Chapter 10 Meiosis

Higher Human Biology

Unit 1: Cell Function and Inheritance

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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.

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• For this process to be possible, the sex cells must be haploid.

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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• Mother cell divides to produce 4 gametes (sex cells).

• Gametes contain half the genetic material of the parent cell (haploid).

• 2 nuclear divisions.

Meiosis: Summary

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Sites of meiosis

Site of meiosis

Diploid gamete mother cell

Haploid gametes formed

Testes (seminiferous tubule)

Sperm mother cell

Sperm

Ovary Egg mother cell 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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Each chromosome

replicates forming 2 identical

chromatids

1st Meiotic Division

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Interphase

Nuclear material becomes visible,

each chromosome has two chromotids attached to a centromere,

Homologous chromosomes

pair up and come to lie alongside one another so

that their centromeres and

genes match exactly

The nuclear membrane disappears,

spindle fibres form, one

chromosome of each pair

moves to one pole and its

partner moves to the opposite

pole.

Members of each

homologous pair begin to repel

one another and move apart at exact points

called CHIASMATA – Crossing over

occurs.

Nuclear membrane

forms round each group of chromosomes

and then the by division of the

cytoplasm resulting in the

formation of two haploid cells.

2nd Meiotic Division

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Each haploid cell now undergoes a

2nd meiotic division

Single chromosomes

(each made of 2 chromatids) line up

at each equator

On separation from its partner each

chromatid is regarded as a chromosome.

Each of the four gametes formed

contains half the number of chromosomes

present in the original gamete mother cell.

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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VariationMuch 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

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• 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.

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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