Meiosis & Sexual Life Cycle

Chapter 13

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AP Essential Knowledge

Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.

b. Mitosis passes a complete genome from the parent cell to daughter cells.4. Mitosis is a continuous process with observable structural

features along the mitotic process. Students must know the order of the processes

(replication, alignment, separation).

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AP Essential Knowledge

c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms. Gametes have a haploid set of chromosomes Homologous chromosomes are paired and then

separated ensuring haploid set of chromosomes During meiosis (Prophase I), homologous chromatids

exchange genetic material via a process called “crossing over” which increases genetic variation in the resultant gametes.

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Homologous Chromosomes

2 chromosomes that have same length, centromere position, and staining pattern

Autosomes Non-sex chromosomes Chromosomes that do not determine gender

Sex Chromosomes Chromosomes that determine gender

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Chromosomes

Human somatic cell = 44 autosomes + 2 sex chromosomes

Human gamete = 22 autosomes + 1 sex chromosome

Sex Chromosomes Can be XX or XY XX = Homologous chromosomes XY = Not homologous chromosomes

Egg must contain X, sperm may contain X & Y Hence, males determine the gender of offspring

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Unnecessary Censorship

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Meiosis

2 Stages of Meiosis Meiosis I & Meiosis II

Much of Meiosis resembles Mitosis

Chromosomes are replicated only once Before Meiosis I

4 daughter cells are produced

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Meiosis: An Overview

Assume that an organism has:1 Homologous Pair = 2 Chromosomes(Diploid cell – 2n)

STEP 1: Each of the chromosomes is replicated in Interphase

STEP 2: Chromosome pairs of copies separate in Meiosis I(Haploid cell – n)BUT 2 copies of each one

STEP 3: Each of the copies (sister chromatids) in a cell separates creating 4 haploid cells(Haploid cell with only 1 copy)

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What is different in Prophase I?

What is different in Anaphase I?

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Meiosis I

Prophase I Longest phase Homologous pairs align Crossing-Over may occur Synapsis – pairing of homologous pairs tied tightly

together Tetrads form (4 chromosomes = 2 pairs)

Each tetrad has 1 or more chiasmata Criss-crossed regions where crossing over has occurred

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Meiosis I (Page 2)

Metaphase I Tetrads are aligned at the metaphase plate Each chromosome pair faces a pole

Anaphase I Homologous chromosomes (composed of 2 copies of

each chromosome called chromatids) are pulled apart

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What is different between Meiosis I & II?

This division is sometimes called the Mitotic division, why?

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Meiosis vs. Mitosis

Tetrads align in Prophase I, Chromosomes align in Prophase mitosis

Chromosomes position @ metaphase plate (Mitosis) Tetrads position @ metaphase plate (Meiosis)

Homologues separate in Meiosis I

Sister chromatids separate in Meiosis II & Mitosis

Crossing over = Meiosis NOT mitosis

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Mitosis Meiosis

DNA replicates in interphase

1 division

No synapsis

2 Diploid cellsGenetically identical cells

Responsible for:-- Zygote growth into multicellular organism

DNA only replicates in Pre-meiotic interphase

2 divisions

Synapsis occurs during prophase I forming tetrads

Crossing over occurs now

4 haploid cellsGenetically different cells

Responsible for:-- Gamete production-- Genetic variation

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Genetic Diversity

The reason for meiosis + sexual reproduction

Mutations are the original source of genetic diversity

3 main sources of Genetic Diversity

1. Independent Assortment of Chromosomes

2. Crossing Over

3. Random Fertilization

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Each daughter cell has a 50% chance of getting maternal chromosome (or its copy)

Similarly, 50% chance of getting paternal chromosome (or its copy)

Independent assortment - each chromosome is positioned independently of the other chromosomes

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When homologous pairs are formed in Prophase I, a recombinant chromosome can be formed -- A chromosome that has DNA from 2 different parents

2 chromosome segments trade places (cross over) producing chromosomes with new combos of maternal & paternal genes

1-3 times per chromosome in humans

Increases genetic variation

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Random Fertilization

Egg + sperm cells are genetically different from parent cells

Their combination (fertilization) increases variation even more