Cellular Reproduction

1
Cellular Reproduction
2
Why Cells Divide

• Cell size is limited
• Cells cannot keep growing indefinitely
• Replacement of damaged cells
• Growth - embryo
• Asexual reproduction
• Single celled life forms
• Plant cuttings
• Sexual reproduction - meiosis

3
Cell Cycle and Mitosis

• Before a cell divides it must copy DNA
• Genes are found on chromosomes
• Chroma - colored body
• Soma - body
• First noticed when staining cells
• To look at under microscope
• Only visible sometimes
• Chromatin - combo of DNA protein

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Fig. 7.5
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Eukaryotic chromosomes

• Chr. Become visible before division
• Long chromatin strands coil
• How many chromosomes do humans have?
• 46 or 23 pairs
• Each new cell must have a complete set of chr.
• Chr. Must be copied

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One chromosome

• Unduplicated (unreplicated)
• Replicated chromosome
• Consists of 2 sister chromatids
• Exact copies of each other
• Connected by a centromere
• Cell division separates chromatids
• Each new cell gets one copy of each chr

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Fig. 7.6
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Cell cycle

• Interphase 90 of dividing cells
• Makes proteins
• Grows
• Copies organelles
• Also copies chromosomes
• G1 phase, S phase, G2 phase
• S- synthesis of DNA
• Sister chromatids form
• Mitotic phase
• Mitosis - nuclear division
• Cytokinesis - division of cytoplasm

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Fig. 7.7A
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Mitosis

• Prophase
• Chromosomes coil -can see them now
• Mitotic spindle forms
• Microtubules radiate out from centrioles
• Cellular poles form
• Nuclear membrane breaks down
• Microtubules capture chr at centromere
• Each chr now connected to both poles
• Microtubules begin moving chr

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Mitosis

• Metaphase (middle)
• Chromosomes lined up at equator
• Microtubules attach chromatids to opposite poles
• Anaphase (apart)
• Sister chromatids separate
• Chromatids now chromosome
• Microtublues pull chr to poles
• Other microtubules elongate cell

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• Teleophase reverses prophase events
• Begins when chr reach poles
• Nuclear envelopes form
• Chromosomes stretch out
• Nucleoli form
• Spindle breaks down
• Cytokinesis
• Division of cellular contents
• Cytoplasm
• Organelles
• Animal cells form cleavage furrow
• Plants form cell plate

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Fig. 7.7B
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Fig. 7.8a
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Fig. 7.8b
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Cancer

• Cell cycle control system
• Enzymes in cell control when and where cells
  divide
• Malfunction in system means cells divide at
  inappropriate times and places
• Benign tumor - abnormal mass of essentially
  normal cells
• Stay at original site, dont move
• Cancer uncontrolled cell division

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Fig. 7.9
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Cancer

• Problem not only uncontrolled division
• Metastisis
• Cancer cells can move to other sites
• New tumor at that site
• Three treatments
• Surgery to remove tumor
• Radiation
• Chemotherapy
• Last two aimed at controlling division

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Cancer treatments

• Radiation
• Disrupts cell division
• Most actively dividing cells are tumor
• Can damage normal cells- ovaries / testes
• Chemotherapy
• Some disrupt cell division
• Taxol freezes spindle
• Vinblastine prevents spindle formation
• Cancer cells immortal in cell culture
• Normal cells stop growing after 50 cultures

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Fig. 7.12
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Homologous chromosomes

• Chr come in pairs homologous chr
• Humans have 23 pairs in somatic cells
• One set from each parent
• Each pair has same genes at same location
• Must separate pairs to make gametes
• Each gamete (sex cell) gets one set of chr
• Process called meiosis
• Chr are numbered according to size
• 1 - largest, 22nd - smallest autosomes
• 23rd pair - sex chromosomes

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Fig. 7.13
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Meiosis

• Most cells have two sets of chr
• This is called diploid (2 sets of 23)
• 2n diploid , humans 2n 46
• Gametes have one set of chr
• This is called haploid cell (1 set of 23)
• n haploid , humans n 23
• Sperm, egg, pollen, ovule are examples
• How chr separate to make haploid cells is meiosis
• Haploid haploid diploid /fertilization

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Fig. 7.14
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Meiosis

• Meiosis has two divisions
• First division separates the pairs
• Each new cell has 1 set of chr. - haploid
• 2nd division separates the chromatids
• Process is like mitosis
• Except for the number of chromosomes
• Each division has same 4 steps
• Prophase, metaphase, anaphase, telophase

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Fig. 7.15A
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Meiosis I

• Prophase I
• Chr condense
• Homologous chr pair up - aka tetrad
• Tetrad - 4 chromatids
• Gene for gene alignment of chr
• Crossing over occurs
• Non sister chromatids exchange pieces
• Nuclear membrane breaks down
• Spindles form
• Capture pairs
• Each homologue attached to opposite pole

