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Chapter 10.2 - Meiosis and Genetic Variation
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The Human Genome• Genome: Complete
complement of an organism’s DNA.– Includes genes
(control traits) and non-coding DNA organized into chromosomes.
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Genes• Eukaryotic DNA is organized in
chromosomes.– Genes have specific places on
chromosomes.
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Heredity
• Heredity – way of transferring genetic information to offspring
• Chromosome theory of heredity: chromosomes carry genes.
• Gene – “unit of heredity”.
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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)
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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)
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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.
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Karyotyping• Shows the
homologous pairs of chromosomes
• Identify sex of offspring
• Identify extra or missing chromosomal disorders
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• Trisomy - Extra chromosome so 3 instead of 2 • Monosomy - Missing a chromosome so 1 instead
of 2
Chromosomal disorders
Down’s Syndrome (trisomy 21)
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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
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Asexual Reproduction• single-celled organisms reproduce by
splitting, budding, parthenogenesis.
• offspring are genetically identical to parent.
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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
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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?
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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.