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BIOLOGY - CLUTCH

CH.13 - MEIOSIS

CONCEPT: SEXUAL REPRODUCTION

Meiosis is a special type of cell division that occurs as part of the sexual life cycle of eukaryotes

● Sexual reproduction – parents donate a unique mixture of their DNA to offspring

□ Heredity – the transmission of genetic traits from parent to offspring

● Assexual reproduction – single parent gives rise to offspring with same genetic makeup

EXAMPLE:

● Allele – a version gene e.g. blood type A, B, O

● Gene locus (loci plural) – the location of a gene on a chromosome

● Homologous chromosomes – chromosomes that the same genes at the same loci

EXAMPLE:

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CONCEPT: SEXUAL REPRODUCTION

A karyotype is a picture of a person’s chromosomes.

□ Humans have 44 autosomes, and 2 sex chromosomes

● Haploid – only one copy of each chromosome

● Diploid – two copies of each chromosome

EXAMPLE:

● Somatic cells – diploid cells of the body of an organism

● Germ cells produce gametes – haploid cells that transfer genetic material in sexual reproduction

● Fertilization is when gametes fuse and create a zygote, the first cell of an organism!

EXAMPLE:

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CONCEPT: SEXUAL REPRODUCTION

● Alteration of generations - unlike animals, plant life cycles alter between multicellular haploid, and diploid forms.

● Most fungi exhibit a life cycle in which the only diploid stage is the zygote.

EXAMPLE:

The Paradox of Sex

● Even though sexual reproduction has its disadvantages, with it comes genetic variation, which can be advantageous.

EXAMPLE:

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PRACTICE: 1. Mitosis is nuclear division that yields _________ cells, while meiosis is nuclear division that produces _______ cells.

a. hapoid diploid c. gametic somatic b. diploid haploid d. gametic haploid

2. Multicellular organisms are made of diploid cells, except for their gametes. a. True b. False

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CH.13 - MEIOSIS

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CONCEPT: MEIOSIS

Meiosis occurs in two stages:

● Meiosis I – Reductional division

● Meiosis II – Equational division

EXAMPLE:

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CONCEPT: MEIOSIS

● Prophase I: like prophase of mitosis, but the chromosomes behave differently

□ Synapsis – homologs pair allowing for exchange of chromosome pieces

□ Tetrads – the two pairs of sister chromatids of homologous chromosomes

EXAMPLE:

● Synaptomal complex – protein complex that holds the homologs together

● Crossing over – the exchange of pieces of chromosomes that occurs in prophase I

● Chiasmata – the point where two homologous, non-sister chromosomes connect

EXAMPLE:

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CONCEPT: MEIOSIS

● Metaphase I:

□ Homologs align on metaphase plate, and each homolog pair attaches to microtubules from opposite poles

□ The microtubules from one pole attach to the kinetochore of both sister chromatids

● Anaphase I:

□ Homologs separate and move to opposite poles

□ Sister chromatids remain attached

EXAMPLE:

● Telophase I and Cytokinesis:

□ Each cell is left haploid with one pair of sister chromatids from each pair of homologous chromosomes

□ No chromosome duplication occurs before meiosis II

EXAMPLE:

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CH.13 - MEIOSIS

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CONCEPT: MEIOSIS

● Prophase II: spindle apparatus forms

● Metaphase II:

□ Sister chromatids align at the metaphase plate (they are not genetically identical)

□ Microtubules from opposite poles attach to the kinetochores of sister chromatids

EXAMPLE:

● Anaphase II: sister chromatids separate and move to opposite poles

● Telophase II and Cytokinesis: results in haploid daughter cell

EXAMPLE:

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CONCEPT: MEIOSIS

● Nondisjunction – when sister chromatids, or homologs fail to separate during cell division

● Aneuploidy – the wrong number of chromosomes, can be as a result of nondisjunction

EXAMPLE:

Genetic Variation:

● Independent assortment – during metaphase I the homologous pairs assort independently of each other

● Recombination – when cross over occurs, genes are swapped between chromosomes

● Random fertilization – which gametes will fuse, is up to chance

EXAMPLE:

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PRACTICE: 1.  Which phase of the cell cycle is lacking between the two meiotic divisions?

a. P phase b. M phase c. S phase d. a full cell cycle

2. A diploid organism has 28 chromosomes. How many chromosomes will the cell have after mitosis but before cytokinesis?

a. 7 b. 14 c. 28 d. 56

3. A scientist observes cells undergoing meiosis. At the end of meiosis I, it is noted that the two resulting daughter cells are both haploid. This is because the cells that resulted from meiosis I are _________ but are composed of two sister chromatids that will be pulled apart during ___________, producing haploid gametes.

a. diploid metaphase I c. diploid meiosis II

b. haploid anaphase I d. haploid meiosis II

4. Why do most genes sort independently?

a. their protein products are unrelated traits

b. their protein products are related traits

c. they are on different chromosomes

d. they are different alleles

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