Meiosis and Sexual Reproduction Review Sheet

5/26/2018 Meiosis and Sexual Reproduction Review Sheet

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Meiosis and Sexual Reproduction Review Sheet

Sexual reproduction requires the processes of

Meiosis(cell division that results in the

production of gametes (sperm and egg cells

containing 1/2 the number of chromosomes)

and fertilization(the fusion of egg and

sperm cells producing a zygote containingthe normal number of chromosomes).

For humans, most cells in the body

contain 46 chromosomes. These are

called somatic cells.

For somatic cells, these chromosomes

can be grouped as matching pairs

(homologous chromosomes).

Homologous chromosomes contain

the same sequence of genes.

One member comes from the mother one

member comes from the father.

However, these genes may have slight variations.

Example: One chromosome may contain the

gene for blue eye color while the other

chromosome may have the gene for green eye

color.

Somatic cells are called diploidcells (2N); Forhumans 2n=46 chromosomes.

Gametes are called haploidcells (1N); For

humans 1n= 23 chromosomes.

Type of

Reproduction

Cell

processes

Offspring

Makeup

Advantages Disadvantages

Asexual

Reproduction

Mitosis Genetic copy of

parent

Speed

Requires no

mate

No Diversity

Sexual

Reproduction

Meiosis &

Fertilization

Combination of

traits from

parents

Genetic

Diversity

Greater

adaptability

Slower

Need to find

mate


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Asexual Reproduction=Binary Fission

Sexual Reproduction:Meiosis

Occurs in the sex organs (testes in males; ovaries in females)

There is oneDNA replication followed by twocell divisions

Produces 4 haploid (1N) daughter cells from one diploid (2N) parental cell

Each daughter cell contains only one member of each homologous pair. Produces

genetic variation by the processes of crossing overand random assortment of

chromosomes(see details below)

Details of MeisosisStep 1: DNA duplicates in S cycle

Step 2: Meiosis 1: Homologous chromosomes separate


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Prophase I:

Duplicated homologous chromosomes condense forminga tetrad

Crossing over occurs: portions of homologous

chromosomes exchange DNA

Nucleus disappears and spindle fibers form (as in

mitosis prophase)

Metaphase I:

Tetrads align in middle of the cell: One duplicated

chromosome from each pair face opposite poles of the

cell

Independent assortment of chromosomes:

o Each pair of duplicated homologous

chromosomes align independently from each

other

Anaphase I:

One member of each pair of duplicated homologous chromosomes migrate to

each pole; sister chromatids remain attached to one another at centromere

Telophase I and Cytokinesis

Completion of chromosome migration and cytokinesis occurs

Have two haploid (1N) cells

Step 3: Meiosis II: Sister chromatids separate; Meiosis II is very similar to mitosis


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Prophase II:

Spindle fibers reform and attach to duplicated chromosomes

Metaphase II:

Chromosomes align in middle of cell

Anaphase II:

Sister chromatids separate and move to opposite poles (like in mitosis)

Telophase II and Cytokinesis

Completion of chromosme migration and cell division

The result is 4 haploid cells (1N)


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Comparison between Mitosis and Meiosis

What happens if meiosis goes bad? Nondisjunction

Chromosomes fail to separate properly (in anaphase I or II)

Results in too many or too few chromosomes

Example 1: Trisomy 21: Downs syndrome; 3 chromosomes 21 instead of 2

Example 2: Abnormalities in sex chromosomes

Documents

Meiosis and Sexual Reproduction Review Sheet