Cell ReproductionMitosis & Meiosis
Why Cells Divide Surface Area/ Volume Ratio
As the cell grows, the volume increases at a greater rate than the surface area
Can't take in enough nutrients, or remove wastes
Therefore the cell must grow or divide Growth and Repair
Replace worn or damaged cellsFrequency of replacement varies:
bacteria ~ every 20 minuteshuman cells ~ every 18-22 hours
Many cells in the body don't divide
Cell Division
Cellular Reproduction When the parent cell divides, it
forms new daughter cells Organisms reproduce in two ways:
Asexual ReproductionSexual Reproduction
Sexual vs. Asexual Reproduction
Asexual Reproductionproduction of offspring from one
parent therefore genetic material is identical
to parent Sexual Reproduction
formation of a new individual from the union of 2 cells
2 parents, therefore offspring have some hereditary material from each
Types of Asexual Reproduction Binary Fission
simplest form; the cell splits in 2 Spore Formation
Begins with replicationSpores can remain inactive until conditions
are favorablemolds, fungi
Yeast reproduce by budding Vegetative Propagation
Some plants, e.g. strawberries Regeneration
planaria, star fish, etc.
Cell Division in
Prokaryotes Binary Fission: The simplest
form of cell division
The cell splits in 2
The Process of Binary Fission First the single circular chromosome
duplicates = Replication Both chromosomes attach to sites on
the cell membrane As the cell grows, a new membrane
forms between attachment sites Membrane pinches off and the new
cells separate
Sexual Reproduction The joining of 2 specialized sex cells
called gametesmale = sperm female = ovum
Process of combining gametes = fertilization
Fertilization produces a zygotehas characteristics of both parents
Human Sexual
Reproduction Male testis produces
sperm Female ovary
produces ova Each has 23
chromosomes Unite to form a
zygote with 46 chromosomes 23 pair
Develops into a fetus
Cell Division
All types of reproduction require cell division 2 processes can be used to divide the cell’s
nuclear material: Mitosis
Occurs in somatic cells (body cells) in eukaryotes As a result of mitosis each daughter cell receives
an exact copy of the chromosomes present in the parent cell
Meiosis Occurs in gametes (sex cells) As a result each daughter cell receives 1 of each
pair of chromosomes present in the parent cell
Mitosis Cell division in eukaryotic cells involves
nuclear division called mitosis Occurs in somatic cells
body cells; not sex cells As a result of mitosis, each daughter cell
receives an exact copy of the chromosomes present in the parent cell
Chromosomes contain genetic materialDNA
Chromosomes During cell division in eukaryotic
cells, the DNA is coiled into chromosomes
Every body cell of the same type of organism has the same number of chromosomeshumans = 46goldfish = 94mosquito = 6
Chromosome Structure Each chromosome is formed from
two joined strands called chromatids Each chromatid is alike
has a long arm & a short arm joined at the centromere
Chromosomes contain DNA and associated proreins
Chromosomal Proteins Each chromosome is a single DNA
molecule and associated proteins Histones –
One type of chromosomal proteinThe DNA wraps tightly around the histonesHistones help maintain the shape of the
chromosome Nonhistone proteins -
control the activity of specific regions of DNA
Picturing Chromosome Structure
Visualizing Chromosomes
Chromosome Make-up Chromosomes of somatic cells are in pairs
One of each pair comes from mother, one from father
The 2 chromosomes in a pair are homologous Alike in appearance and type of genetic
information carried Humans have 23 pairs of chromosomes
22 pairs of autosomes Autosomes are all but the sex
2 sex chromosomes ( X & Y)
Sex Chromosomes
Determine the sex of the organism
Also carry other genetic information
In humans, either X or Y
Females are XX, males are XY
Thus the male determines the sex of the offspring
Haploid vs. Diploid Cells with two copies of each
chromosome = diploid Autosomal cells are diploid Gametes (sex cells) have only one of
each type of chromosome Cells with one copy of each chromosome
= haploid
Karyotypes A picture of paired human chromosomes Used to to detect certain genetic
diseases
Mitosis The process of dividing the nuclear
material in a somatic cell in eukaryotes Necessary for cell division
Preparation for Mitosis
Interphase The time between the formation of a cell
through mitosis and the next mitosis Most of the cell cycle is interphase During this phase cell prepares by:
replicating genetic materialproducing organellesassembling structures needed for mitosis
Chromosomes & Interphase
During interphase chromosomes cannot be distinguished under the light microscope They appear as chromatin
