BO101 Molecular Genetics Lecture 2 PDF

Sexual Life Cycles & Meiosis BO101 - Molecular Genetics - Lecture 2 Dr Andrew Flaus, Biochemistry Flickr - Moss Sporophytes - BlueRidgeKitties Chromosomes and concept of sexual lifecycle Karyotype: Set of chromosomes in cell...

Sexual Life Cycles & Meiosis BO101 - Molecular Genetics - Lecture 2 Dr Andrew Flaus, Biochemistry Flickr - Moss Sporophytes - BlueRidgeKitties Chromosomes and concept of sexual lifecycle Karyotype: Set of chromosomes in cell metaphase chromosomes ๏ Aligned chromosome set ๏ Sex chromosomes ‣ Determine gender (XY) ๏ Autosomes ‣ All non-sex chromosomes human karyotype = 23 pairs = 46 chromosomes Campbell g 13.3 fi Karyotype: Set of chromosomes in cell ๏ Karyotype ‣ Aligned chromosome set in cell ๏ Two types of copy ‣ Homologous chromosome pairs Parents contribute one of each chromosome: 23+23=46 ‣ Sister chromatids Replicated copies of each chromosome Generated in S phase Campbell g 13.3 fi Chromosome sets in human cells ๏ Diploid cell ‣ Cell with homologous chromosome pairs ‣ Human diploid cells have 46 chromosomes in 23 pairs 22 autosomes XX or XY as sex chromosomes ‣ Diploid number ‘2n’ Campbell g 13.4 fi Chromosome sets in human cells ๏ Diploid cell ‣ Cell with homologous chromosome pairs ‣ Human diploid cells have 46 chromosomes in 23 pairs ๏ Haploid cell ‣ Cell with only one of each chromosome type Human gametes (egg and sperm cells) have 23 chromosomes Haploid number ‘n’ Campbell g 13.4 fi Clicker question 1-2 help ๏ Homologous chromosomes ‣ Pair of chromosomes with same length encoding same genes (diploid, see Mitosis lecture) ๏ Sister chromatids ‣ Two copies of a duplicated chromosome https://en.wikipedia.org/wiki/Sister_chromatids Sexual life cycles Human sexual life cycle ๏ Humans develop by mitosis ‣ Almost all cells are diploid ‣ 46 chromosomes ‣ 23 homologous chromosome pairs Campbell g 13.5 fi Review of stages of MITOSIS G2 Prophase Prometaphase Metaphase Anaphase Telophase & (interphase) Cytokinesis Campbell g 12.7 fi Stages of MITOSIS BioFlix 12_07 Mitosis Human sexual life cycle ๏ Humans develop by mitosis ‣ Almost all cells are diploid ๏ Gametes ‣ Transmit gene copies between generations ‣ Fertilisation by two gametes (n) produces a diploid zygote (2n) Campbell g 13.5 fi Human sexual life cycle ๏ Humans develop by mitosis ๏ Gametes ‣ Transmit gene copies between generations ‣ Gametes arise in germ cells of ovaries and testes by meiosis ‣ Gametes are haploid One of each chromosome type 23 chromosomes, n = 23 in humans Campbell g 13.5 fi Other sexual lifecycles animals ๏ Fertilisation and meiosis alternate ๏ Multicellular forms of organisms plants ‣ Animals: diploid only ‣ Plants: both diploid and haploid ‣ Most fungi: haploid only fungi Campbell g 13.6 fi Meiosis generates haploid cells Meiosis ๏ Specialised cell division to produce gametes ‣ Reduces number of chromosome sets to haploid ๏ Similar to mitosis except TWO divisions occur without replication Campbell g 13.7 fi Meiosis ๏ TWO divisions ๏ Meiosis I ‣ Homologous chromosome pairs separate ‣ Cells now haploid with sister chromatids ๏ Meiosis II ‣ Sister chromatids separate ‣ Only a single copy of each DNA remains Campbell g 13.7 fi Stages of MEIOSIS BioFlix 13.08 Meiosis Stages of meiosis Campbell g 13.8 fi Prophase I ๏ Chromosomes begin to condense and homologous chromosome pairs align ๏ Synapsis ‣ Homologous pairs are in contact ๏ Chiasmata ‣ Crossing over between homologues ๏ Sister chromatids remain associated throughout Campbell g 13.8 fi Prophase I ๏ Chromosomes begin to condense and homologous chromosome pairs align ๏ Synapsis ‣ Homologous pairs are in contact ๏ Chiasmata ‣ Crossing over between homologues ๏ Sister chromatids