Transcript Chromosomes and Cell Reproduction

Chromosomes and Cell
Reproduction
Chapter 6
Chromosomes
DNA must be present in any new cell
that is formed so it must be copied
and distributed so each cell has a
complete set
Prokaryotic Cell Reproduction
Single circular DNA attached to inner cell
membrane
Binary Fission – asexual reproduction
producing identical offspring
2 Stages of Binary Fission
Stage 1: DNA is copied
Stage 2: Cell divides by adding a new cell
membrane between 2 DNA copies
Squeezes in middle
New cell wall forms until pinched in 2
Eukaryotic Cell Reproduction
DNA is organized into genes
These are segments that code for a
protein of RNA molecule
Thousands of genes in DNA
When genes are being used the DNA is
stretched out
Chromosomes
Chromosomes are
the DNA and the
proteins associated
with it
As a eukaryote
prepares to divide,
chromosomes
become visible (DNA
is copied before this)
Chromatids – 2 exact
copies of each
Chromosome attached
at point called
centromere
During cell division
chromatids separate
and are placed into
new cells
Each Human
somatic cell
(anything besides
sperm and egg)
has 2 copies of 23
different
chromosomes =
46 chromosomes
These 23 pairs
are different in
size, shape, and
the genes they
carry
Sets of Chromosomes
23 pairs made of 2 homologous
chromosomes (a.k.a. homologues)
which are similar in size and shape,
and genetic content
Each Homologue comes from 1
parent
Again, you have 46 chromosomes, 23
from each parent
Diploid or Haploid?
Diploid – somatic cells contain 2 sets
Haploid – gametes contain 1 set
n = 1 set of chromosomes
n = 23 = human haploid #; gametes
2n = 46 = human diploid #; somatic cell
Fertilization is when 2 haploid
gametes fuse
Forms a diploid zygote (fertilized
egg), the first cell of an individual
Each organism has a characteristic
number of chromosomes
Some may have 1 pair, some plants have over
500
Sex Chromosomes
23 chromosome pairs in human
somatic (body) cells
22 of these are autosomes, not
directly involved in determining
gender
The other pair are sex chromosomes
that contain genes that determine
gender
Humans have X and Y chromosome
Y chromosomes
have genes that
cause fertilized egg
to develop into a
male (XY)
Females can only
donate X so sex is
determined by man
Sex chromosomes are different in
other organisms
Male grasshopper = X0 (0 = missing)
Female grasshopper = XX
Change in Chromosome Number
All 46 chromosomes are needed for normal
development and function
If you have more than 2 copies of a
chromosome – trisomy – don’t develop
properly
Karyotype
Photo of chromosomes in a dividing cell
that shows them in order by size
Change in Chromosome Structure
These are called mutations
Breaking of chromosome in 4 different
ways
Deletion
Duplication
Inversion
Translocation
Deletion
Piece breaks off
completely
Certain genes are
missing in new cell
Usually fatal to
zygote
Duplication
Chromosome
fragment attaches
to homologous
chromosome
giving it 2 copies of
certain genes
Inversion
Chromosome
reattaches to
original but in
reverse order
Translocation
Reattaches to
nonhomologous
chromosome
6.2 The Cell Cycle
A repeating sequence of growth and
division
90% of the time cells are in 1st three
phases called Interphase
Last 2 phases only when it’s about to
divide
5 Phases of Cell Cycle:
Phases 1 through 3 = Interphase
G1 = 1st Growth Phase
Cell grows rapidly
Carries out normal functions
Major portion of life
Stay here if not dividing
S = Synthesis Phase
DNA is copied
At end each chromosome is
2 chromatids attached by/at
centromere
G2 = 2nd Growth Phase
Prepares for nucleus to
divide
Microtubules are rearranged
preparing for next stage =
Mitosis
Mitosis
Nucleus divides into
2 nuclei with same
number and kind of
chromosomes
Cytokinesis
Cytoplasm divides
These 2 stages
produce new cells
identical to original
and allow growth,
repairs and in some
cases asexual
reproduction
Control of Cell Cycle
Feedback information tells cell what
to do
Checkpoints/Inspection Points
Feedback can trigger next phase or
delay the next phase
The cycle is controlled by many
proteins and has 3 principal
checkpoints
G1 Checkpoint - Growth
Decides whether cell will divide
Proteins will stimulate cell to begin S
phase if conditions are right
G2 Checkpoint – DNA Synthesis
DNA replication is checked by DNA
repair enzymes
Once passed, proteins trigger mitosis
Mitosis Checkpoint
Triggers an exit from mitosis
Cancer
Uncontrolled growth of cells
May promote growth-promoting
molecules
May inactivate off/slow switch
6.3 Mitosis and Cytokinesis
When cells divide the
chromatids on each
chromosome are
moved to opposite
sides with the help of
a spindle
A spindle is a cell
structure made up of
both centrioles and
individual microtuble
fibers
Forming The Spindle
Centrosomes found at cells
poles organize assembly of
spindle
In animal cells there are a pair
of centrioles found inside each
centrosome (plants don’t have
centrioles)
Centrioles and spindle fibers are
both made of microtubules
(hollow tubes of protein)
Each spindle fiber is made of 1
microtubule
Each centriole is made of 9
triplets of microtubules arranged
in a circle
Separation of Chromatids by
Attaching Spindle Fibers
Some microtubules interact, others
attach to centromere
2 sets of microtubules extend out
from poles
When poles and centomeres are
attached the 2 chromatids can be
separated
Once separated called chromosomes
One chromosome goes to one pole
while the other is pulled towards the
other
They move along microtubule paths
and move closer to poles as
microtubules are broken down
Mitosis – 4 Stages
Prophase
Chromosomes coil and
become visible
Nuclear envelope dissolves
Metaphase
Chromosomes line up along
equator
Chromatids linked to poles by
spindle fibers
Anaphase
Centromeres divide
Chromosomes move toward
poles
Telophase
Nuclear envelope reforms
Chromosomes uncoil
Spindle dissolves
Cytokinesis
Cytoplasm divided in half
Cell membrane grows to enclose each cell
Animal Cells – pinched in half by belt of
protein threads
Plant Cells – Golgi apparatus forms
vesicles that fuse at midline forming cell
plate (cell wall in middle of cell)
Each offspring is equal in size, amount of
cytoplasm, number of organelles and has
an identical copy of chromosomes