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Meiosis I continued

• Metaphase I
• Homologous pairs line up at equator
• Pushed/pulled into place by microtubules
• Anaphase I
• Homologous pairs separate
• Move toward opposite poles
• Sister chromatids still together
• Telophase I
• Chr arrive at poles
• New nuclear membrane forms
• Spindles break down
• Cytokinesis - division of cytoplasm/organelles

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Fig. 7.15B
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Meiosis II

• Prophase II
• Chr condense
• Spindles form
• Nuclear membrane breaks down
• Microtubules capture chr at centromere
• Metaphase II
• Chr line up at equator
• Anaphase II
• Sister chromatids separate
• Move toward opposite poles

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Meoisis II continued

• Telophase II
• Chr arrive at poles
• New nuclear membrane forms
• Spindles break down
• Cytokinesis
• Cytoplasm divides
• Includes cytosol, and organelles

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Fig. 7.16
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Mitosis vs Meiosis

• Mitosis
• 1 division
• 2 daughter cells
• Exact copies of parent cells
• Diploid to diploid
• Purpose
• Growth
• Repair
• Asexual reproduction

• Meiosis
• 2 divisions
• 1st separates pairs
• 4 daughter cells
• Each unique
• Diploid to haploid
• Purpose
• Make gametes/ sex cells
• Leads to genetic variation

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Fig. 7.17
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Genetic variation

• Sexual reproduction results in variation
• Natural selection acts on variation
• Meiosis promotes variation
• 1. Independent assortment
• Only one chr of each homologous pair goes into
  gametes
• Which one goes into gametes is random
• Could be one from your father or from mother
• Depends on how they line up at metaphase I

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Genetic variation cont.

• Two pairs can produce 4 different gametes
• Possible combinations 2n
• n 23 in humans
• 223 over 8.3 million possible combinations!
• 2. Random fertilization
• Which sperm fertilizes which egg is random
• One man one woman have over 64 trillion
  possible offspring!

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Fig. 7.18
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Genetic variaiton cont.

• 3. Crossing over
• Prophase I of meiosis
• Homologous pairs synapse
• Gene for gene alignment
• Non sister chromatids exchange parts
• Recombines chr from your mom and dad
• Recombinant chr contains some genes from mom and
  some from dad
• Its amazing that siblings look as much alike as
  they do

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Fig. 7.20
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Errors in meiosis

• Trisomy 21 -aka Down syndrome
• Most meiosis errors do not result in a viable
  pregnancy - miscarriage
• Most common serious birth defect
• 1 in 700 births
• Varying degrees of mental retardation
• 1/2 eggs of female will carry extra 21
• 1/2 will be normal
• Risk increases with age of mother

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Causes of errors in meiosis

• Spindles are highly accurate at separating pairs
  into different cells
• Nondisjucntion
• If chr/chromatids are connected to same pole
• Can happen in either meiosis I or II
• Some gametes will have extra chr
• Some gemetes will have missing chr
• Meiosis in woman begins before birth
• Completed after fertilization!
• The longer in meiosis the greater the risk

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Fig. 21
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Detection of errors

• Karyotype - picture of chromosomes arrested
  during mitosis
• From a karyotype can determine
• of chromosomes
• Sex of individual
• Extra or missing pieces of chromosomes
• First step in genetic testing
• Becoming more and more common

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Fig. 7.22
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Fertilization after nondisjunction

• Nondisjunction results in gamete with extra chr
• If other gamete is normal zygote will have 2n 1
• Most of the time an extra chr prevents
  development from occurring

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Table 7.1
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Sex chr disorders

• Klinefelter syndrome
• XXY - male
• Could be from nondisjunction in either parent
• XYY - nondisjunction in male
• XXX - metafemale
• XO - only instance of development missing chr
• Multiple extra chr results in more problems -
  mental retardation

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Fig. 7.23
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Problems with crossing over

• Occur when gene for gene alignment is off
• Deletion - cystic fibrosis, cri du chat
• Missing piece of chr
• Only one copy of some genes
• Duplication - Huntingtons
• Extra piece of chr
• May have 3 copies of some genes
• Inversion - least damage
• Every gene still there
• Translocation
• Crossing over between non homologous chr
• One cause of Down syndrome

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Fig. 7.24
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Somatic cell problems

• Philadelphia chromosome
• Reciprocal translocation
• Pieces of 9 and 22 trade places
• Turns on a cancer causing gene
• Problem in mitosis
• Crossing over not typical in somatic cells
• CML chronic myelogenous leukemia
• In bone marrow cells

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Fig. 7.25