Ch. 9

 Gametes are reproductive cells (eggs and sperm) that contain half the complement of chromosomes found in somatic cells
◦ the gametes fuse to form a new cell called a zygote , which contains two complete copies of each chromosome
 the fusion of gametes is called fertilization , or syngamy egg + sperm = Zygote

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Typically, each cell of your body has 46 chromosomes..23 from each parent
So, you have what we call a
Diploid value of 46
Or, referred to as 2N = 46
Your gametes, however, have
1N values
1N = 23….This is a Haploid condition
All your normal body cells are diploid, only your gametes are haploid
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THIS IS TRUE FOR MOST
ORGANISMS!!!!!!
Sex Cells

• Every organisms inherits chromosomes from their parents.
• Asexually reproducing organisms pass on their full set of chromosome creating offspring that are essentially clones of themselves
• Sexually reproducing organisms pass on half of their chromosomes through their sex cells. The offspring created inherits half of its chromosomes from each parent.

 Gamete- egg and sperm
 Zygote- cell produced when egg and sperm fuse
 Fertilization/ syngamy- fusion of gametes

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Inheritable traits are carried on factors called genes and are passed down from generation to generation .
Genes are located on DNA within the nucleus of the cell.
There are two alleles for every gene. One allele is inherited from each parent
• Alleles are alternate versions of a gene
The process by which chromosomes are separated in the production of sex cells is known as Meiosis.

• Homologous
Chromosomeschromosomes that carry the alleles for the same gene.
• Diploid Cells- (2N) “Two sets”; have both sets of homologous chromosomes (All body cells are diploid)
• Haploid Cells- (1N) “one set”; has a single set of genes. (All sex cells are haploid)

Meiosis - is the process by which the homologous chromosomes are separated in a diploid cell to produce a haploid cell
Meiosis involves two divisions, meiosis I and meiosis II
◦ meiosis I separates the homologues in a homologous pair
◦ meiosis II separates the replicate sister chromatids

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Meiosis I is traditionally divided into four sequential stages
1.
Prophase I
 Homologous chromsomes pair up and exchange segments
2.
Metaphase I
 The paired homologous chromosomes align on a central plane
3.
Anaphase I
 Homologues are pulled apart and move to opposite poles of the cell
4.
Telophase I
 Individual replicated chromosomes gather at each of the two poles

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Prior to the start of Meiosis, chromosomes are replicated during Interphase.
During Prophase I homologous chromosomes pair up to form a tetrad (there are 4 chromatids in a tetrad)
Crossing over may occur when the tetrads exchange a portion of the chromatids
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During crossing-over chromatids break and may be reattached to a different homologous chromosome.
Human female eggs remain in
Meiosis I until puberty… 12-13 years

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Each homologous set of chromosomes line up along cell center
Orientation on the metaphase plate is random…with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

 During metaphase I, the orientation of the homologous chromosome pairs is a matter of chance
◦ each possible orientation of which homologue in a homologous pair faces which pole results in gametes with different combinations of parental chromosomes
◦ this process is called independent assortment

 Homologs are pulled apart to opposite poles
 Chromosomes, each with two chromatids, move to separate poles.
 Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

 Homologous chromosomes have fully separated
 Results in a Haploid (1N) set of chromosomes at each pole
 Nuclear envelopes may reform, or the cell may quickly start meiosis 2.

Meiosis II is also divided into four stages
1.Prophase II new spindle forms to attach to chromosome clusters
2.Metaphase II spindle fibers bind to both sides of the centromere and individual chromosomes align along a central plane
3.Anaphase II sister chromatids move to opposite poles
4.Telophase II the nuclear envelope is reformed around each of the four sets of daughter chromosomes

What does Meiosis II look like????

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Sexual reproduction has an enormous impact on how species evolve because it rapidly generates new genetic combinations
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Three mechanisms help produce this variety
1.
Independent assortment
2.
Crossing over
3.
Random fertilization

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Gametogenesis the process of forming gametes
(haploid, 1n) from diploid cells (somatic cells) of the germ line.
Spermatogenesis is the process of forming sperm cells by meiosis. In spermatogenesis all 4 meiotic products develop into gametes and human males produce 200,000,000 sperm per day
Oogenesis is the process of forming an ovum
(egg) by meiosis in specialized gonads known as ovaries. Human females female produce one egg
(usually) each menstrual cycle