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Meiosis and genetic variation

Honors BiologyUnit 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

Genome > Chromosome > DNA > Genes

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Eukaryotic DNA is organized in chromosomes.◦ Genes have specific places on

chromosomes.

Genes

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Heredity – way of transferring genetic information to offspring

Chromosome theory of heredity: chromosomes carry genes.

Gene – “unit of heredity”.

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.

Reproduction

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Fusion of two gametes to produce a single zygote.

With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

Sexual reproduction

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Total = 46 chromosomes (23 pairs)44 autosomes + 2 sex chromosomes

In humans …

Two kinds of Chromosomes Autosomes:

◦ Code for most genes in your body (not sex determining chromosomes)

◦ In humans chromosome #1-22

Two kinds of Chromosomes Sex Chromosomes: ◦ chromosomes that

determine what gender you will be.

◦ #23 in humans

◦ Females are: XX◦ Males are XY

Meiosis KM 11

Gametes: sperm/egg ◦haploid (n)◦ Haploid= Contains a single set of chromosomes (23)

Zygote: fertilized egg◦ now diploid (2n)◦ Diploid= Contains a two sets of chromosomes (23x2=46)

Somatic cell: any cell other than gametes, most of the cells in the body. – All are diploid (2n)

Important Vocabulary

Sex Chromosomes Mammals use a chromosomal method of

determining sex: XX is female and XY is male.

Birds use a ZW system: ZZ is male and ZW is female.◦ the evolutionary origin of mammalian and bird sex

chromosomes is different

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)

Homologues

Exception: Sex chromosomes (X, Y).

All autosomes are in homologous pairs.

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Homologous chromosomes after replication

• 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 15

Meiosis KM 16

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Chromosome numbersAll 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

Why does it occur: Meiosis is used to produce the haploid(n) gametes (sperm and eggs)

Meiosis: What is it?

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Meiosis reduces the number of chromosomes by half.

Daughter cells differ from parent, and each other.

Meiosis involves two divisions, Mitosis only one.

Meiosis: Key differences from mitosis

Meiosis KM 20

Meiosis KM 21

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First division of meiosis

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.

Meiosis 1

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Crossing over• produces recombinant chromosomes, mixing the

genes of the mother and father, recombining them.• Occurs during Prophase 1

Meiosis KM 25

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Harlequin chromosomes

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Second division of meiosis: Gamete formation

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.

Meiosis II – Similar to mitosis

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Meiosis KM 29

Mitosis vs. meiosis

Meiosis KM 30

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During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)

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.

Meiosis creates genetic variation

Meiosis KM 32

Independent assortment

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Number of combinations: 2n

Independent assortment

e.g. if there are 2 chromosomes in haploid2n = 4; n = 22n = 22 = 4 possible combinations

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In humans

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 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!!!

Random fertilization

Gamete Formation in Animals

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).

Male gamete formation - Spermatogenesis

Gamete Formation in Animals

In females, most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg.

The other 3 meiotic cells are small “polar bodies”, which degenerate.

Female gamete formation - Oogenesis

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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.

Why Sexual Reproduction …

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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?

Review Questions

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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

Warm-up