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Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis Blueprint of Life Topic 11: Chromosomes, Meiosis and Gamete Formation.

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Presentation on theme: "Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis Blueprint of Life Topic 11: Chromosomes, Meiosis and Gamete Formation."— Presentation transcript:

1 Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis Blueprint of Life Topic 11: Chromosomes, Meiosis and Gamete Formation

2 DOT POINT(s) explain the relationship between the structure and behaviour of chromosomes during meiosis and the inheritance of genes explain the role of gamete formation and sexual reproduction in variability of offspring

3 Introduction Today we know that every species has a characteristic number of chromosomes in every body cell (eg: 46 chromosomes in humans). This number does not change from one generation to the next.

4 Introduction In order to maintain the constant chromosome number from one generation to the next, a mechanism called meiosis (reduction division) occurs to halve the chromosome number when gametes are produced. Every parent cell contains two sets of chromosomes— one paternal set and one maternal set, resulting in the diploid number.

5 Introduction During meiosis, homologous pairs of chromosomes segregate so that each gamete receives only one copy of every chromosome— that is, the gametes are haploid.

6 Introduction The diploid number is restored when the gametes fuse in fertilisation to form a zygote.

7 Introduction Meiosis has the added role of mixing the paternal and maternal chromosomes (and even the paternal and maternal genes when crossing over occurs) and these recombined chromosomes are passed into gametes, thereby increasing the genetic variation in offspring produced.

8 Chromosomes Chromosomes are compact coils of thread-like molecules called DNA (deoxyribonucleic acid), organised around proteins called histones. A helpful analogy is to imagine the DNA as cotton thread that can be wound around a ‘cotton reel’ of protein.

9 Chromosomes In this packaged form, the ‘reels of cotton’ represent chromosomes; in the unwound form, the network of threads created would be the chromatin material (dispersed DNA) seen in the nucleus of a working (non-dividing) cell. a

10 Chromosomes It is much easier for DNA to be transported (e.g. from one cell to another during cell division) in its compact chromosome form. This is why chromosomes become visible at the start of mitosis and meiosis.

11 Chromosomes Chemically, each gene is made up of a portion of DNA that stores information as a coded sequence, and each coded sequence/gene is located at a particular site or locus on the chromosome. The coded information within genes determines how living things look, behave and function—that is, it influences particular characteristics (phenotypes). A chromosome can therefore be described as a linear sequence of genes.

12 Genetic Variation During meiosis, genetic variation arises as a result of the behaviour of chromosomes at two stages: 1. during crossing over 2. when chromosomes randomly segregate and paternal and maternal chromosomes assort independently of each other.

13 Genetic Variation During meiosis I: 1. chromosomes line up in homologous pairs (one maternal and one paternal chromosome in each pair) during prophase I Handout11.1.2 Differences between mitosis and meiosis flow diagram

14 Genetic Variation During meiosis I: 2. crossing over occurs—arms of homologous chromosomes exchange genetic material (during metaphase). This introduces genetic variation— genes that occur on the same chromosome are said to be linked. Crossing over (synapsis) ensures that linked genes on a chromosome can be inherited independently of each other.

15 Genetic Variation During meiosis I: The exchange of genetic material between homologous chromosomes during crossing over causes the mixing of paternal and maternal genes and the result is an increased number of combinations of genes that may be transmitted by gametes to offspring, thereby increasing genetic variation.

16 Genetic Variation During meiosis I: 3. the chromosomes in each pair of chromosomes separate (during anaphase I), so that one entire chromosome of each pair moves into a daughter cell.

17 Genetic Variation During meiosis I: The manner in which these chromosomes separate is termed: independent assortment: the paternal and maternal chromosomes sort themselves independently of each other. This separation of chromosomes, also referred to as random segregation, ensures the chromosome number in the resulting gametes will be half that of the original cell.

18 Genetic Variation During meiosis I: For example, the maternal chromosomes do not all move into one gamete and the paternal into another. Which chromosome of each pair ends up in a gamete is random and determined completely independently of the separation of any other gene pair.

19 Genetic Variation During meiosis I: As a result, mixing of maternal and paternal chromosomes occurs and so independent assortment leads to further genetic variation. There are 223 possible different combinations of chromosomes in the formation of a human gamete.

20 Genetic Variation During meiosis II: The two daughter cells that result from meiosis I each undergo meiosis II, which is similar to mitosis, and the behaviour of chromosomes in the second meiotic division does not further affect genetic variation.

21 Genetic Variation Mendel did not know about genes and chromosomes; Sutton and Boveri noted the similarity between the behaviour of chromosomes and Mendel’s laws (1904). Units of heredity on chromosomes were later termed genes (1909) and their inheritance patterns are now used to explain the ratios derived in Mendel’s laws.

22 Genetic Variation The term variability means something different to variation. In the preliminary course, we learnt that variation is evident in individuals (e.g. differences in colour and height). Variability in genetics relates to the different forms of a gene within a population (that is, the total of all alleles present in the gene pool of a population).

23 Genetic Variation Both variation and variability may have one of three origins: ■ genetic ■ environmental ■ a combination of both genes and the environment. Both variation and variability are of evolutionary advantage only if they have a genetic basis.

24 Gamete Formation and Variation We know gametes form by meiosis, where recombination of genetic material takes place as a result of crossing over and random segregation.

25 Gamete Formation and Variation Gametes that arise from genetically dissimilar parents (cross- fertilisation as opposed to self-fertilisation) are likely to differ from each other more than those produced by self fertilisation. Cross- fertilisation produces a greater variety of gametes, increasing variability.

26 Fertilisation and Variability Gametes contain different, recombined genetic material and the possibility of many different combinations of gametes fusing (that is, which sperm cell fuses with which egg cell) increases variation. New combinations of genes occurring in the offspring lead to greater variability within a population.

27 Fertilisation and Variability Example: One parent may have blue eyes and fair hair, the other dark eyes and dark hair. If the traits for hair colour and eye colour assort independently from each other, their gametes may combine to produce offspring that have blue eyes and dark hair or brown eyes and fair hair. In this case, the offspring have a different combination of the parents’ genes, increasing variation.

28 Fertilisation and Variability Further variability arises in a population if a greater number of alleles are present for each gene. If within the population there are individuals with red hair and green eyes, there is greater variability and an even greater opportunity for more gene combinations to arise in gametes produced by individuals.

29 Fertilisation and Variability Variability may be increased as a result of: ■ a recombination of genetic material ■ an increased number of alleles for a particular gene.

30 Importance of Sexual Reproduction Sexual reproduction is important in increasing genetic variability (the amount by which individuals in a population vary from each other genetically). The study of this phenomenon is called population genetics.

31 Importance of Sexual Reproduction Mechanisms of sexual reproduction, such as gamete formation (meiosis) and fertilisation, increase gene recombination and therefore variability in a population. Evolutionary studies show that greater variability improves the ability of a population to adapt to changes in the environment, resulting in an increased chance of survival.

32 Importance of Sexual Reproduction If there is little or no variability within a population, the result is a static or unchanging population that is less likely to be able to adapt to sudden changes in the environment and is more likely to be wiped out.

33 Activity/Homework -Long PPT, NO Activity!! a

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