Cellular Basis of Inheritance

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Chapter 9

• Cellular Basis of Inheritance

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All Cells come from Cells

• Repair and Growth
• New Skin cells will replace dead or damaged cells
• Reproduction
• Asexual Organisms make new organisms through
  simple cell division. Requires only one parent
• Sexual involves 2 parents in the production of
  offspring through the union of sex cells

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Chromosomes and Cell Division

• Genes of a eukaryotic cell--in the nucleus.
• Chromatin a combination of DNA and protein
• Chromosomes chromatin condenses into visible
  structures for cell division

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• The cell will duplicate its chromosomes
• Each chromosome now consist of 2 identical joined
  copies called sister chromatids
• The region where the two meet is called the
  centromere. It will serve as a sort of dividing
  line.

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Why Cells Must Divide

• In multi-celled organisms (like humans) cells
  specialize for specific functions thus the
  original cells must divide to produce different
  kinds of cells
• Cells can only take in nutrients and excrete
  waste products over the surface of the membrane
  that surrounds them. The surface to volume ratio
  decreases with the square of the volume

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The Cell Cycle

• Interphase
• Carries out its metabolic processes here
• Spends 90 time to perform functions
• Before and after the S phase, the interphase
  periods are called G1 and G2
• Mitotic Phase
• When the cell is actually dividing.
• Includes mitosis and cytokinesis

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The Cell Cycle

• The cell cycle is well defined and can be divided
  into four stages
• Gap 1 (G1) - The growth phase in which most cells
  are found most of the time
• Synthesis (S) - During which new DNA is
  synthesized
• Gap 2 (G2) - The period during which no
  transcription or translation occurs and final
  preparations for division are made
• Mitosis - Cell division

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From Birth to Rebirth, a Cell Progresses Through
Characteristic Stages That Constitute the Cell
Cycle
In multicellular organisms like us, progress
through the cell cycle is carefully regulated.
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The Plant Cell Wall Forces Cytokinesis to Play by
Different Rules
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DNA is Packaged into Chromosomes
DNA in the cell is virtually always associated
with proteins.
The packaging is impressive 2 meters of human
DNA fit into a sphere about 0.000005 meters in
diameter.
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9.5 Meiosis and Homologous Chromosomes

• Meiosis is a type of cell division that results
  in 4 daughter cells
• The role of chromosomes are extremely important
  in passing info. from parent to offspring.
• Homologous chromosomes are 2 chromosomes of a
  matching pair
• A karyotype is a display of your 46 chromosomes.
• You inherit one chromosome of each pair from your
  mom and dad

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A Karyotype is an Informative, Arranged Picture
of Chromosomes At Their Most Condensed State
Note that almost all chromosomes come in
homologous pairs.
Boy or girl?
A normal human karyotype
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Homologous chromosomes contd

• Each homologous chromosome carries the same
  sequence of genes for characteristics
• Humans have 23 homologous pairs of chromosomes
• The 23rd pair of chromosomes are the sex
  chromosomes
• XX is for female
• XY is for male
• Note that most genes are carried on the X
  chromosome

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Diploid Vs. Haploid Cells

• Diploid contains 2 homologous sets of
  chromosomes
• 46 chromosomes in a human is the diploid
• Haploid contains a single set of chromosomes
• The haploid is 23 which are processed through
  meiosis
• Fertilization is the fusion of gametes to form a
  zygote.
• The zygote is diploid

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Meiosis

• Consists of 2 divisions which 4 hap. cells
• Meiosis I
• Homologous chromosomes are separated
• Prophase I, Metaphase I, Anaphase I, Telophase I
  and Cytokinesis
• Meiosis II
• Sister chromatids are separated like mitosis,
  except that the cells are haploid
• Prophase II, Metaphase II, Anaphase II,
  Telophase II and Cytokinesis

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The Mechanics of Meiosis Following a Single
Chromosome Pair

• Meiosis produces haploid daughter cells from a
  diploid cell
• Each daughter cell will contain one set of
  chromosomes from the parents
• It has 2 meiotic divisions
• There is an exchange of genetic material between
  homologous chromosome

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The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis
The first (and distinguishing) division of meiosis
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Meiosis increases Genetic Variation

• Assortment of Chromosomes
• The chromosomes will line up in the cell and
  separate at metaphase I
• The assortment of these chromosomes happens
  randomly
• If you know the haploid number, you can calculate
  the possible number of combinations which are
  equal to 2N where n is the haploid number

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Mitosis Vs. Meiosis
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One Way Meiosis Makes Lots of Different Sex Cells
(Gametes) Independent Assortment
Independent assortment produces 2n distinct
gametes, where n the number of unique
chromosomes.
In humans, n 23 and 223 6,000,0000.
Thats a lot of diversity by this mechanism alone.
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Mitosis vs. Meiosis
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Cell division requires coordinated division of
chromosomes (mitosis) ..
and division of the cytoplasm (cytokinesis).
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Crossing Over

• The number of different chromosome combinations
  in gametes is one way to increase genetic
  variation
• The second factor is crossing over which involves
  the exchange of genetic material between
  homologous chromosomes
• Segments of 2 chromatids can be exchanged at one
  or more sites.
• A single chromosome that contains a new
  combination of genetic information from
  differenet parents, is called genetic
  recombination

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Crossing over
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Cell Division Demands Coordination of DNA
Replication, Mitosis and Cytokinesis
Whats so important about cell division?
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The Link Between DNA Replication and Chromosome
Duplication