Honors Biology
Unit 5
2013-2014
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Genome: Complete complement of an organism’s DNA.
◦ Includes genes (control traits) and non-coding
DNA organized in chromosomes.
Genome > Chromosome > DNA > Genes
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Eukaryotic DNA is organized in chromosomes.
◦ Genes have specific places on chromosomes.
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Heredity – way of transferring genetic information to offspring
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Chromosome theory of heredity: chromosomes carry genes.
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Gene – “unit of heredity”.
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Asexual: self-reproducing, produce clones offspring genetically identical to parent )
◦ Many single-celled organisms reproduce by splitting, budding, parthenogenesis.
◦ Some multicellular organisms can reproduce asexually.
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Fusion of two gametes to produce a single zygote.
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With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.
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7

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Total = 46 chromosomes (23 pairs)
44 autosomes + 2 sex chromosomes
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Autosomes:
◦ Code for most genes in your body (not sex determining chromosomes)
◦ In humans chromosome #1-22

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Sex Chromosomes:
◦ chromosomes that determine what gender you will be.
◦ #23 in humans
◦ Females are: XX
◦ Males are XY

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Gametes: sperm/egg
◦ haploid (n)
◦ Haploid = Contains a single set of chromosomes (23)
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Zygote: fertilized egg
◦ now diploid (2n)
◦ Diploid = Contains a two sets of chromosomes (23x2=46)
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Somatic cell: any cell other than gametes, most of the cells in the body. – All are diploid
(2n)
Meiosis KM 11

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Mammals use a chromosomal method of determining sex: XX is female and XY is male.
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Birds use a ZW system: ZZ is male and ZW is female.
◦ the evolutionary origin of mammalian and bird sex chromosomes is different
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Some reptiles use developmental temperature to determine sex: depends on the species, but hot is male and cold is female in some.

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Chromosomes exist in homologous pairs in diploid cells.
homologous pairs: chromosomes that code for the same genes but possibly have different alleles (get one from each parent)
Exception: Sex chromosomes (X, Y).
All autosomes are in homologous pairs .
13

• Now each homologous chromosome has a sister chromatid
• Still considered diploid
(2n) cell because still just two copies of each chromosome (even though they now have a twin)
Meiosis KM 14

Meiosis KM 15

Meiosis KM 16

All are even numbers – diploid (2n) sets of homologous chromosomes!
Ploidy = number of copies of each chromosome.
Diploidy
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Meiosis: cell division process by which the number of chromosomes per cell is cut in half
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Why does it occur: Meiosis is used to produce the haploid(n) gametes
(sperm and eggs)
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Meiosis KM 20

Meiosis KM 21

First division of meiosis
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Prophase 1: Duplicated homologous chromosomes match up forming tetrads .
Crossing-over occurs at the chiasmata .
Metaphase 1: Tetrads align at the equator of the cell.
Anaphase 1: Homologous pairs separate with sister chromatids remaining together.
Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
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23

• produces recombinant chromosomes , mixing the genes of the mother and father, recombining them.
• Occurs during Prophase 1
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Meiosis KM 25

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Second division of meiosis: Gamete formation
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Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes line up at the equatorial.
Anaphase 2: Sister chromatids move separately to opposite ends of the cell.
Telophase 2: Cell division is complete. Four haploid(n) daughter cells are produced.
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Meiosis KM 29

Meiosis KM 30

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During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
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Meiosis results in genetic variation by:
◦ Shuffling of maternal and paternal chromosomes and crossing over.
◦ No daughter cells formed during meiosis are genetically identical to either mother or father
◦ During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.
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Meiosis KM 32

Number of combinations: 2 n e.g. if there are 2 chromosomes in haploid
2n = 4; n = 2
2 n = 2 2 = 4 possible combinations
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e.g. 23 chromosomes in haploid
2n = 46; n = 23
2 n = 2 23 = ~ 8 million possible combinations!
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At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!
Meiosis KM 35

Male gamete formation - Spermatogenesis
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In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

Female gamete formation - Oogenesis
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In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg.
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The other 3 meiotic cells are small “polar bodies”, which degenerate.

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More genetic diversity: more potential for survival of species when environmental conditions change.
◦ Shuffling of genes in meiosis
◦ Crossing-over in meiosis
◦ Fertilization: combines genes from 2 separate individuals
DNA back-up and repair.
◦ Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. (some organisms have more)
◦ Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.
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1. How does metaphase of mitosis differ from metaphase I of meiosis?
2. What happens as homologous chromosomes pair up during prophase I of meiosis?
3. What is the sole purpose of meiosis?
4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis?
5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced.
b. the amount of DNA in the daughter v. parent cell c. mechanism for introducing genetic variation.
6. What is a zygote and how is it formed?
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1.
Summarize the process of meiosis in 5 sentences or less (include the following in your summary)
1.
What is the sole purpose of Meiosis?
2.
What does it start with and what does it end with (include males and females)
3.
Use and underline vocab words gamete, diploid, haploid, crossing over, and homologous pairs, and sister chromatids
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