remain associated throughout Campbell g 13.9 fi Metaphase I ๏ Homologous chromosome pairs align ๏ Sister chromatids of each homologous pair are linked to a single pole ‣ Sister chromatids move together Campbell g 13.8 fi Anaphase I ๏ Homologous chromosome pairs separate and move to opposite poles ๏ Sister chromatids remain associated and move together Campbell g 13.8 fi Telophase I & Cytokinesis ๏ Each half of cell has haploid number of chromosomes ๏ Each chromosome has two sister chromatids ๏ Cytoplasm divides (cytokinesis) to form two cells ๏ No replication occurs ‣ Cells are haploid ‣ Sister chromatids still together Campbell g 13.8 fi Prophase II ๏ Spindle apparatus reforms Campbell g 13.8 fi Metaphase II ๏ Individual chromosomes position at metaphase plate ๏ Sister chromatids are attached to opposite spindle poles Campbell g 13.8 fi Anaphase II ๏ Sister chromatids separate and move to opposite poles ๏ Sister chromatids are not identical due to crossing over Campbell g 13.8 fi Telophase II & Cytokinesis ๏ Nuclei reform ๏ Cytokinesis occurs ‣ Each of the four daughter gametes is non-identical and distinct from parents ‣ Each daughter cell has a haploid chromosome number Campbell g 13.8 fi Meiotic products ๏ Each of the four daughter gametes is ‣ Non-identical ‣ Distinct from parents ๏ Each daughter cell has a haploid chromosome number Campbell g 13.8 fi Stages of MEIOSIS BioFlix 13.08 Meiosis Genetic variation contributes to evolution Asexual v sexual reproduction ๏ Alternative ways of creating offspring 1. Asexual reproduction ‣ Gives clones with identical genetic makeup Genetic variation by mutation etc all offspring identical 2. Sexual reproduction ‣ Gives unique gene combinations from both parents ‣ Sexual reproduction generates all offspring unique genetic variation Campbell g 45.4, concept 13.3 fi  possibility 1 Equally probable arrangements of chromosomes at metaphase I Basis of variation metaphase I 1. Independent assortment of metaphase II chromosomes in gametes ‣ Each homologous pair separates gametes independently Multiple possibilities possibility 2 ‣ Chromosomes from parents can metaphase I assort in new combinations ‣ Each cell receives some chromosomes from mother and some from father after meiosis I metaphase II gametes Campbell g 13.11 fi Basis of variation 1. Independent assortment of chromosomes in gametes 2. Crossing over of homologous chromosomes to exchange arms ‣ Happens in meiosis I ‣ In humans 1-3 crossovers occur per homologous pair ‣ Possible because of synapsis in metaphase I of meiosis Campbell g 13.12 fi Basis of variation Any chromosomes can assort in gamete 1. Independent assortment of chromosomes in gametes 2. Crossing over of arms 3. Gametes randomly combine at fertilisation ‣ Two random gametes combine as zygote ‣ Gamete assortment from meiosis I mixing gives 223 possibilities ‣ Fertilisation of 2 random gametes Any gamete pair can fertilise in zygote gives zygote from 246 possibilities Campbell g 13.11 fi Summary of lecture ๏ Offspring acquire genes by inheriting chromosomes ๏ Fertilisation and meiosis alternate in sexual life cycles ๏ Meiosis reduces chromosome sets from diploid to haploid ๏ Genetic variation via sexual life cycles  Learning outcomes for lecture ๏ On successful completion of this lecture, you will be able to: ‣ De ne sexual reproduction and contrast it with asexual reproduction ‣ Describe the alternation between haploid and diploid states in sexual life cycles, with the help of diagrams and examples ‣ Explain the mechanism of meiosis, emphasising stages in meiosis I ‣ Compare and contrast meiosis with mitosis ‣ Outline the multiple ways in which the mechanism of sexual reproduction contributes to genetic variation fi