CELL GROWTH AND
DIVISION Mitosis and Meiosis
Mitosis Animations Animation: How Meiosis Works Mitosis/Meiosis Comparison
LIMITS TO CELL GROWTH
The larger the cell, the more trouble the cell
has moving nutrients and waste across the
cell membrane.
DNA overload – As a cell increases in size,
it does not make extra copies of its DNA.
Cell size is limited by its DNA, if the cell gets
too big the DNA would no longer be able to
serve the needs of the cell.
LIMITS TO CELL GROWTH
Exchanging materials – materials enter and exit a
cell through the cell membrane.
Surface area – the total area of the cell membrane
The rate at which materials can be exchanged depends
on the surface area.
The rate at which materials are used up and waste is
produced depends on the cell’s volume.
LIMITS TO CELL GROWTH
Ratio of Surface Area to Volume
Surface area – L x W x number of sides
Volume – L x W x H
If a cell has a length of 1 cm, width of 1 cm,
height of 1 cm, and 6 sides,
its surface area would be 6 cm2
its volume would be 1 cm3
LIMITS TO CELL GROWTH
The ratio of surface area to volume in this example cell would be 6:1.
If the cell increase in size, its volume increases faster than the surface area.
This makes it more difficult for the cell to move needed materials in and waste products out.
Goal of the cell: have a large ratio of surface area to volume.
Prevention from getting too
big
Before a cell gets too big, it will split or
divide in half.
A parent cell forms two new daughter cells.
A cell must copy all its DNA (through
replication) before division to ensure that
each cell is the same, thus each daughter
cell has their own copy of DNA, both
identical to the other.
CELL DIVISION
CHROMOSOME
STRUCTURE
DNA contains the
information needed to
direct a cell’s activities.
DNA is composed of genes
– segments of DNA that
encode a protein.
Are transmitted info from
parent to offspring.
1 DNA strand contains 1000’s
of genes.
CHROMOSOME
STRUCTURE
Most prokaryotes have a single circular
DNA molecule.
Most eukaryotes have as much as 1000
times the amount of DNA as prokaryotes.
CHROMOSOME
STRUCTURE Eukaryotic DNA is generally located in the
nucleus in the form of several chromosomes.
Chromosomes—contain the genetic information that is passed on from one generation of cells to the next.
DNA molecules are extremely long.
In prokaryotes, the DNA molecules have to fold into a space about 1/1000th smaller than their length.
CHROMOSOME
STRUCTURE
Eukaryotes have even more DNA so the
cell had to have a way to fit it all in
Chromosomes contain both DNA and
proteins
Chromatin – DNA that is tightly coiled
around proteins called histones
CHROMOSOME
STRUCTURE Nucleosomes – Beaded structure composed of multiple
histones with associated DNA.
Nucleosomes pack together to form a thick fiber that is shortened by a system of loops and coils.
During mitosis the histones cause the fibers of the chromosome to coil up and packed into the structures that you can see.
CHROMOSOME STRUCTURE
DNA is coiled into rod shaped structures
called chromosomes.
Chromosomes are made of chromatin.
Chromatin is DNA and proteins wrapped together.
Proteins make coiling into chromosomes possible.
CHROMOSOME STRUCTURE
Chromosomes are 40% DNA and 60% protein.
Chromosomes copy themselves during DNA replication
forming sister chromatids.
Chromosomes must copy themselves so that the new
cell gets the same info as the old cell.
Sister chromatids are attached by a centromere.
PROKARYOTIC CELLS SIMPLY
SPLIT
Bacteria are prokaryotes lacking nuclei.
Bacterial DNA is a circular chromosome.
DNA unzips making 2 strands and each strand
is copied giving 2 identical copies of DNA.
Bacterial cell grows and then splits into
equal halves.
This is called binary fission.
Product of binary fission – 2 identical bacterial
cells.
EUKARYOTIC CELL CYCLE
Eukaryotic cells have a nucleus, so they must
undergo nuclear division.
Cell cycle – repeating sequences of growth and
division through which many kinds of eukaryotic
cells pass.
INTERPHASE
G1 – rapid cell growth, a cell is in this part
of the cell cycle for the longest period of
time between cell divisions.
S – DNA is copied, chromosome replicated
is now two sister chromatids joined at
centromere.
INTERPHASE
G2 – Organelles replicate, microtubules are reassembled to form spindle apparatus that will move chromosomes, cell is now prepared for mitosis. This is the shortest phase of interphase.
An eukaryotic cell spends most of its time in Interphase.
