Transcript chromosomes - cloudfront.net

Chromosomes
and Cell
Reproduction
Chapter 6
Section 6-1
chromosomes
Bacterial cells divide to
reproduce
 Bacteria
reproduce by binary fission
 Binary fission is a form of asexual
reproduction that produces identical
offspring
 There are two stages:


DNA is copied
Bacteria divides
Eukaryotic cells form chromosomes
 DNA
is organized into individual units called genes
 A gene is a segment of DNA that codes for a
protein or RNA
 As a cell preps to divide, DNA is copied and coils up
into chromosomes
 The two exact copies of DNA that make up a
chromosome are called chromatids
 The chromatids are attached at a point called a
centromere
Chromosome Number and
Structure Affect Development
 Each
human somatic cell (non-sex cell)
has 2 copies of 23 chromosomes
 Each of the 23 pairs of chromosomes
consist of 2 homologous chromosomes
 Homologous chromosomes have a similar
size, shape, and genetic content
 Each homologue comes from each
parent
Chromosomes
A
somatic cell is diploid, because it contains 2 sets
of chromosomes
 A gamete, which has one set of chromosomes, is
said to be haploid
 The fusion of 2 haploid cells is called fertilization and
results in a zygote or a fertilized egg
Chromosomes determine sex
are chromosomes that aren’t
involved in determining gender
 The sex chromosome is 1 of 23 pairs of
chromosomes and determines gender
 Y = Boy, X= Girl
 Males carry XY, females carry XX
 Autosomes
Change in Chromosome
number
The
presence
of all 46
chromosomes
is essential for
normal
development
The Cell Cycle
Section 6-2
The Cell Cycle
 The
cell cycle is a repeating sequence of
cellular growth and division
 A cell spends 90% of its time in the first 3
phases of the cycle called interphase
 There are 5 phases of the cell cycle
Cell Cycle
1.
2.
3.
4.
5.
First Growth (G1) phase: cells grow
rapidly and carry out routine functions. If
a cell isn’t dividing, it remains in G1
Synthesis (S) phase: DNA is copied.
Second Growth (G2) phase: Nucleus
prepares to divide
Mitosis: nucleus of cell is divided
Cytokinesis: Cytoplasm divides
Cell Cycle is Controlled
 The
cell has checkpoints and feed back
signals that trigger the next phase of the
cell cycle
 Proteins control the cell cycle in
eukaryotes
Cell Cycle is Controlled

Cell Growth (G1) checkpoint: this checkpoint
decides if the cell will divide.



Nerve and muscle cells remain in this stage and
never divide
DNA Synthesis (G2) checkpoint: DNA
replication is checked by repair enzymes and
triggers mitosis
Mitosis Checkpoint: this triggers the exit from
mitosis and signals the beginning of growth
cycle
When Control is Lost
 Certain
genes make the proteins that
regulate cell growth
 Mutations can disrupt the regular function
of these proteins
 Cancer is the uncontrolled growth of cells
 It is a disorder of cell division
Mitosis and Cytokinesis
Section 6-3
In Mitosis, Chromatids are
Pulled by Microtubules
 During
mitosis, the nucleus divides into
two nuclei, each contains a complete set
of chromosomes
 Spindles
are cell structures made of
centrioles and microtubules that move
chromosomes during division
Forming the spindle
 Animal
cells have centrioles that sit at a
right angle to each other
 Centrioles are replicated to make 2 pairs
 Centrioles move to opposite poles of the
cell and spindles form

Made of microtubules
 Plant
cells do not have centrioles but still
form spindles
Separating chromatids by
attaching spindle fibers
 The
spindles attach to the poles of the cell
and to the centromere

The chromatids may now separate
 The
spindle fibers break down by the pole
and “reel” the chromatids closer to the
pole
Mitosis
1.
2.
3.
4.
Prophase: chromosomes coil up and become
visible while spindles form
Metaphase: chromosomes line up at center of cell
and the spindle fibers attach to the chromatids
and poles
Anaphase: centromeres divide and chromatids
move toward the poles
Telophase: a nuclear envelope forms around the
chromosomes at the pole and spindles disappear
Cytokinesis
 As
mitosis ends, cytokinesis begins
 Cytoplasm is divided in half and cell membranes
grow to enclose each cell
 Animal cells use proteins to pinch the membrane in
half
 Plant cells fuse vesicles together to make a cell
plate and cell walls form on each side to allow cell
to split