How Cells Reproduce

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How Cells Reproduce

• Chapter 7

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DNA Terminology
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• Chromatin DNA not condensed into chromosmes.

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• Chromosomes DNA condensed into rod like
  structures

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one chromosome (unduplicated)
one chromosome (duplicated)
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centromere
one chromatid
its sister chromatid
One chromosome in the duplicated state
Chromatid one half of a chromosome with
duplicated DNA Centromere connection between
sister chromatids of a chromosome
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The Cell Cycle (3 parts)
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interphase
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cytokenesis
cytokinesis
G1
S
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telophase
mitosis
anaphase
G2
metaphase
prophase
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Interphase (3 parts)

• G1 gap phase of cell growth before
  chromosomes duplicate
• S DNA duplication occurs
• G2 Cell prepares to divide

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Mitosis (4 stages)

• Prophase
• Metaphase
• Anaphase
• Telophase
• Mitosis is division of the nucleus

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Roles of Mitosis

• Multicelled organisms
• Growth
• Cell replacement
• Some protistans, fungi, plants, animals
• Asexual reproduction

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Early Prophase Mitosis Starts

• Duplicated DNA (chromatin) begins to condense
  into chromosomes

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Late Prophase

• New microtubules are assembled
• One centriole pair is moved toward opposite pole
  of spindle
• Nuclear envelope starts to break up

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Transition to Metaphase

• Spindle forms
• Spindle microtubules become attached to the two
  sister chromatids of each chromosome

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Metaphase

• All chromosomes are lined up at the spindle
  equator
• Chromosomes are maximally condensed

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Anaphase

• Sister chromatids of each chromosome are pulled
  apart
• Microtubules of the spindle shorten moving
  chromosomes to opposite poles of the cell

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Telophase

• Chromosomes return to chromatin
• Two nuclear membranes form, one around each set
  of unduplicated DNA

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Results of Mitosis

• Two daughter nuclei
• Each with same chromosome number as parent cell
• Chromosomes are in unduplicated form

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Cytoplasmic Division

• Usually occurs between late anaphase and end of
  telophase
• Two mechanisms
• Cell plate formation (plants)
• Cleavage (animals)

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Cytokinesis in Plant Cells
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Cytokinesis in Animal Cells
Stepped Art
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Mitosis Maintains Chromosome Number
chromosomes (unduplicated) in parent cell at
interphase
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Chromosome Number

• Sum total of chromosomes in a cell
• Somatic cells (all but sperm and egg)
• Chromosome number is diploid (2n)
• Two of each type of chromosome
• Gametes (sperm and egg)
• Chromosome number is haploid (n)
• One of each chromosome type

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Human Chromosome Number

• Diploid chromosome number (2n) 46
• Two sets of 23 chromosomes each
• One set from father
• One set from mother
• Mitosis produces diploid cells from diploid cells

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Homologous Chromosomes

• Pairs of chromosomes carrying genetic information
  for the same traits
• One chromosome in each pair from mother, other
  from father
• Paternal and maternal chromosomes carry different
  forms of a trait
• Diploid cells contain homologous pairs

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Karyotype of a Human Female
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?????

• How are cells with the haploid number of
  chromosomes produced?

Meiosis
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Meiosis Two Divisions

• Meiosis cuts the number of chromosomes in half
• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form

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Prophase I

• Each duplicated chromosome pairs with homologue
• Homologues swap segments
• Each chromosome becomes attached to spindle

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Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• The spindle is fully formed

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Anaphase I

• Homologous chromosomes segregate
• The sister chromatids remain attached

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Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

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Prophase II

• Microtubules attach to the duplicated chromosomes

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Metaphase II

• Duplicated chromosomes line up at the spindle
  equator, midway between the poles

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Anaphase II

• Sister chromatids separate to become independent
  chromosomes

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Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• Four haploid cells

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Mitosis
Meiosis I
Meiosis II
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Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs insomatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

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How Meiosis Puts Variation in Traits
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Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Chromatids of different chromosomes exchange
  segments

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Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Creates new allele combinations in offspring

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Random Alignment

• During transition between prophase I and
  metaphase I, chromosomes attach to microtubules
  of the spindle
• Initial contacts between microtubules and
  chromosomes are random producing different
  combinations of chromosomes in gametes

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Possible ChromosomeCombinations
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2
3
or
or
or
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Fertilization Also Creates Variation

• Male and female gametes uniteand nuclei fuse
• Fusion of two haploid nuclei produces diploid
  nucleus in the zygote
• Which of the two gametes unite is random
• Adds to variation among offspring

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Factors Contributing to Variation among Offspring

• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

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The Cell Cycle and Cancer
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Cancer Characteristics

• Cells grow and divide abnormally
• Plasma membrane and cytoplasm altered
• Weakened capacity for adhesion cells can move to
  new tissues
• Cells loose their specilization

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The End