copyright © 2012 pearson education, inc. chapter 19 chromosomes and cell division betty mcguire...
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Chapter 19
Chromosomes and Cell Division
Betty McGuireCornell University
Lecture Presentation
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Chromosomes and Cell Division
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 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 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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