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!