Download Cell Division

Chapter 8
Cell Reproduction
8-1 Chromosomes
Recall:
•DNA: long, thin
molecule that stores
genetic information
Chromosomes
• DNA during cell division
• DNA coiled into very
compact rod-like
structures with proteins
• Can be seen if you stain a
cell as it is undergoing cell
division
Histones
• Chromosomes =
DNA + proteins
• Eukaryotic DNA
wrapped tightly
around histones
– Help maintain
shape of
chromosome
– Aid in tight
packaging
Nonhistones
• Do not help in packing DNA
• Involved in controlling activity of specific
regions of DNA
Chromatids
• Chromosome: two identical halves
• Each half is a chromatid
• Form as DNA makes copy of itself
before division
• When cell divides each of two
new cells will receive one
chromatid from each
chromosome
Centromere
• Construction area of each
chromatid
• Holds the two chromatids
together until they
separate in cell division
When the cell is not dividing…
• Chromatin:- less tightly coiled DNA
• DNA regions are uncoiled so that they can be
read
• Why read?
– Information used
to direct cells
activities
Prokaryotes
• Much simpler
• Usually just one chromosome attached to
inside of cell membrane
• Chromosome = One Circular DNA molecule +
proteins
Chromosome Number
• Each species has
characteristic chromosome
number
• Some species have same
number of chromosomes
• Ex:
– Potatoes, plums, and
chimpanzees all have 48
chromosomes
Sex Chromosomes
• Chromosome that determines sex of an
organism
• May carry genes for other characteristics
• X or Y
– Female: XX
– Male: XY
Autosomes
• All other chromosomes
• Every cell of an organism produced from
sexual reproduction has two copies of each
autosomes in each cell
– One from mother
– One from father
Homologous Chromosome
• aka Homologues: Two copies of autosomes
• Same size and shape
• Carry genes for same traits
– Two genes for eye color;
Two genes for hair texture
– Two for every trait that
makes you, YOU!
Karyotype
• Photomicrograph of chromosomes in a
dividing cell
• Shows all
chromosomes
• 46 chromosomes
of humans look like
22 homologous
pairs PLUS two sex
chromosomes
Diploid
• Cells having two sets of chromosomes
• Have both chromosomes of each homologous
pair
• ALSO have two sex chromosomes
• ALL human cells EXCEPT sex cells (sperm, egg)
• 2n
• Human 2n=46
Haploid
• Cells that contain
only one set of
chromosomes
• Half number of
chromosomes as
diploid cells
• 1n
• Ex: sperm, egg
Let’s talk numbers…
• Haploid + Haploid =
Diploid
• 1n + 1n = 2n
• Egg + Sperm = New
organism
• If reproductive cells were
diploid, new cell would
have too many
chromosomes and not be
functional
8-2: Cell Division
Quick Look:
• “All cells come from preexisting cells”
• Cell Division: process by which cells produce
offspring cells
• Different in
prokaryotes and
eukaryotes
Prokaryotes
• Binary fission: division
of prokaryotic cell into
two offspring cells
• Three stages
Stages
1. Chromosome (attached in
inside of cell membrane)
copies itself
2. Cell continues to grow until
it is 2x the normal size
3. Cell wall forms between two
chromosomes and cell splits
RESULT: TWO cells with
identical chromosomes
Eukaryotes
• Both cytoplasm and nucleus divide
• More complicated
• Types:
– Mitosis
– Meiosis
Mitosis
• Division of nucleus
• New cells with genetic material identical to
original cell
• Occurs in reproduction of unicellular organisms
and in addition of cell or tissue in multicellular
organism
Meiosis
• Reduces
chromosome
number by half
• New cells join
together later in
organism’s life
cycle to produce
cells with complete
set of
chromosomes
The Cell Cycle
• Repeating set of events that make up life of a
cell
1. Cell division:
1. M Phase or Mitosis: nucleus divides
2. Cytokinesis: cytoplasm divides
2. Interphase: time between cell divisions
Interphase
• Most of cells life here!
• After cell division, offspring cells about ½
usual cell size
• Phases
– G1
–S
– G2
– G0 *ONLY when
cell is exiting cycle*
G1 Phase
• Offspring grows to
mature size
• “G1” stands for
time gap following
cell division and
before DNA
replication
S Phase
• After cell has
reached mature
size
• DNA is copied
or synthesized
G2 Phase
• Time when cell
prepares for cell
division
• “G2” stands for time
gap following DNA
synthesis and before
cell division
G0 Phase
• ONLY when cell is exiting
cycle
• Usually from G1
• Cells do not copy DNA or
prepare for cell division
• Many cell in human
body are in G0 phase
• Ex: fully developed cells
in central nervous
system
Mitosis Stages
• Continuous process
• Can see stages under
light microscope
• REMEMBER: PMAT
1.Prophase
2.Metaphase
3.Anaphase
4.Telophase
Prophase
• Begins with shortening and tight coiling of
DNA
– DNA Chromosomes
• Nucleolus and nuclear membrane break down
and disappear
• Centrosomes: Two
pairs of dark spots
appear next to
nucleus
More prophase…
• Centrioles: cylindrical bodies that exist as
pairs in centrosomes
• Centrosomes move towards opposite poles of
cell
• Spindle fibers made
of microtubules
stretch from
centrosomes to
prepare for mitosis
Spindle Fibers
• Mitotic spindle: array of spindle fibers
• FUNCTION: equally divide chromatids
between two offspring cells during mitosis
• Types:
– Kinetochore fibers: extend from kinetochore (disk
shaped protein of centromere) from each
chromatid to centrosome
– Polar fibers: extend across dividing cell from
centrosome to centrosome
Metaphase
• Kinetochore fibers move chromosomes to center of
dividing cell
• Once in center, each chromosome held in place by
kinetochore fiber
• Chromosomes seen
most easily NOW!
