Normal Cells are diploid (2n)

There are 2 of each chromosome

Means there are 2 copies of every gene

Useful b/c 2 copies of every gene means there is a backup if one gets mutated

Mutation are frequent in large organisims

We would not survive with just one copy

New organisms arise from the merger of 2 parental cells

If these parental cells were diploid the offspring would have twice as much DNA as the parents

This would mean that over 3 generations the offspring would have 368 chromosomes

Parents only pass on half of genetic material

Parental cells that merge to give rise to the offspring are not diploid

Having only one memeber of each chromosome pair

Process where a diploid cell with two pairs of each chromosome gives rise to haploid cells with

only one of each chromosome.

Pair of chromosomes with the same sequence of genes (one

chromosome in a homologous pair come from the mother the other comes from the father).

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Just because homologous chromosomes have the same order of genes, that is not to say that the genes are

identical.

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e.g., if a specific spot on the homologous pairs has a gene for blood type, one homologue's gene may

code for the A blood type, the other homologue's gene may code for B blood type

Process where a diploid cell with two pairs of each chromosome gives rise to haploid cells with only one of each chromosome.

Homologous Chromosomes (homologues)

Pair of chromosomes with the same sequence of genes (one

chromosome in a homologous pair come from the mother the other comes from the father).

Meiosis I

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Replication of Chromosomes

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Crossing over

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Reduction of chromosome # (from 2n to 1n)

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Homologous chromosomes are separated from one another

Division of chromosomes

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Sister chromatids are separated from one another

There is more going on in meiotic Prophase than in mitotic Prophase

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Nuclear membrane & nucleoli disappear

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Chromatin condenses to form chromosomes

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Centrioles migrate to opposite ends and spindle fibers form

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Homologous chromosomes pair up and cross over

 Consists of G1, S and G2 phases

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Remember, the chromosomes are replicated during the S phase

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Crossing over is the exchange of genetic material between homologous chromosomes

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Homologous chromosomes pair up to form tetrads and exchange pieces of their DNA.

 Two homologous chromosomes paired up

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This process allows for a reshuffling of genetic material.

Metaphase I

The tetrads separated, but the centromeres remain in tact.

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This means that the sister chromatids remain attached to one another and it is the homologous pairs that

are separated.

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In this step, the chromosome number is reduced by half. The cell went from being diploid to being

haploid.

Telophase I

◦ Nuclear membrane reforms.

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Chromosomes decondense to form chromatin.

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Nucleolus reappears

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Spindle apparatus breaks down.

Cytokinesis I

◦ The cell divides to form two cells.

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Each of these cells contains one chromosome of each homologous pair.

Interphase II (Interkinesis)

May be long, brief or non-existent.

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The DNA is NOT replicated during this phase if it occurs.

Prophase II

◦ Nuclear membrane & nucleoli disappear

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Chromatin condenses to form chromosomes

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Sister chromatids are attached to one another by a centromere

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Centrioles migrate to opposite ends & spindle fibers form

Metaphase II

Anaphase II:

◦ The centromere of each chromosome splits

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The sister chromatids are separated and pulled to opposite ends by the spindle fibers.

Telophase II

◦ Nuclear membrane reforms.

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Chromosomes decondense to form chromatin.

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Nucleolus reappears

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Spindle apparatus breaks down.

Cytokinesis II

Meiosis in Human Males

• In males, each of the four cells produced by meiosis will become sperm cells.

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The process of making sperm is ongoing and continuous from puberty onward.

Meiosis in Human Females

• Meiosis starts when the female is a fetus, but is halted during prophase I.

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During puberty, when ovulation occurs, the egg that is to be released finishes meiosis I.

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This produces two different types of cells: an oocyte and a polar body. The polar body may or may not

undergo meiosis II; either way, it will eventually die.

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The oocyte will begin meiosis II, but stops after prophase II. It will not finish meiosis II until it is fertilized

by a sperm. If the oocyte is fertilized, the cell contents are unevenly divided to give rise to two different

cells: a large ovum (which will develop into a new human being) and a small polar body which will die.

Haploid (1n)