Cell Reproduction
Chromosomes
• Recall that DNA is a long thin molecule of
nucleotides that stores genetic information
– During cell division, DNA in eukaryotic cell’s
nucleus is coiled into very compact structures
called chromosomes
• Chromosomes are rod-shaped structures made of
a single DNA molecule & proteins
Proteins in Chromosomes
• DNA in eukaryotic cells wraps tightly
around proteins called histones
– Histones help maintain shape of chromosome
& aid in the tight packing of DNA
• Nonhistone proteins are involved in
controlling the activity of specific regions of
the DNA
Anatomy of a Chromosome
• Chromosomes are made of 2 identical halves
• Each half of chromosome is called a chromatid
– Chromatids form as DNA makes a copy of itself
before cell division
– When the cell divides, each of two new cells will
receive one chromatid from each chromosome
• Centromere holds two chromatids together until
they separate during cell division
Centromere
Chromatid
Chromosomes: Prokaryotes Vs
Eukaryotes
• Chromosomes in prokaryotes are simpler
than chromosomes in eukaryotes
– DNA of most prokaryotes is made of only 1
chromosome
• Eukaryotic DNA is made of multiple chromosomes
– Prokaryotic chromosomes consist of circular
DNA molecule & its proteins attached to the
inside of the cell membrane
• Eukaryotic chromosomes are rod-shaped with
associated DNA and proteins found within cell’s
nucleus
Chromosome Numbers
• Each species has a
characteristic number
of chromosomes in
each cell
Organism
# of
Chromosomes
Adder’s
tongue fern
Cat
Dog
Lettuce
Human
1,262
32
78
18
46
Sex Chromosomes
• Sex chromosomes are chromosomes that
determine the sex of an organism & may
also carry genes for other characteristics
– In humans, sex chromosomes are either X or
Y
• Normal females have two X chromosomes
• Normal males have an X and a Y chromosome
• Humans have 2 sex chromosomes (out of their 46
total)
MALE
XY
FEMALE
XX
Autosomes
• Autosomes are all of the other chromosomes
in an organism
– Humans have 44 autosomes (out of their 46 total)
• Each cell of an organism produced by sexual
reproduction has two of each autosome
– One autosome from each parent
– Homologous chromosome, or homologues, refer
to the two copies of each autosome
• Homologous chromosomes are same size, shape, &
carry genes for same trait
Karyotype
• Karyotype photomicrograph of
chromosomes in a dividing cell
– Represents an organized profile of a person’s
chromosomes
– Arranged from largest to smallest
• 46 Human chromosomes exist as 22
homologous pairs of autosomes and two
sex chromosomes
Karyotype
Diploid Vs Haploid Cells
• Diploid cells contain both chromosomes of
a homologous pair
– All normal human cells, except reproductive
cells are diploid
– Abbreviated 2n
• Haploid cells contain only one set of
chromosomes
– Human reproductive cells—sperm & egg
– Abbreviated 1n
Cell Division
• All cells come from the division of
preexisting cells
– Principle of the Cell Theory
• Cell division is process by which cells
produce offspring or daughter cells
– Process differs in prokaryotes & eukaryotes
Cell Division in Prokaryotes
• Binary fission is the division of a
prokaryotic cell into two offspring cells
• 3 stages:
– 1. Chromosome makes a copy of itself
– 2. Cells grows to it reaches about twice cell’s
original size
– 3. Cell wall forms between two chromosomes
& cell splits into two new cells each containing
one of the identical chromosomes
Binary Fission
Prokaryotic Cell
DNA is copied
Cell begins to divide
Two identical cells
Cell Division in Eukaryotes
• 2 Types—Mitosis & Meiosis
– Mitosis results in 2 new daughter cells with
genetic material that is identical to that of
original cell
• Occurs in reproduction of unicellular organisms &
addition of cells to tissues or organs in multicellular
organisms
– Meiosis results in 4 new haploid daughter
cells by reducing chromosome number by half
in new cells
• Occurs in reproduction of multicellular organisms
Cell Cycle
• Cell cycle is the life cycle of a cell
– Repeating set of events that make up life of a
cell
– Stages:
• 1. Interphase: time between cell divisions
• 2. Cell Division
– M phase or mitosis: nucleus of cell divides
– Cytokinesis: division of cytoplasm of cell
The Cell Cycle
Cytokinesis
M phase
G2 Phase
S Phase
G1 Phase (cell growth)
G0 Phase
Interphase
• Cell spends most of its life in interphase
• Interphase is divided into 3 phases:
– 1. G1 phase: offspring cells grow to mature
size
• Represents time gap following cell division &
before DNA replication
– 2. S phase: cell’s DNA is copied
– 3. G2 phase: cell prepares for division
• Represent time gap following DNA synthesis (S
phase) & before cell division
Interphase Continued
• Cells may exit cell cycle, usually from G1
phase & enter Go phase
– These cells do not copy their DNA & do not
prepare for cell division
– These cells may reenter cell cycle
Mitosis
• Mitosis is division of the nucleus
• Mitosis is a continuous process
• Divided into 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
Prophase
• First phase of mitosis
• Begins with shortening & tight coiling of DNA into
rod-shaped chromosomes
• Nucleolus & nuclear membrane break down &
disappear
• Two dark sports called centrosomes appear next
to disappearing nucleus
– In animal cells, each centrosome contains a pair of
small, cylindrical bodies called centrioles
• Centrioles are not found in plant cells
Prophase Cont’d
• Centrosomes move toward opposite poles of the cell
• As centrosomes separate, spindle fibers made of
microtubules radiate from centrosome in preparation for
mitosis
– Array of spindle fibers forms mitotic spindle, which serves to
equally divide chromatids between two offspring cells during cell
division
– Made of 2 types of spindle fibers
