Transcript Meiosis: Creation of Haploid Gametes

Chapter 19
Chromosomes
and Cell
Division
Lecture Presentation
Betty McGuire
Cornell University
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Chromosomes and Cell Division
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Two types of cell division
Form of chromosomes
The cell cycle
Mitosis: Creation of genetically identical
diploid body cells
 Cytokinesis
 Karyotypes
 Meiosis: Creation of haploid gametes
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Two Types of Cell Division
 The human life cycle has two types of cell
division
 Meiosis
 Mitosis
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Two Types of Cell Division
 Meiosis
 Gives rise to gametes that have half the
number of chromosomes as the original cell
 In females
 Occurs in ovaries
 Produces eggs
 In males
 Occurs in testes
 Produces sperm
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Two Types of Cell Division
 Mitosis
 Results in identical body cells
 Occurs during growth and repair
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[INSERT FIGURE 19.1 ON THIS SLIDE]
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Two Types of Cell Division
Web Activity: The Human Life Cycle
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Form of Chromosomes
 Chromosomes
 Location: nucleus
 Structure and function: each is a tightly
coiled combination of a DNA molecule and
specialized proteins called histones
 DNA contains genetic information, which
directs body development and
maintenance
 Histones help with support and control of
gene activity
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Form of Chromosomes
 Gene
 Specific segment of the DNA
 Directs synthesis of a protein, which plays
a structural or functional role in the cell
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Form of Chromosomes
 Somatic cells
 All cells except eggs and sperm
 In humans, have 46 chromosomes
 Two sets of 23 chromosomes
 One set of 23 from each parent
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Form of Chromosomes
 Each somatic cell contains two
chromosomes with genes for the same
traits
 Called homologous pairs of chromosomes
 One chromosome of the pair is from the
mother
 One chromosome of the pair is from the
father
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Form of Chromosomes
 Diploid
 A cell with two sets of chromosomes (2n)
 Genes also occur in pairs in diploid cells
 Members of each gene pair are located at
the same position on homologous
chromosomes
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Form of Chromosomes
 Of the 23 pairs of chromosomes
 Sex chromosomes make up one pair
 Determine gender
 Two types: X and Y
 XX = genetic female
 XY = genetic male
 Autosomes make up 22 pairs
 Determine expression of most of a
person’s inherited characteristics
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The Cell Cycle
 Mitosis
 The process of cell division for body cells
 When one nucleus divides into two daughter
nuclei with the same number and kinds of
chromosomes
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The Cell Cycle
 Cell cycle
 Predictable set of events that a cell goes
through from its origin to its own division into
two daughter cells
 Two major phases
 Interphase
 Cell division
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The Cell Cycle
 Interphase
 Period of growth and preparation for cell
division; not a “resting period”
 Three phases
 G1
S
 G2
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The Cell Cycle
 Interphase
 G1
 First “gap”
 Time of major growth before DNA
synthesis begins
 Chromosomes consist of a strand of DNA
and proteins
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The Cell Cycle
 Interphase
S
 DNA is replicated, and the two copies of
the chromosome, called chromatids,
remain attached at the centromere
 Two attached chromatids are
genetically identical
 Called sister chromatids
 Growth continues
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The Cell Cycle
 Interphase
 G2
 Second “gap”
 Period after DNA is synthesized and
before mitosis begins
 Growth continues
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The Cell Cycle
 Cell division consists of two processes
 Mitosis
 Division of the nucleus
 Cytokinesis
 Division of the cytoplasm
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The Cell Cycle
 Developing embryo and fetus
 Body cells divide continually
 Children
 Body cells divide in growth and repair
 Adults
 Some cells lose ability to divide (most neurons)
 Some cells stop dividing but retain ability to divide
should the need arise (liver cells)
 Some cells actively divide throughout life (skin cells)
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
 Mitosis occurs in four phases
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Prophase
Metaphase
Anaphase
Telophase
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
 Prophase
 Chromatin condenses and forms
chromosomes as DNA wraps around
histones
 Nuclear membrane begins to break down
 Mitotic spindle forms
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
 Metaphase
 Chromosomes attach to mitotic spindles and
form a line at the center of the cell
 Alignment ensures each daughter cell
receives one chromatid from each of the
46 chromosomes
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
 Anaphase
 Sister chromatids of each chromosome
begin to separate, splitting at centromere
 Now separate entities, the sister chromatids
are considered chromosomes
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
 Telophase
 Nuclear envelope forms around each group
of chromosomes at each pole
 Mitotic spindle disassembles
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Mitosis: Creation of Genetically Identical
Diploid Body Cells
Web Activity: Mitosis
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Cytokinesis
 Cytokinesis
 Division of the cytoplasm
 Begins during telophase
 Ring of microfilaments contracts at midline of
cell, eventually pinching cell in two
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Karyotypes
 Karyotype
 Constructed by arranging chromosomes
