Planet Earth and Its Environment A 5000-million year Journey

Biology in Focus, HSC CourseGlenda Childrawi, Margaret Robson and Stephanie Hollis

Blueprint of LifeTopic 11: Chromosomes, Meiosis and Gamete Formation1DOT 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 offspringIntroduction 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. youshotmedown.wordpress.com

IntroductionIn 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.

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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.en.wikipedia.org

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

IntroductionMeiosis 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. www.sciencelearn.org.nzChromosomesChromosomes 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.ghr.nlm.nih.gov

ChromosomesIn 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

ChromosomesIt 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.course1.winona.edu

ChromosomesChemically, 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 functionthat is, it influences particular characteristics (phenotypes). A chromosome can therefore be described as a linear sequence of genes.biology.unm.edu

Genetic VariationDuring meiosis, genetic variation arises as a result of the behaviour of chromosomes at two stages:1. during crossing over2. when chromosomes randomly segregate and paternal and maternal chromosomes assort independently of each other.www.studyblue.com

Genetic VariationDuring meiosis I:1. chromosomes line up in homologous pairs (one maternal and one paternal chromosome in each pair) during prophase Icourse1.winona.edu

Handout11.1.2 Differences between mitosis and meiosis flow diagram Genetic VariationDuring meiosis I:2. crossing over occursarms 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.course1.winona.edu

Genetic VariationDuring 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.www.phschool.com

Genetic VariationDuring 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.biology.unm.edu

Genetic VariationDuring 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.

www.phschool.comGenetic VariationDuring 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.

Genetic VariationDuring 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.

Genetic VariationDuring 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.biology.unm.edu

Genetic VariationMendel did not know about genes and chromosomes; Sutton and Boveri noted the similarity between the behaviour of chromosomes and Mendels 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 Mendels laws.ar.wikipedia.org

Genetic VariationThe 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).evolution-khc.wikispaces.com

Genetic VariationBoth 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.www.biologyaspoetry.com

Gamete Formation and VariationWe know gametes form by meiosis, where recombination of genetic material takes place as a result of crossing over and random segregation.biology.unm.edu

Gamete Formation and VariationGametes 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.www.thehorse.com

Fertilisation and VariabilityGametes 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.drugline.org

Fertilisation and VariabilityExample: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. www.teach4real.com

Fertilisation and VariabilityFurther 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.www.bbc.co.uk

Fertilisation and VariabilityVariability may be increased as a result of: a recombination of genetic material an increased number of alleles for a particular gene.morningside.australialisted.com

Importance of Sexual ReproductionSexual 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.ajilbab.com

Importance of Sexual ReproductionMechanisms 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. www.photographyblogger.net

Importance of Sexual ReproductionIf 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.www.gizmodo.com.au

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