Meiosis and genetic variation A study in creating sex cells Meiosis KM

Genome Genome: Complete complement of an organism’s DNA. Includes genes (control traits) and non-coding DNA organized in chromosomes. Meiosis KM

Genes Eukaryotic DNA is organized in chromosomes. Genes have specific places on chromosomes. Meiosis KM

Heredity Heredity – way of transferring genetic information to offspring Chromosome theory of heredity: chromosomes carry genes. Gene – “unit of heredity”. Meiosis KM

Reproduction Asexual Many single-celled organisms reproduce by splitting, budding, parthenogenesis. Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).

Reproductions: some vocabulary Gamete: the name given to sex cells (either a sperm or an egg) Fertilization: the process when the sperm cell and the egg cell fuse together Zygote: The cell created when the egg and sperm cell fuse together

Sexual reproduction Fusion of two gametes to produce a single zygote. Gametes have half the number of chromosomes In humans: 23 chromosomes (1n) The zygote is formed when the gametes fuse In humans, this forms 46 chromosomes (2n) Introduces greater genetic variation, allows genetic recombination. With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

Importance of Sex Cells You need to reduce the number of chromosomes in sex cells to half (1n) Otherwise when they fused, you would keep doubling the number of chromosomes Example: In humans, generation 1 would have 46 Generation2 would have 92 Generation3 would have 184, etc. These organisms would not survive

Importance of Sex Cells Sexual repro starts with sex cell & ends with fertilization Zygote is formed--in human now is diploid or 2n with 46 chromosomes

Why do we need to create sex cells? During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of the DNA. This means that the genes have more possibili No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

In humans If you have 1 chromosome, you have 2 possibilities With 23 chromosomes in in each gamete 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … 64 trillion combinations for a diploid zygote!!! That’s more than all humans that have ever lived. Meiosis KM

SEX IS COSTLY Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required Intimate contact provides route for infection by parasites and disease (AIDS, syphillis, etc.) Genetic costs: in sex, we pass on only half of genes to offspring. Males are an expensive luxury - in most species they contribute little to rearing offspring. QUESTION: Why is genetic diversity so important?

But … More genetic diversity = more potential for survival of species when environmental conditions change. Shuffling of genes Fertilization: combines genes from 2 separate individuals DNA back-up and repair. Asexual organisms don't have back-up copies of genes Sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. Sexual mechanisms, especially recombination, are used to repair damaged DNA The undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.

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In humans … 23 chromosomes donated by each parent (total = 46 or 23 pairs). Gametes (sperm/ova): Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid number “n” = 23 in humans). Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Only gametes are produced by meiosis. Meiosis KM

MEIOSIS I Unlike mitosis, meiosis occurs in two parts Meiosis I is similar to mitosis where you are creating a 2n number of chromosomes Meiosis II is the process where you create the 1n number of chromosomes in the sex cells

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Meiosis I --Sex Cell Formation In meiosis, there are 2 divisions of the nucleus: meiosis I & meiosis II Prophase I: double stranded chromosomes and spindle fibers appear; nuclear membrane and nucleolus fade (CROSSING OVER occurs here) Meiosis KM

Meiosis I --Sex Cell Formation Metaphase I: chromosome pairs (chromatids) line up spindle fibers attach to centromeres and centrioles Anaphase I: chromotids separate from matching pair (independent assortment occurs here) Meiosis KM

Meiosis I --Sex Cell Formation Telophase I: cytoplasm divides and 2 cells form Meiosis KM

Meiosis 1 First division of meiosis Prophase 1: Each chromosome dupicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Summary of Meiosis I Meiosis KM

Meiosis II At the end of meiosis I, you have created 2 cells with a 2n number of chromosomes The purpose of meiosis II is to take those 2 cells and create a total of 4 cells, each with n number of chromosomes Meiosis KM

Meiosis II --Sex Cell Formation Prophase II: chromatids and spindle fibers reappear Meiosis KM

Meiosis II --Sex Cell Formation Metaphase II: chromatids line up in the center of the cell spindle fibers attach to centromere & centriole Anaphase II: centromere divides chromosomes split and move to opposite poles Meiosis KM

Meiosis II --Sex Cell Formation Telophase II: spindle fibers disappear nuclear membrane forms around chromosomes at each end of cell each nucleus has half the # of chromosomes as the original (haploid) now there are 4 sex cells (daughter cells) Meiosis KM

Meiosis II Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Summary of Meiosis II

Meiosis I and Meiosis II OVERVIEW OF MEIOSIS Meiosis I and Meiosis II

Animation Meiosis KM

Meiosis creates genetic variation There are a couple of methods to increase genetic variability in sex cells Crossing-over Occurs mainly during prophase I When adjacent chromatids break of a portion of their structure and switch places Independent assortment Occurs mainly during anaphase I When homologous chromosomes separate randomly and move to opposite poles of a dividing cell

Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes. Meiosis KM

Independent assortment Meiosis KM

Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations Meiosis KM

REMINDER: KARYOTYPES Karyotype: ordered display of an individual’s chromosomes. Collection of chromosomes from mitotic cells. Staining can reveal visible band patterns, gross anomalies.

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Down's Syndrome Meiosis KM

Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues. Meiosis KM

DIFFERENCES BETWEEN MITOSIS AND MEIOSIS Meiosis KM

Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). In Metaphase I, homologous pairs line up at metaphase plate. In Anaphase I, independent assortment of chromosomes. Meiosis KM

Mitosis vs. meiosis Meiosis KM

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