meiosis & sexual life cycle chapter 13. slide 2 of 27 definitions genetics – scientific study...
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Slide 2 of 27
Definitions
Genetics – scientific study of heredity and hereditary variation Heredity – transmission of traits from one generation to the
next one
Genes – Hereditary units that code for proteins
Gametes – Actual mechanism for hereditary transmission
Fertilization – Combining gametes
Locus – gene’s location on a chromosome
Slide 3 of 27
Asexual Reproduction
Single Parent
Reproduction occurs by mitosis, binary fission, budding, etc.
Offspring is exact copy (genetically) Can be called a clone or a “Mini-me
Can get genetic variation, but rarely Due to mutations
Common among unicellular organisms, but also found in multicellular organisms as well Budding
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Sexual reproduction
It takes 2 to tango = 2 parents
Unique combination of genes
Vary genetically from both parents and their siblings
May exhibit similarities to parents
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Homologous Chromosomes
2 chromosomes that have same length, centromere position, and staining pattern
Autosomes Non-sex chromosomes Chromosomes that do not determine gender
Sex Chromosomes Chromosomes that determine gender
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Chromosomes
Human somatic cell = 44 autosomes + 2 sex chromosomes
Human gamete = 22 autosomes + 1 sex chromosome
Sex Chromosomes Can be XX or XY XX = Homologous chromosomes XY = Not homologous chromosomes
Egg must contain X, sperm may contain X & Y Hence, males determine the gender of offspring
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Karyotype
Ordered display of chromosomes used to distinguish the number and size of homologous chromosomes
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Meiosis
2 Stages of Meiosis Meiosis I & Meiosis II
Much of Meiosis resembles Mitosis
Chromosomes are replicated only once Before Meiosis I
4 daughter cells are produced
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Meiosis: An Overview
Assume that an organism has:1 Homologous Pair = 2 Chromosomes(Diploid cell – 2n)
STEP 1: Each of the chromosomes is replicated in Interphase
STEP 2: Chromosome pairs of copies separate in Meiosis I(Haploid cell – n)BUT 2 copies of each one
STEP 3: Each of the copies (sister chromatids) in a cell separates creating 4 haploid cells(Haploid cell with only 1 copy)
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Repeat the Diagram but with 4 Chromosomes
The cell before interphase has 4 chromosomes and is diploid
Indicate how many chromosomes are present:
1. After interphase but before Meiosis
2. After Meiosis I
3. After Meiosis II
Slide 13 of 27
Questions?
If a cell has 10 chromosomes and is diploid, how many chromosomes (include what the book calls chromatids) are found at:
A) the end of Meoisis I
B) the end of Meiosis II
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Meiosis I
Prophase I Longest phase Homologous pairs align Crossing-Over may occur Synapsis – pairing of homologous pairs tied tightly
together Tetrads form (4 chromosomes = 2 pairs)
Each tetrad has 1 or more chiasmata Criss-crossed regions where crossing over has occurred
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Meiosis I (Page 2)
Metaphase I Tetrads are aligned at the metaphase plate Each chromosome pair faces a pole
Anaphase I Homologous chromosomes (composed of 2 copies of
each chromosome called chromatids) are pulled apart
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What is different between Meiosis I & II?
This division is sometimes called the Mitotic division, why?
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Meiosis vs. Mitosis
Tetrads align in Prophase I, Chromosomes align in Prophase mitosis
Chromosomes position @ metaphase plate (Mitosis) Tetrads position @ metaphase plate (Meiosis)
Homologues separate in Meiosis I
Sister chromatids separate in Meiosis II & Mitosis
Crossing over = Meiosis NOT mitosis
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Mitosis Meiosis
DNA replicates in interphase
1 division
No synapsis
2 Diploid cellsGenetically identical cells
Responsible for:-- Zygote growth into multicellular organism
DNA only replicates in Pre-meiotic interphase
2 divisions
Synapsis occurs during prophase I forming tetrads
Crossing over occurs now
4 haploid cellsGenetically different cells
Responsible for:-- Gamete production-- Genetic variation
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Genetic Diversity
The reason for meiosis + sexual reproduction
Mutations are the original source of genetic diversity
3 main sources of Genetic Diversity
1. Independent Assortment of Chromosomes
2. Crossing Over
3. Random Fertilization
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Each daughter cell has a 50% chance of getting maternal chromosome (or its copy)
Similarly, 50% chance of getting paternal chromosome (or its copy)
Independent assortment - each chromosome is positioned independently of the other chromosomes
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When homologous pairs are formed in Prophase I, a recombinant chromosome can be formed -- A chromosome that has DNA from 2 different parents
2 chromosome segments trade places (cross over) producing chromosomes with new combos of maternal & paternal genes
1-3 times per chromosome in humans
Increases genetic variation