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Cell Reproduction
Chromosome Structure
 _DNA_ is a long, thin molecule that  During interphase, before cell division begins,
stores the information needed to direct
the chromosomes double. The two copies of
the activities of cells.
each chromosome are called _chromatids, and
are held together by a protein disk called a
 Genes are pieces of DNA that code
centromere.
information for particular activities.
Genes are transmitted from parent cell  Humans have 23_ sets of two chromosomes in
to daughter cells.
each of their body (somatic) cells. _Gametes_
(egg and sperm) have only one of each of the
 When a cell begins to divide, its DNA
23 different kinds of chromosomes.
and associated proteins coil together
into thick rod-shaped structures called  Each set of two chromosomes is called a
_chromosomes_.
homologous pair. Homologous chromosomes
are similar in shape and size, and contain genes
 Your chromosomes are about 60_%
for the same_ traits.
DNA and40% protein.
·
Chromatid
Chromatid
Centromere
Two chromatids of a doubled chromosome
Chromosomes Affect Development
 Each human chromosome holds
 Pieces of chromosomes can be missing
__________________ of genes.
(________________) or copied
_________ of these genes must be
(____________________). Chromosome
present to form a complete human.
fragments can also rejoin the chromosome
backwards (_________________) or join a
 Extra or missing chromosomes cause
non-homologous chromosome
severe problems.
(_____________________).
____________________, an extra
chromosome #21, causes Down
 Chromosomes determine your sex. If you
syndrome.
have two X chromosomes, you’re
______________, but an X and a Y make
 ________________________, either by
you a __________.
amniocentesis or by chorionic villi
sampling, can determine if a baby has
Down syndrome or other chromosomal
abnormalities.
 Although rare, other changes can
happen to the genes in chromosomes.
These changes are called
___________________.
·
Mitosis and Cell Division
 When bacteria reproduce, they simply  Eukaryotic cells undergo nuclear
split in half. This process is called
division, called mitosis_.
_binary fission_.
 The life cycle of eukaryotic cells is a
 During binary fission, the bacterium
repeating series of steps called the cell
copies its _DNA_, forms a new cell
cycle_____________.
membrane and a new cell wall.
G1 Phase - cell growth
Afterwards, the cell splits in two.
The cell carries out its normal
Cyto-
activities. This is most of the
time between cell divisions.
kinesis
Interphase
Mitosis
G2 Phase - more
growth and
preparation for
mitosis
S Phase - DNA is
copied.
Chromosomes exist
as chromatids.
·
What Happens In Mitosis?
 ________________ cells spend
 When the spindle fibers are complete,
most of their lives in the G1, S, and
each chromatid is attached to
G2 phases, which together are
_________________ poles of the cell.
called___________________.
 Next, the ________________ holding
 At the end of interphase, the cell
together the chromatids separate, and
begins to build the structures needed
the ___________________________,
to move chromosomes to opposite
pulling the chromatids apart, and
ends of the cell.
toward the poles of the cell. Each pole
of the cell will receive a complete set
 In animal cells, a pair of organelles
of chromosomes.
called __________________ move
to opposite ends of the cell.
 The last step of cell division is
formation of two new
 As the centrioles separate, protein
__________________________
strands called
around the two sets of chromosomes,
____________________ form
and a pinching inwards of the cell
between them. Other spindle fibers
membrane to split the cell in half,
extend outwards from centromeres
called ________________________.
binding
·
the______________________
Stages of Mitosis
Anaphase - Chromatids separate
Prophase - Doubled chromosomes become
visible, nuclear envelope disappears, spindle while centromeres divide, separated
fibers begin to appear.
chromatids move towards opposite poles
of the cell
 Metaphase - Chromosomes move to
the center of the cell and are aligned
along the cell’s “equator.”
·
Telophase - Chromosomes uncoil and
nuclear envelopes form while spindle
fibers disappear. Mitosis is complete.
Cytokinesis now begins.
What Happens in Meiosis?
 In organisms that reproduce sexually,  During meiosis,
the sex cells (_______________) fuse
______________________ can
together to produce the offspring or
happen. This is when the arms of
_____________. Gametes must have
paired chromosomes cross over each
________ of each type of
other and pieces of the chromosomes
chromosome, not two.
break off, then reattach to the “wrong”
chromosome in the pair. The result of
 In the sex organs of animals, gametes
cross-over is
are produced by ____________,
_______________________________
which is also called reduction
Meiosis I
__________________________.
division.
 In plants, meiosis is usually used to
produce _________________.
Meiosis II
 Meiosis consists of ________
successive nuclear divisions, but the
DNA is only copied in ________ of
them. Thus, one diploid cell
·
produces four haploid
cells.
Meiosis I
The two nuclear divisions of meiosis are divided into eight stages. Although they have
the same names as the stages of mitosis, what happens in meiosis is different.
 Prophase I - Chromosomes condense
and nuclear envelope breaks down, just like
in mitosis. But homologous chromosomes
also pair up and crossover can occur!
 Anaphase I - The homologues separate and
are pulled by spindle fibers to opposite poles,
however, chromatids do not separate. Each
chromosome still consists of two joined
chromatids. Nondisjunction can occur!.
 Metaphase I - Pairs of homologous
chromosomes (called tetrads) are moved by  Telophase I - Individual chromosomes gather
spindle fibers to the cell’s equator.
at each pole, and in most organisms, the
cytoplasm divides. Daughter cells are haploid!
·
Meiosis II
 Prophase II - As in mitosis Chromosomes  Anaphase II - Centromeres divide, and
condense, nuclear envelope disappears, and
chromatids (now called chromosomes) move
spindle apparatus appears. Chromosomes do towards opposite poles of the cell.
not double in meiosis II! Each of two
dividing daughter cells from meiosis I is
haploid.
 Metaphase II - Chromosomes are at
the cell’s equator.
·
 Telophase II - Chromosomes uncoil at cell’s
poles. Nuclear membranes and organelles
reappear. Spindle apparatus disappears. The
four daughter cells produced are haploid.