CELL CYCLE - MITOSIS
Mitosis – the process by which the nucleus
of a cell is divided into two nuclei, each with
the same number of chromosomes.
Misconception – mitosis IS NOT cell
division, it is a part of cell division!
CYTOKINESIS
Cytokinesis – the division of the cytoplasm.
This is the last step of cell division.
After cytokinesis is complete, the cell will be
in Interphase again.
MITOSIS
Biologists divide the events of mitosis into
four phases:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
PROPHASE
Chromosomes condense
and become visible.
Centrioles form and take up
positions on opposite ends of
the nucleus.
Spindle becomes visible.
Nuclear membrane breaks
down, and the nucleolus
disappears.
METAPHASE
Spindle fibers assist in moving the
chromosomes to the equator
(middle) of the cell.
The centromeres of all the sister
chromatids line up.
The imaginary line that bisects each
of the chromatids through the
centromere is called the metaphase
plate.
ANAPHASE
Sister chromatids separate from
each other at the centromere.
The spindle now pulls each
chromosome to opposite ends of
the cell (toward the centrioles).
The spindle is taken apart as the
chromosomes move.
Each pole now has one complete
set of chromosomes.
TELOPHASE
Chromosomes uncoil,
spindle fibers
disappear, and the
nuclear membrane
reforms.
Mitosis is complete.
CYTOKINESIS
Cytoplasm of original cell is split in half.
Cell membrane grows to enclose both cells.
Animal cells pinch in the membranes
forming a cleavage furrow.
CYTOKINESIS (continued)
Plant cells form a cell plate at the equator of the cell
where new cell wall forms on both sides of the plate.
The plate is formed from secretions of the golgi.
The product is two identical cells.
Following cytokinesis, the cell re-enters interphase at the
G1 phase, and the cell cycle continues.
Meiosis - the process of cell division that produces 4 gametes with half
the number of chromosomes
Discovery of Meiosis
• In 1882, British cytologist Pierre-Joseph
van Beneden found different numbers of chromosomes in different cells
Gametes(sperm and egg) are haploid (n) – contain half the number of chromosomes compared to somatic cells (nonreproductive cells)
Somatic cells are diploid (2n)
Fertilization
• Van Beneden then proposed that an egg and a sperm fuse to produce a zygote .
• The zygote contains two copies of each chromosome (one copy from the sperm and one copy from the egg).
• Fertilization is the name for the fusion of gametes.
Reduction Division
• Since the sperm and the egg contain only half the number of chromosomes, they cannot be formed from mitosis.
• Meiosis - the process of cell division that produces gametes with half the number of chromosomes
The Sexual Life Cycle
Unique Features of Meiosis
Feature #1 – Synapsis
Following chromosome replication, the homologous chromosomes pair all along their length. This process is called synapsis.
Unique Features of Meiosis
Feature #2 – Crossing Over
While the homologous chromosomes are joined, crossing over occurs. Crossing over is the exchange of genetic material from homologous chromosomes.
This causes genetic variations.
Synapsis and Crossing Over
Unique Features of Meiosis
Feature #3 – Reduction Division
The chromosomes are not copied in between the two divisions. At the end of meiosis, each cell contains one half the genetic material.
Reduction Division
Prophase I
• Individual chromosomes first become visible
– homologous chromosomes become closely associated in synapsis
– crossing over occurs
• Crossing over is a complex series of events in which DNA segments are exchanged between nonsister or sister chromatids.
Prophase I
Metaphase I
• The homologous chromosomes line up in the center of the cell and are still held together
Anaphase I • Spindle fibers shorten
• The homologous chromosomes are separated (the sister chromatids are still paired)
• Independent assortment – random chromosomes move to each pole; some may be maternal and some may be paternal
Telophase I • The nuclear membrane reforms around each
daughter nucleus
• Each new cell now contains two sister chromatids that are NOT identical due to crossing over
At the end of Meiosis I…
• You have made 2 cells
• Each cell contains a diploid number of chromosomes
• No DNA replication occurs between Meiosis I and Meiosis II
• Meiosis II resembles normal, mitotic division
Prophase II
• Nuclear envelope breaks down again
Metaphase II
• The chromosomes line up in the middle of the cell.
Anaphase II
• The spindle fibers shorten and the sister chromatids move to opposite poles.
Telophase II
• Nuclear envelope re-forms around the four sets of daughter chromosomes.
At the end of Meiosis II…
• At the end of Meiosis II, there are 4 haploid cells.
• No two of these haploid cells are alike due to crossing over.
– This is why you and your siblings are genetically unique!