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Meiosis

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Terminology

Heredity – continuity of biological traits from one generation to the next:

Results from transmission of hereditary units (genes) from parents to offspring.

Variation – inherited differences among individuals of the same speciesGenetics - Scientific study of heredity and hereditary variation

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TerminologyDNA – type of nucleic acid that is a polymer of four different kinds of nucleotidesGenes – units of hereditary information that are made of DNA and are located on chromosomes.Chromosomes – organizational unit of heredity material in the nucleus of eukaryotic organisms

Consist of a single long highly folded DNA molecules that are coiled along with proteinsContain genetic information arranged in a linear sequence and coded by genes

Locus – specific location on a chromosome that contains a gene

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Comparison of asexual versus sexual reproduction

Single individual is the sole parent

Single parent passes on all its genes to its offspring

Offspring are genetically identical to the parent

Results in a clone, or genetically identical individual. Rarely, genetic differences occur as a result of mutation, a change in DNA

Two parents give rise to offspring.

Each parent passes on half its genes, to its offspring

Offspring have a unique combination of genes inherited from both parents

Results in greater genetic variation; offspring vary genetically from their siblings and parents

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Asexual reproduction of

hydraIn asexual reproduction – a single individual is the sole parent and passes copies of all its genes on to its offspring.

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DefinitionsSexual reproduction occurs only in eukaryotes.

During the formation of gametes (reproductive cells),

the number of chromosomes is reduced by half (haploid),

and returned to the full amount (diploid) when the two gametes fuse during fertilisation.

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DefinitionsPloidy is a term referring to the number of sets of chromosomes.

Haploid organisms have one set of chromosomes; diploid – two.

Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete".

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Definitions

Somatic (most) cells in the human body are produced by mitosis.

Germ line cells (gametes) are produced by meiosis.

The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.

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Life cycle In each generation, the doubling of chromosome number that results from fertilization is offset by the halving of chromosome number that results from meiosis.

For humans, the number of chromosomes in a haploid cell is 23 ( n=23); the number of chromosomes in the diploid zygote and all somatic cells arising from it is 46 (2n = 46).

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Life cycle Three sexual life cycles differing in the timing of meiosis and fertilization.

The common feature of all three cycles is the alternation of these two key events, which contribute to genetic variation among offspring.

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Overview of meiosis

Paternal homolog

Maternal homolog

Premeiotic cell

chromatid Replicatedchromosomes

Synapsis

Cell division I

Daughter cells

Cell division II

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Meiosis I (reduction)

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Prophase IProphase I has a unique event - the pairing of homologous chromosomes.

The process of linking of the replicated homologous chromosomes is called synapsis.

The resulting chromosome composed of two chromatids from each chromosome is termed a tetrad. At this point crossing-over may occur.

During crossing-over chromatids break and may be reattached to a different homologous chromosome.

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Metaphase and anaphase

In metaphase tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair.

In anaphase I the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact.

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Meiosis II (division)

During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform.

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Meiosis II (division)

During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.

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Meiosis II (division)

Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division).

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Comparison of mitosis and meiosisMitosis maintains ploidy level, while meiosis reduces it.

Meiosis may be considered a reduction phase followed by a slightly altered mitosis.

Meiosis occurs in a relative few cells of a multicellular organism, while mitosis is more common.

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Comparison of mitosis and meiosis

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Comparison of mitosis and meiosis

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Homologous chromosome rearrangementsIn this figure we consider the consequences of meiosis in a hypothetical organism with a diploid chromosome number of 4 (2n = 4).

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Homologous chromosome rearrangementsThe arrangement of chromosomes at metaphase I determines which chromosomes will be packaged together in the haploid daughter cells.

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The results of crossing over during meiosisDuring prophase of meiosis I,

nonsister chromatids of homologous chromosomes exchange corresponding segments.

Following these chromosomes through meiosis, we can see that crossing over gives rise to individual chromosomes that have some combination of DNA originally derived from two different parents.

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Gametogenesis (spermatogenesis and oogenesis)

Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line.

Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (termed testes in males ). After division the cells undergo differentiation to become sperm cells.

Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries.

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Gametogenesis (spermatogenesis and oogenesis)

In spermatogenesis all 4 meiotic products develop into gametes;

in oogenesis most of the cytoplasm is placed into one large egg and the other cells, the polar bodies, do not develop.

Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.

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Gametogenesis

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Gametogenesis

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Summary1. Offspring derive from parents by inheriting chromosomes

2. Asexual versus sexual reproduction: offspring either from one or two parents

3. Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete".

4. Meiosis reduces chromosome number from diploid to haploid.

5. Sexual life cycles produce genetic variation among offspring: • independent assortments of chromosomes• crossing over between homologous chromosomes

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ReadingCh. 13, pp. 248-261