At the start of mitosis, the chromatin thickens, and chromosomes become visible
The Cell Cycle The sequence of cell growth and division The cell cycle can last several hours to
several days Can be affected by environmental factors, like
temperature Has 4 stages:
mitosis & division of cytoplasm (cytokinesis)
The other 3 are part of interphase:G1SG2
Picturing the Cell Cycle
G1 - Growth After mitosis, a period of intense
cellular activity and growth The cell doubles in size Enzyme production is high Cells that stop growing remain in G1
S- Synthesis Cells that divide enter S, or
synthesis, phase The chromosomes replicate
G2 – Further Growth A second period of growth Structures used in mitosis are
assembled
The Phases of Mitosis
Mitosis is actually a continuous process But we divide it into 4 phases:
ProphaseMetaphaseAnaphaseTelophase
Prophase 60% of the period of mitosis is prophase Divided into 3 parts: early, middle, & late Chromosomes begin to coil into short rods Nucleoli break down & begin to disappear 2 pairs of dark spots called centrosomes
appear outside the nuclear membrane In animal cells, the centrosomes contain centrioles,
formed from microtubules Plant cells have no centrioles
The centrosomes move to opposite sides of the cell
Mid Prophase At the beginning of mid-prophase
spindle fibers form between the centrioles
Additional fibers radiating out from each centriole form the aster
The nuclear membrane has broken down and disappeared
The Mitotic Spindle Spindle fibers made of microtubules radiate
from the centrosomes This array of spindle fibers = the mitotic
spindle 2 types of spindle fibers: Kinetechore fibers
Attach to a disk-shaped protein called a kinetecore Found in the centromere of each chromosome Extend from the kinetechore of each chromatid to
one of the centrosomes Polar fibers
Extend across the dividing cell from one centrosome to the other
Late Prophase The centrosome pairs are at opposite
ends of the cell The centosomes are fully formed Chromosomes are attached to the
centrosomes by spindle fibers Other spindle fibers stretch across the
cell from one centriole to the other
Metaphase The chromosomes are pushed and
pulled by spindle fibers along cell's the midplanecalled the equator
Anaphase
Begins with the separation of chromatids in each chromosome
Spindle fibers appear to shorten, pulling the chromatids apart at the centomere
Each chromatid is now a chromosome 2 sets of separated chromosomes then
move through the cytoplasm to opposite poles of the cell
Telophase The last stage of mitosis After the individual chromosomes have
reached opposite poles of the cell, spindles disappear
A nuclear membrane forms around each set of chromosomes
Chromosomes return to a thread-like mass Centrioles duplicate
2 centrioles formed in each daughter cell Nucleoli re-form within each newly formed
nucleus
Cytokinesis The division of the cytoplasm Follows mitosis Cytokinesis begins during telophase In animal cells, the cell membrane
pinches together The area that pinches in and separates
is called the cleavage furrow In plants, a cell plate is formed, dividing
the two halves
Picturing Cytokinesis
Chromosome Number Cells formed thru mitosis have the same
number of chromosomes as the parent cells
If combined in sexual reproduction, the offspring would have 2x chromosomes!
Therefore gametes have only half the number of chromosomes of somatic cellsGametes = sex cells
Meiosis
Gametes need another process for nuclear division
Meiosis reduces the number of chromosomes to 1/2 the number in somatic cells
Meiosis I & II Forming haploid
daughter cells from diploid parent cells requires two successive cell divisions:First = Meiosis I –
homologous chromosomes separate
Second = Meiosis II chromatids of
each chromosome separate
Meiosis I Preceded by replication of DNA that
forms the chromosome Synapsis = pairing of homologous
chromosomes Each pair of homologous chromosomes
twists around each other, forming a structure called a tetrad
Meiosis can be divided into same 4 phases as mitosis: Prophase, Metaphase, Anaphase, Telophase
Prophase I
Chromatin begins to coil into short rods Homologous chromosomes are formed Spindle fibers appear Nucleoli break down By the end, the nuclear membrane has
dissolved, and tetrads are visible
Crossing Over During synapsis, (prophase I) the
chromatids of homologous pairs twist around each other
A portion of one chromatid may break off and reattach, “trading” with the same piece from its homologous partner
The exchange of genes by reciprocal segments of homologous chromosomes during meiosis = “crossing-over”
Crossing over causes exchange of genetic material between maternal & paternal chromosomes
Results in genetic recombination
Genetic recombination is less likely in genes that are closer together.