• 1. Kinetochore fibers extend from kinetochore of each chromatid to
one of the centrosomes
– Kinetochore is a disk-shaped protein found in centromere region of
each chromosome
• 2. Polar fibers extend from centrosome to centrosome
Metaphase
• Second phase of mitosis
• Chromosomes are easiest to identify using
a microscope in this phase
• Kinetochore fibers move chromosomes to
center of dividing cell
• Once in center of cell, each chromosome
is held in place by kinetochore fibers
Anaphase
• Third phase of mitosis
• Chromatids separate at centromere
– Each chromatid is now a chromosome
• Chromosome slowly move, centromere
first, toward opposite poles of dividing cell
Telophase
• Fourth phase of mitosis
• After chromosomes reach opposite ends
of cell, spindle fibers disassemble &
chromosomes return to less tightly coiled
chromatin state
• Nuclear envelope forms around each set
of chromosomes
• Nucelolus forms in each of newly forming
cells
Cytokinesis
• During telephase, cytoplasm of cell divides by
process called cytokinesis
– In animal cells, cytokinesis begins with pinching
inward of cell membrane midway between the
dividing cell’s two poles
• Cleavage furrow is area of cell membrane that pinches in &
eventually separates dividing cell into two daughter cells
– In plant cells, vesicles formed by Golgi apparatus fuse
at midline of dividing cell, forming membrane-bound
cell wall called cell plate
• Cell plate separates cell into two daughter cells
Meiosis
• Meiosis is process of nuclear division that
reduces the number of chromosomes in
new cells to half the number of the original
cell
– Halving of chromosome # counteracts fusion
of cells later in the life cycle
• In humans, meiosis produces haploid reproductive
cells galled gametes
– Human gametes are sperm & egg cells, each of which
contains 23 (1n) chromosomes
– Fusion of sperm & egg results in a zygote that contains
46 (2n) chromosomes
First Stage: Meiosis 1
• Prophase 1:
– DNA coils tightly into chromosomes
– Spindle fibers appear
– Nucleus & nucleolus disassemble
– Every chromosome pairs up with its
homologue—process called synapsis
• Tetrad name given to each pair of homologous
chromosome
Crossing-Over in Prophase I
– Crossing-over may
occur where portions
of chromatids may
break off & attach to
adjacent chromatids of
homologous
chromosomes
• Results in genetic
recombination of
genetic material from
the exchange of genetic
material between
maternal & paternal
chromosomes
First Stage: Meiosis 1
• Metaphase 1:
– Tetrads line up randomly along midline of
dividing cell
– Spindle fibers from one pole attach to
centromere of one homologous chromosome
– Spindle fibers from opposite pole attach to
other homologous chromosome of the pair
First Stage: Meiosis 1
• Anaphase 1:
– Each homologous chromosome moves to an
opposite pole of the dividing cell
– Homologous chromosomes undergo random
separation called independent assortment
• Results in random separation of maternal &
paternal chromosomes—genetic variation
First Stage: Meiosis 1
• Telephase 1:
– Final phase of meiosis 1
– Chromosomes reach opposite ends of cell &
cytokinesis begins
– New cells contain haploid number of
chromosomes
Summary of Meiosis 1
• Original cell produces two new cells
• Each new cell contains one chromosome
from each homologous pair
• New cells contain half the number of
chromosomes of original cell
• Each new cell contains two copies of the
chromosome because original cell copied
its DNA before meiosis I
Second Stage: Meiosis II
• Occurs in each cell formed during meiosis
I & is not preceded by copying of DNA
• Prophase II
– Spindle fibers form & begin to move
chromosome toward midline of dividing cell
Second Stage: Meiosis II
• Metaphase II
– Chromosomes move to midline of dividing
cell, facing opposite poles of dividing cell
Second Stage: Meiosis II
• Anaphase II
– Chromatids separate & move toward opposite
poles of cell
Second Stage: Meiosis II
• Telephase II
– Nuclear membrane forms around
chromosomes in each of the four new cells
Second Stage: Meiosis II
– Cytokinesis II occurs resulting in 4 new cells,
each of which contains half original cell’s
number of chromosomes
Meiosis
Formation of Gametes
• In animals, meiosis produces haploid
reproductive cells called gametes
• Meiosis then only occurs within the
reproductive organs in animals
– In humans, meiosis occurs in testes & ovaries
Spermatogenesis
• Refers to the production of sperm cells or
spermatozoa
– Occurs in the testes
– In the development of sperm cells, a diploid
reproductive cell divides meiotically to form
four haploid cells called spermatids
– Each spermatid then develops into a mature
sperm cell
SPERMATOGENESIS
SPERM CELLS
Oogenesis
• Refers to the production of mature egg
cells or ova
• Diploid reproductive cell divides meiotically
to produce one mature egg cell
– During cytokinesis I and II, cytoplasm of
original cell is divided unequally between new
cells
– Produces 1 mature egg cell that contains
most of the cytoplasm of original cell
– Also produces 3 polar bodes that degenerate
OOGENESIS
EGG CELL
POLAR BODIES
Asexual Reproduction
• Production of offspring from one parent
– Does not usually involve meiosis or union of
gametes
– In unicellular organisms, like bacteria, new
organisms are created by either binary fission
or mitosis
– Multicellular organisms use process of
budding
– Offspring from asexual reproduction are
genetically identical to the parent
Sexual Reproduction
• Production of offspring through meiosis &
union of a sperm & egg
• Offspring produced are genetically
different from parent because genes are
combined in new ways in meiosis
– Exception: identical twins
• Evolutionary advantage of sexual
reproduction is it enable species to adapt
rapidly to new conditions