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Structure of DNA

• Double Helix• Bases- Cytosine, Thymine, Adenine,

Guanine• T-A ; C-G• Each strand is complementary to the other-

Allow for precise copying of the DNA• DNA also has many proteins associated- All

together = Chromatin

Fig. 9.5 Nitrogenous Bases

Fig. 9.7 The Double Helix

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DNA DNA Xerox Machine: DNA Polymerase

DNA Polymerase

Cellular DivisionThree main functions of cell divisions

1. Reproduction2. Growth and development3. Tissue renewal

ProkaryoticChromosomes and Cell Division

• Prokaryotic Genomes– Bacterial

Chromosomes- Single strands of DNA

– Reproduction- Binary Fission (asexual)

– Simple compared to Eukaryotic cell division

EukaryoticChromosomes and Cell Division• Eukaryotic Genomes

– Organization of Eukaryotic Genomes: Complexity!• Genomic size- each human cell has 3 meters

of DNA!!• Chromosomal structure- Fold that 3 m into

something manageable– Each Chromosome is one long strand of DNA

• Biparental inheritance - diploidy

Why is cell division in eukaryotes more complicated than binary fission in

prokaryotes?

• Nuclear and chromosomal packaging– Histone proteins and supercoiling– Chromatin, chromosomes and chromatids

• Organismal and cellular complexity– Genome size– Biparental inheritance and diploidy

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From chromatin to chromosome: DNA Structure

Human female chromosomes shown by bright field G-banding Human female karyotype shown by bright field G-banding of chromosomes

In complex organisms chromosomes exist in pairs:

Chromosome duplication and distribution during mitosis

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Chromosomes and Cell Division

• Mitosis and the Cell Cycle– Prophase, Prometaphase, Metaphase,

Anaphase, Telophase – Interphase: Gap 1, S, Gap 2

The Cell Cycle The Cell Cycle

• Cell Cycle– Interphase- 95%+ of most cells life

• Gap 1, Synthesis, Gap 2, and

– Mitotic Phase• What is Mitosis?

Mitosis– The Process of cell division that results in the

production of 2 daughter cells that are genetically identical to each other and to the parent cell from which they arose.

– Mitosis is for cell growth, development and repair

• (and reproduction in single cell organisms)

– Occurs in Somatic Tissues

Chromatin duplicates, then condenses into chromosome

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Parts of the Replicating Chromosome

•Homologous chromosomes from parents = similar

•Sister chromatids are copies of each other = identical

Phases of Mitosis

Interphase Prophase Metaphase

Anaphase Telophase

Summary of Mitosis

• Homologous chromosomes duplicate

• Sister chromatids split• Each of the two new

cells

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Cytokinesis- Animals Cytokinesis- Plants

Cytokinesis

Mitosis Uncontrolled: Cancer Genetic Variation

• Lets look around the class at our variation in Phenotype (how our genes are expressed on the outside)

• Lots of variation exists…why?– Helps with survival-

• Hot Gene and Cold Gene…

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Genetic Variation

• So variation is good for a population, how do we get it?

• Genetic Variation in Bacteria– Antibiotic resistance from random mutations

Genetic Variation

• Antibiotic Resistance• 1 in a million genes per generation will mutate• E. coli reproduces in 20 minutes (Generation Time)• 1…2…4…8…16…32…64…………

7 hours = 1,000,000 cells!!!• If one of these cells mutated so it was resistant, how

many resistant cells in 7 more hours?

Genetic Variation

• So, for organisms with very short generation times, depending on mutations for genetic variation can be OK

• What is your generation time?• Can’t depend on mutations for

variation…too slow• Why is there Sex?

– Provides a way to mix genes and get variation

Meiosis

• The process of cell division that results in the production of 4 haploid gametes that are genetically different from one another and from the parents.

• In Mitosis we made exact copies of the parent cells, with two sets of chromosomes

• In Meiosis the result is different cells with only one set of chromosomes

Meiosis

• Gametogenesis- formation of the gametes– Sperm= Spermatogenesis

• 1 cell = 4 sperm

– Egg= Oogenesis• 1 cell = 1 egg +3 polar bodies

– Need to get Chromosome number in half!

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Gametic Cells: Haploid Meiosis- Overview

Meiosis I Meiosis II

How do we get our Genetic Variation?

• Random Assortment– We have a set of chromosomes from mom and a set from

dad, which of these gets into the gametes is random– Four chromosome example- 22=4 combos– Humans have 23 chromosomes-

223= over 8 million different combos from each parent. 223 x 223 =?

– Over 64 trillion different combinations of your parents genes!!

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Random AssortmentHow do we get our Genetic Variation?

• Crossing Over– The exchange of corresponding segments

between two homologous chromosomes.– Scrambles up the genes– Happens in Prophase I of Meiosis– Figure 7.18- example with one chromosome

Crossing Over When Meiosis Has Errors

• Extra Chromosomes– Down Syndrome

• 3 copies of Chromosome 21

• 1 in 700 births

Extra Chromosomes Problems with Sex Chromosomes

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When Meiosis Has Errors

• Alteration of Chromosome Structure– Four main types of structural

changes– Deletion, Duplication,

Inversion, and Translocation

Reciprocal Translocation: Cancer connection

Errors are a source of Variation

• Most errors are “bad”, but some can be good.

• New species can be developed from extra chromosome numbers (polyploidy).

• Beneficial changes will persist, damaging ones will die out.