• Karyotypes usually
made from cells
during metaphase
Anaphase
• Chromatids of each chromosome separate at
centromere
• Then move to opposite poles of cell
• When chromatids
separate they are
considered to be
individual
chromosomes
Telophase
• After chromosomes reach opposite sides of
dividing cell, spindle fibers disassemble
• Chromosomes Chromatin
• Nuclear envelope
surround each set
of chromosomes
• Nucleolus forms in
new cells
Cytokinesis
• Splitting of
cytoplasm
during
telophase
• Different in
plants and
animals
Animal cytokinesis
• Area of cell membrane pinches and separates
cell into two cells
• Cleavage furrow: area that pinches cell into
two cells through action of microfilaments
Plant cytokinesis
• Vesicles from Golgi body fuse at midline of
dividing cell forming
– cell plate:
membrane
bound cell
wall
• Then, cell
plate separates
cell into two
cells
8-3: Meiosis
Quick Look:
• “Process of nuclear division that reduces the
number of chromosomes in new cells to half
the number in the original cell”
• 2n  1n
• Produces gametes
Gametes
• Haploid reproductive
cells
• Human: egg and
sperm cells
• Fusion of gametes 
zygote
• Human: 23 + 23 = 46
Meiosis
• Prepare: G1, S, G2 Stages of interphase
• SO! DNA is replicated
• SO! Cells must divide TWICE to become
haploid
• 2n 4n 2n 1n
• First stages: Meiosis I
• Second Stages: Meiosis II
Prophase I
•
•
•
•
•
DNA coils into chromosomes
Spindle fibers appear
Nucleus and nucleolus dissemble
Chromosomes line up next to homologues
Synapsis: pairing of homologous
chromosomes
Tetrads
• Each pair of homologous chromosomes
• Chromosomes aligned lengthwise so genes on
one chromosome are adjacent to genes on
corresponding chromosome
Crossing-over
• During synapsis, chromosomes
twist around one another
• Portions may break off and
attach to adjacent chromosome
• Permits exchange of genetic
material between maternal and
paternal chromosomes
• Result: genetic recombinationnew mixture of genetic material
produced
Metaphase I
• Tetrads line up randomly along midline of
dividing cell
• Spindle fiber from each pole attach to one
homologous chromosome each
Anaphase I
• Each homologous chromosome moves to an
opposite pole of dividing cell
• Independent assortment: random separation
of homologous chromosomes
• RESULT: maternal and paternal chromosomes
separated; Genetic
recombination
Telophase I
• Final phase of meiosis I
• Chromosomes reach opposite ends and
cytokinesis begins
• New cells will contain diploid numbers
• 1 cell  2 cells
• One 4n  Two 2n
Meiosis II
• VERY similar to meiosis I EXCEPT:
• NO DNA synthesis before
• Some species start meiosis II after nuclear
membrane re-forms
• Other species begin meiosis II directly after
meiosis I
Prophase II
• Spindle fibers form
and begin to move
chromosomes toward
midline of dividing
cell
Metaphase II
• Chromosomes move
to midline of dividing
cell facing opposite
poles of dividing cell
Anaphase II
• Chromatids separate
and move toward
opposite poles of cell
Telophase II
• Nuclear membrane forms around
chromosomes in EACH of FOUR new cells
Cytokinesis II
• Occurs during telophase II
• FOUR new cells
• Each of FOUR new cells contain half of the
original cell’s number of chromosomes
Formation of Gametes
• In animals: produces gametes
• Meiosis occurs only within reproductive
organs
• Humans: testes and ovaries
Male Gamete Formation
• Spermatozoa: sperm cells
• Diploid reproductive cell
divides meiotically to form
four haploid cells called
spermatids
• Spermatid develops into
mature sperm cells
• Spermatogenesis:
production of sperm cells
Oogenesis
• Production of mature egg cells
• Diploid reproductive cell divides meiotically to
produce ONE ovum (mature egg cell)
• During cytokinesis I and cytokinesis II of
oogensis cytoplasm of cell is unequally
divided
• RESULT: One ovum
and THREE polar bodies
Asexual Reproduction
•
•
•
•
Production of offspring from one parent
NO meiosis or union of gametes
Usually binary fission or mitosis
RESULT: offspring genetically
identical to parent
Sexual Reproduction
• Production of offspring through meiosis and
union of sperm and egg
• RESULT: offspring genetically different from
parents
• All offspring contain
unique combination
of parental genes
Sexual Reproduction and Evolution
• Sexual reproduction enables species to rapidly
adapt to new conditions
• Lots of genetic combinations
= Lots of different kinds of
organisms
• Lots of different kinds of
organisms = lots of possible
survivors to specific
evolutionary change
in environment
• No genetic variation = ALL the same
• If all the same is not good for evolution, all die