chromosomes line up along equator
7 Assortment is independent or explanation (labelled diagram accepted)
8 Describe numerically. eg 23 pairs gives over 8 million different combinations
9 Non-disjunction
10 This is a form of mutation
11 Description of non-disjunction
12 Example of non-disjunction - eg Down’s syndrome
1 mark for coherence and 1 mark for relevance -Maximum total = 10 marks
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