Chapter 10 Meiosis Higher Human Biology Unit 1: Cell Function and Inheritance 01/10/20151Mrs Smith

Learning Intentions To revise chromosomes To revise mitosis To examine meiosis To look at effect of chromosomes crossing over during meiosis 01/10/20152Mrs Smith

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. 01/10/20153Mrs Smith

Homologous chromosomes In pairs. Not identical, but the genes “describe” the same characteristics. e.g. Eye colour can have blue or brown alleles. 01/10/20154Mrs Smith

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. 01/10/2015Mrs Smith5 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. 01/10/2015Mrs Smith

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 01/10/20157Mrs Smith

Sites of meiosis Site of meiosis Diploid gamete mother cell Haploid gametes formed Testes (seminiferous tubule) Sperm mother cell Sperm OvaryEgg mother cellEggs (ova) 01/10/2015Mrs Smith8

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. 01/10/2015Mrs Smith9

Each chromosome replicates forming 2 identical chromatids 1 st Meiotic Division 01/10/201510Mrs Smith 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.

2 nd Meiotic Division 01/10/201511Mrs Smith Each haploid cell now undergoes a 2 nd 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 01/10/201512Mrs Smith

Meiosis 01/10/201513Mrs Smith

Chromosome Complement shows the number of sets of chromosomes, where n=1 set 01/10/201514Mrs Smith

Task: TYK Qu’s Torrance pg 76 01/10/2015Mrs Smith15

Variation Much of the VARIATION that exists amongst the members of the species is inherited and determined by alleles of genes. 01/10/2015Mrs Smith16

Variation Caused by: Idependent assortment of chromosomes Chiasmata (crossing over of genetic information) Results in variation between organisms of same species 01/10/201517Mrs Smith

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. 01/10/2015Mrs Smith18

The potential for infinite variation In the next diagram this is shown, here it gives rise to 2 2 (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 2 23 (i.e ) different combinations. 01/10/2015Mrs Smith19

Independent Assortment 01/10/201520Mrs Smith

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. 01/10/2015Mrs Smith21

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. 01/10/2015Mrs Smith22

Crossing over -Chiasmata 01/10/201523Mrs Smith 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 01/10/2015Mrs Smith24

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. 01/10/2015Mrs Smith25

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. 01/10/201526Mrs Smith

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. 01/10/2015Mrs Smith27

Task: TYK Qu’s Torrance pg 78 01/10/2015Mrs Smith28

Task: Applying YK Qu’s Torrance pg /10/2015Mrs Smith29

Essay Question 2004 (2A) Describe the events in meiosis which give rise to variation in gametes (10). 01/10/2015Mrs Smith30

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 01/10/2015Mrs Smith31