Chapter 10.2 - Meiosis and Genetic Variation

The Human Genome• Genome: Complete

complement of an organism’s DNA.– Includes genes

(control traits) and non-coding DNA organized into chromosomes.

Genes• Eukaryotic DNA is organized in

chromosomes.– Genes have specific places on

chromosomes.

Heredity

• Heredity – way of transferring genetic information to offspring

• Chromosome theory of heredity: chromosomes carry genes.

• Gene – “unit of heredity”.

All sexually reproducing organisms have two types of cells in their bodies

• Diploid cells– Normal body cells – Produced during

mitosis– 2 sets of

chromosomes paired together (2n)

• Haploid Cells– Sex cells (gametes)– Produced during

meiosis– Only 1 set of

chromosomes (n)

Diploid cellsA normal diploid human body cell has 46

chromosomes paired together (23 pairs)• The paired chromosomes match each other

in gene type and location• They are called homologous pairs

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Problem-solving Lab 10.2 on page 264.

• Answer in your notes

• 1)

• 2)

• 3)

• 4)

Looking at Chromosomes in a cell• Karyotype:

– ordered display of an individual’s chromosomes

– Chromosomes are stained to reveal visible band patterns and major abnormalities.

Karyotyping• Shows the

homologous pairs of chromosomes

• Identify sex of offspring

• Identify extra or missing chromosomal disorders

• Trisomy - Extra chromosome so 3 instead of 2 • Monosomy - Missing a chromosome so 1 instead

of 2

Chromosomal disorders

Down’s Syndrome (trisomy 21)

Genes are passed on through Reproduction

• Asexual

– Without sex cells

– Produces identical copies of parent (clones)

– Mitosis is asexual reproduction

• Sexual– With sex cells – Produces genetically

variable offspring• This is good for

survival!– Meiosis is sexual

reproduction

Asexual Reproduction• single-celled organisms reproduce by

splitting, budding, parthenogenesis.

• offspring are genetically identical to parent.

Sexual reproduction

• Important vocabulary– Gametes = sex cells (egg and sperm) that are

haploid– Haploid = one set of chromosomes– Diploid = two sets of chromosomes – Zygote = fertilized egg

• Fusion of two gametes to produce a single fertilized egg (zygote).

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Meiosis (sexual) vs. Mitosis (asexual)

• Meiosis reduces the number of chromosomes by half.– Produces 4 haploid

cells• Daughter cells differ

from parent cell • Meiosis involves two

divisions

• Mitosis keeps the same number of chromosomes– Produces 2 diploid

cells• Daughter cells

identical to parent (clones)

• Mitosis involves only one division

Sexual Reproduction is Important!

• Because a zygote has genes from two different parents…..– Introduces greater genetic variation for a

species

– Allows for genetic recombination

– Increase species survival!

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Meiosis I - First division of meiosis

• Interphase - all chromosomes replicate (just like in mitosis)

• Prophase 1: Homologous chromosomes begin to pair up. Crossing-over can occur during the latter part of this stage.

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Crossing over - occurs during Prophase I of meiosis

Genetic recombination that is an exchange of genetic material between homologous chromosomes

Crossing over produces recombinant chromosomes and increases genetic variation!

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Harlequin chromosomes - crossing over

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Meiosis I continued

• Metaphase 1: Homologous chromosomes align at the equatorial plate. (in pairs next to each other)

• 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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Meiosis IISecond division of meiosis: Gamete formation

• Prophase 2: DNA does not replicate again.

• Metaphase 2: Chromosomes align at the equatorial plate.

• Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.

• Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

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Meiosis creates genetic variation

• 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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Independent assortment - chromosomes separate randomly during meiosis

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In humans

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

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Random fertilizationAt least 8 million combinations from Mom,

and another 8 million from Dad …

>64 trillion combinations for a diploid zygote!!!

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Meiosis & sexual life cycles• Life cycle = sequence

of stages in organisms reproductive history; conception to reproduction.

• Somatic cells = any cell other than gametes, most of the cells in the body.

• Gametes produced by meiosis.

Generalized animal life cycle

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Sex is costly!• Large amounts of energy required to find a

mate and do the mating: specialized structures and behavior required

• Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.)

• Genetic costs: in sex, we pass on only half of genes to offspring.

• Males are an expensive luxury - in most species they contribute little to rearing offspring.

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But …• 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.

– 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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Study Questions• 1. What happens as homologous

chromosomes pair up during prophase I of meiosis?

• 2. How does metaphase of mitosis differ from metaphase 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?

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5. Compare mitosis and meiosis on the following points:

• a. number of daughter cells produced.

• b. the amount of DNA in the daughter cells in contrast to the original cell.

• c. mechanism for introducing genetic variation.

• 6.  What is a zygote and how is it formed?

• 7. What is the main advantage of sexual reproduction?

Meiosis – key differences from mitosis

• Meiosis reduces the number of chromosomes by half (haploid cells).

• Daughter cells differ from parent cell and each other.

• Meiosis involves two divisions, Mitosis only one.

• Meiosis produces 4 haploid cells, Mitosis produces 2 diploid cells.