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Chapter 9
The Cellular Basis of
Inheritance
Reproduction
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Asexual – process in which a single cell or set
of cells produces offspring that inherit all their
genetic material from one parent
 Ex. Binary fission – bacteria
Sexual – process in which genetic material
from two parents combines and produces
offspring that differ genetically from either
parent
 Fertilization – egg fuses with sperm (zygote)
Cellular division – division of the genetic
material of a cell in which two new cells are
created
Chromosome Structure
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Nucleus – control center of cell
 Contains DNA
Chromatin – thin fibers of protein and DNA
found in a non-dividing cell
Chromosomes – Condensed, coiled,
chromatin, that forms in dividing cells
Chromatid (sister) – each half of a duplicated
chromosome
Centromere – Site of attachment for
chromatids
Chromosome Structure
Chromosome Numbers
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Each species has a characteristic number of
chromosomes
 Humans – 46, 23 pairs
 Fruit Flies – 8
 Chimps – 48
 Carrot - 18
Chromosome Numbers
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Humans
 Sex Chromosomes (2) Determine the sex
 Male XY
 Female XX
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Autosomal Chromosomes (44)
Chromosome Numbers
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Homologous
Chromosomes – One of a
pair of morphologically
similar chromosomes
(mother, father)
 Carry genes for the
same traits
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Karyotype – a technique
for examining the
chromosome makeup of
an individual
Chromosome Numbers
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Diploid (2N)
 Cells that contains 2 of each
type of chromosome
 Somatic cells – body cells
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Haploid (1N)
 Cells that contain only 1 set
of chromosomes
 Germ cells – those that
give rise to gametes
Cell Cycle
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Cell Cycle
 Sequence of events from the production of a
eukaryotic cell to the time the cell itself
reproduces
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Three Stages in the cell cycle
 Interphase
 Mitosis
 Cytokinesis
Cell Cycle
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Interphase
 Period of cell growth and development before
cell division (90% cells life)
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3 Phases of Interphase
 G1 – Time of cell growth
 S – Cells DNA is copied (replicated)
 G2 – Cell prepares for division
Cell Cycle
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Mitosis
 Cell (nuclear) division in which the number of
chromosomes is kept the same from parent cells
to daughter cells
 2N parent cell creates 2 diploid daughter cells
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4 stages of Mitosis
 Prophase
 Metaphase
 Anaphase
 Telophase
Cell Cycle
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Mitosis
 Prophase
 1. Each duplicated chromosome (consisting
of two sister chromatids) condenses from
thread-like form to rod-like form
 2. Nuclear envelope and nucleolus disappear
 3. Centrioles form and move to opposite
ends of the cell and give rise to spindle
fibers
 Spindle – football shaped framework of
microtubules
Cell Cycle
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Mitosis
 Metaphase (midway between)
 Spindle fibers (microtubules) attach to the
centromeres of the chromosomes and
align them in the center of the cell
(spindle equator)
Cell Cycle
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Mitosis
 Anaphase
 Sister chromatids of each chromosome
separate and move to opposite poles
 Each chromatid is now its own “daughter”
chromosome
Cell Cycle
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Mitosis
 Telophase
 1. Chromosomes return to threadlike form
 2. Nuclear envelopes begin to reform and
separate the 2 new sets of chromosomes
 3. Spindle fibers disassemble
 4. Nucleolus forms in each of the newly
forming cells
Cell Cycle
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Cytokinesis
 Division of the cytoplasm
 Splitting of a parent cell into daughter cells
Plants
 Cell plate – vesicles containing cell wall
building substances fuse together and
separate the two new cells
Animals
 Cleavage furrow – area of cell membrane that
pinches inward until 2 new cells are formed
Mitosis Video Clip
Meiosis
The Formation of Gametes
Meiosis
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Meiosis
 Cell (nuclear) division in which the
chromosome number is cut in half
 2N cell divides to form 4 (1N) cells
Two cell divisions – 2 stages
 Meiosis I
 Prophase I, Metaphase I, Anaphase I,
Telophase I
 Meiosis II
 Prophase II, Metaphase II, Anaphase II,
Telophase II
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Meiosis
Prophase I
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1. Chromosomes condense to rod-like form
2. Nucleolus and nuclear membrane disappear
3. Centrosomes form and move to poles, give
rise to spindle fibers
4. Homologous chromosomes pair up creating a
tetrad (4 chromatids)
 Crossing over – part of chromatid may break
off and attach to adjacent chromatid on the
homologous chromosome
 Genetic recombination – regrouping of genes
due to an exchange of maternal and paternal
genetic material
Meiosis
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Metaphase I
 Spindle fibers attach to the centromeres, and
homologous pairs line up side by side
(tetrad) on the spindle equator
Meiosis
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Anaphase I
 Homologous chromosomes are separated
and moved to opposite poles
Meiosis
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Telophase I
 Cytokinesis occurs and produces 2 haploid
(1N) cells
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Each new cell contains one chromosome
from each homologous pair
Meiosis
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Prophase II
 Spindle fibers form and attach themselves to
the centromeres of the chromosomes
Meiosis
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Metaphase II
 Chromosomes are moved to the equator of
the cell
Meiosis
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Anaphase II
 Chromatids separate and move toward
opposite poles of the cell
Meiosis
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Telophase II
 4 daughter nuclei
 Each cell has a haploid chromosome number
and all are in the unduplicated state
 Gametes (sperm, egg)
Meiosis Video Clips
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