from photographs based on size and
centromere location
 Can be examined for defects in number and
structure of chromosomes
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Meiosis: Creation of Haploid Gametes
 Gametes
 Differ from somatic cells
 Haploid (n)
 Have only one member of each
homologous pair of chromosomes
 23 chromosomes
 Formed by meiosis
 Two divisions that result in up to four
haploid daughter cells
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Meiosis: Creation of Haploid Gametes
 Functions of meiosis
 Keeps the number of chromosomes in a
body cell constant from generation to
generation
 Increases genetic variability in population
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Meiosis: Creation of Haploid Gametes
 Meiosis involves two cell divisions
 Meiosis I – separates homologues
 Meiosis II – separates sister chromatids
 Each division has its own
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Prophase
Metaphase
Anaphase
Telophase
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Meiosis: Creation of Haploid Gametes
 Meiosis I
 Preceded by interphase, during which each
chromosome was copied and now consists
of two attached chromatids
 Reduction division because it produces two
haploid cells
 Each daughter cell has 23 chromosomes
 One member of each homologous pair
 Each chromosome consists of two
sister chromatids
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Meiosis: Creation of Haploid Gametes
 Prophase I
 Chromosomes condense
 Homologous chromosomes pair
 Phenomenon called synapsis
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Meiosis: Creation of Haploid Gametes
 Metaphase I
 Matched homologous pairs line up at the
midline of cell and attach to spindle fibers
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Meiosis: Creation of Haploid Gametes
 Anaphase I
 Homologous pairs of chromosomes separate
and move to opposite ends of cell
 Each homologue still consists of two sister
chromatids
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Meiosis: Creation of Haploid Gametes
 Telophase I
 Nuclear envelope forms around
chromosomes at each end of cell
 Cytokinesis occurs to form two haploid cells
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Meiosis: Creation of Haploid Gametes
 Interkinesis
 Brief interphase-like period before meiosis II
 Differs from mitotic interphase in that there is
no replication of DNA
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Meiosis: Creation of Haploid Gametes
 Meiosis II
 Prophase II
 Chromosomes condense
 Metaphase II
 Chromosomes line up along midline
of cell
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Meiosis: Creation of Haploid Gametes
 Meiosis II (cont.)
 Anaphase II
 Centromere holding two sister chromatids
separates
 Chromatids of each pair now considered
chromosomes
 Chromosomes move to opposite poles
of cell
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Meiosis: Creation of Haploid Gametes
 Meiosis II (cont.)
 Telophase II
 Nuclear membrane forms around
chromosomes at each end of cell
 Cytokinesis occurs in both daughter cells,
forming four haploid daughter cells
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Meiosis: Creation of Haploid Gametes
 The important feature of meiosis II
 It separates the two sister chromatids of
each chromosome
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Meiosis: Creation of Haploid Gametes
Web Activity: Comparing Mitosis and Meiosis
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Meiosis: Creation of Haploid Gametes
 Following meiosis, changes in shape and
function of the haploid cells result in
functional gametes
 Spermatogenesis produces four sperm cells
specialized for transporting the male’s
genetic information to the egg
 Oogenesis produces up to three polar bodies
and one ovum packed with nutrients to
nourish the early embryo
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Meiosis: Creation of Haploid Gametes
 During meiosis, genetic variation is
created through
 Crossing over
 Independent assortment
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Meiosis: Creation of Haploid Gametes
 Crossing over
 Occurs when corresponding pieces of
chromatids of maternal and paternal
homologues are exchanged during synapsis
 Prophase I
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Meiosis: Creation of Haploid Gametes
 Independent assortment
 The relative positioning of homologous
maternal and paternal chromosomes with
respect to poles of the cell is random
 Members of each homologous pair orient
independently of other pairs
 Metaphase I
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Meiosis: Creation of Haploid Gametes
 Nondisjunction
 Failure of homologous chromosomes to
separate during meiosis I or of sister
chromatids to separate during meiosis II
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Meiosis: Creation of Haploid Gametes
 Nondisjunction results in too many, or too
few, chromosomes in a cell
 The imbalance of chromosome numbers
usually causes abnormalities in development
and miscarriage
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Meiosis: Creation of Haploid Gametes
 Trisomy
 Three representatives of a chromosome in
a cell
 Monosomy
 Only one representative of a chromosome
in a cell
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Meiosis: Creation of Haploid Gametes
 Down syndrome
 An infant born with three copies of
chromosome 21
 Risk of having a baby with Down syndrome
increases with maternal age
 Multiple physical and mental abnormalities
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Meiosis: Creation of Haploid Gametes
 Nondisjunction also can occur with sex
chromosomes (X and Y)
 Nondisjunction of sex chromosomes during
sperm formation
 Resulting sperm will carry both X and Y or
no sex chromosome at all
 Nondisjunction of sex chromosomes during
egg formation
 Resulting egg will have two X
chromosomes or none at all
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Meiosis: Creation of Haploid Gametes
 Turner syndrome
 XO
 Klinefelter syndrome
 XXY
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