Chromosome Mapping The likelihood that recombination will occur due
to crossing-over depends on the genes’ distance from each other on the chromosome
Scientists can determine how frequently genes for particular traits occur together in offspring
This can be used to create a map of the chromosome
1% recombination (crossing-over) = 1 map unit
Metaphase I Tetrads line up along the equator of the
cell Each tetrad becomes attached to
spindle fibers
Anaphase I
Homologous chromosomes that form each tetrad are pulled apart in pairs
One pair goes to one end of the cell, the other to opposite end
Telophase I Chromosomes reach ends of the cell Cell divides into 2 daughter cells
Independent Assortment During Anaphase I, one member of each
homologous chromosome pair moves to one end of the cell, the other moves to the opposite end
The separation of homologous chromosomes is random
More, or fewer maternal (or paternal) chromosomes may end up on one side or the other
Each separation is independent of the others This is the principal of independent assortment
of chromosomes Results in genetic variation
Meiosis I Summary Meiosis I is a Reductive Division It reduces the number of chromosomes
from diploid "2n" to haploid "n"
Meiosis II Similar to mitosis but not preceded by
replication of DNA 4 Stages:
Prophase IIMetaphase IIAnaphase IITelophase II
Prophase II & Metaphase II Prophase II
A new spindle forms around paired chromatids
Metaphase IIChromosomes line up along the equatorThey are attached at the centromere to
spindle fibers
Anaphase II Centromeres duplicate & the chromatids
separate Resulting single chromatids move to
opposite poles Chromatids are now called
chromosomes
Telophase II A nuclear membrane forms around each
set of chromosomes The spindle breaks down and
cytokinesis occurs Result: 4 haploid daughter cells
Males vs. Females In males, all 4 daughter cells
differentiate to become sperm In females, the cytoplasm divides
unevenly in Meiosis I The smaller cell = first polar body doesn't survive
In Meiosis II, the division is again unequalsmaller half is second polar body
So only 1 of 4 daughter cells survivesrich in cytoplasm, has many nutrients to
nourish the young organism
Comparing Mitosis & Meiosis MEIOSIS
2 4
diploid
haploid
different
MITOSIS
1
2
diploid
diploid
identical
# of nuclear divisions:
# of daughter cells:
Parent cell type:
Daughter cell type:
Genetic likeness to parent:
Control of Cell Division Timing and rate of cell division
varies in different cell types Control of rate of division is critical Some cells require regulatory
substances to begin division = growth factors
Effect of Growth Factors
Effect of Density
Density of cells also effects the rate of division
Crowding inhibits cell division
The Restriction Point
A crucial checkpoint occurs late in the G1 phase of the cell cycle
Point of decision to divide = restriction point Cell cannot turn back after this point If it is “yes,” cell goes to S phase and copies
DNA If “no,” it goes to non-dividing state (G0)
Most cells are in G0
MPF After S, the cell will
enter G2 The “OK” signal
that causes the cell to proceed from G2 to mitosis = mitosis promoting factor (MPF)A complex of
proteins
MPF is an enzymeProtein kinase
Abnormal Cell Division Cancer cells do not respond normally to the
body’s control mechanisms for cell division Cancer cells divide excessively Can invade other body tissues When a cell divides abnormally =
transformed Abnormal cells are usually destroyed by
the immune system
Cancer
If abnormal cells are not destroyed and reproduce, they may form a mass of abnormal cells = tumorBenign tumor = abnormal cells remain at
the original siteMalignant tumor = cells spread to other
parts of the body Metastasis = spread of cancer cells in
the body
Breast Cancer Cell