Chapter 10 Meiosis

1. Unit 1: Cell Function and Inheritance Chapter 10 Meiosis Higher Human Biology Mrs Smith

2. Learning Intentions • To revise chromosomes • To revise mitosis • To examine meiosis • To look at effect of chromosomes crossing over during meiosis Mrs Smith

3. 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. Mrs Smith

4. Homologous chromosomes • In pairs. • Not identical, but the genes “describe” the same characteristics. • e.g. Eye colour can have blue or brown alleles. Mrs Smith

5. 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. Mrs Smith

6. 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. 46 92 46 Mrs Smith

7. 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. Mrs Smith

8. Sites of meiosis Mrs Smith

9. 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. Mrs Smith

10. 1st Meiotic Division 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. Homologous chromosomes pair up and come to lie alongside one another so that their centromeres and genes match exactly Members of each homologous pair begin to repel one another and move apart at exact points called CHIASMATA – Crossing over occurs. Nuclear material becomes visible, each chromosome has two chromotids attached to a centromere, Each chromosome replicates forming 2 identical chromatids Interphase Mrs Smith

11. 2nd Meiotic Division 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. Mrs Smith

12. Meiosis Summary Mrs Smith

13. Meiosis Mrs Smith

14. Chromosome Complement shows the number of sets of chromosomes, where n=1 set Mrs Smith

15. Task: TYK Qu’s Torrance pg 76 Mrs Smith

16. Variation Much of the VARIATION that exists amongst the members of the species is inherited and determined by alleles of genes. Mrs Smith

17. Variation Caused by: • Idependent assortment of chromosomes • Chiasmata (crossing over of genetic information) Results in variation between organisms of same species Mrs Smith

18. 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. Mrs Smith

19. 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. Mrs Smith

20. Independent Assortment Mrs Smith

21. 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. Mrs Smith

22. 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. Mrs Smith

23. 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. Mrs Smith

24. 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 Mrs Smith

25. 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. Mrs Smith

26. 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. Mrs Smith

27. 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. Mrs Smith

28. Task: TYK Qu’s Torrance pg 78 Mrs Smith

29. Task: Applying YK Qu’s Torrance pg 78-79 Mrs Smith

30. Essay Question 2004 (2A) • Describe the events in meiosis which give rise to variation in gametes (10). Mrs Smith

31. 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 Mrs Smith