Cell Growth

Most organisms grow by producing more cells, not by

producing larger cells

There are two main reasons why cells divide

The larger a cell becomes, the more demands the cell places on its DNA

Also, a larger cell has more trouble keeping up with the needs of the cell

moving nutrients in andexporting wastes outthrough the cell membrane

The process by which cells divide

Cells divide to form two “daughter” cellsThis is called cell divisionallows organisms to grow larger while,

allowing cells to remain smallBefore a cell divides it must copy its

genetic information, so each daughter cell can get a copy

The cells genetic information is carried by the chromosomes

Chromosomes are composed of bundles of DNA

Cells of different organisms have different numbers of chromosomes

Fruit fly cells have 8 chromosomes

Carrot cells have 18 chromosomes

Human cells have 46 chromosomes

The DNA of a cell is not visible until it begins to condense into chromosomes for cell division

Before DNA condenses into chromosomes it is replicated

The copies condense into two identical sister chromatids

Attached at an area called the centromere

When the cell divides these sister chromatids separate so that each new

cell formed gets one set of chromosomes

The cell cycle

The steps that a cell goes through as it grows and divides

During this cycle the cell growsPrepares for cell divisionDivides to form two identical daughter

cellsThese new cells begin the cycle againThe cell cycle consists of four phases

The first group of phases is called interphase and has three stages

S phase follows and is where DNA gets replicated and proteins associated with

chromosomes are synthesized

G1 phase is where cell growth occurs

G2 phase is where organelles and molecules required for cell division are produced

The time frame for the cell cycle

Most of the cell cycle is spent in interphase

The last phase is called the M phase

This phase consists of mitosis and cytokinesis

Mitosis is divided into four steps

Step one of mitosis is prophase

Chromosomes condense and become visible and form

structures like this

Centrioles produce spindle fibers that separate the sister chromosomes later on in

mitosis

Nucleolus disappears and nuclear envelope breaks down

Step 2 of mitosis is metaphase

The chromosomes line up across the center of the cell

Spindle fibers are connected to each chromosome

Step 3 of mitosis is anaphase

Centromeres that join the sister chromatids splits

Chromatids separate and are pulled to either side of

the cell by the spindles

Step 4 of mitosis is telophase

Condensed chromosomes begin to disperse

Spindle fibers break down

Nuclear envelope begins to develop around each set of

separated chromosomes

At this time mitosis is complete but cell division in not completed

Because DNA is replicated in interphaseEach copy is condensed and separated

in mitosisIf the original parent cell has 4

chromosomes, then mitosis will produce 2 daughter cells each containing 4 chromosomes

The second part of the M phase is called cytokinesis

This process divides the cytoplasm between the two newly formed nuclei

In animal cells, the cell membrane is pinched in the center until it touches to form two separate cells

In plant cells, a cell plate forms between the two new nuclei and then a cell wall forms around it

What controls cell growth and division so carefully?

Cells in a petri dish begin to divide

Until they fill the dish and come in contact on all sides

and then stop dividing

If some cells are removed from the

center

The cells bordering the open space begin to

divide again

Until they come in contact again

The controls on cell growth and division can be turned on and off by the cells

One protein that regulates the timing of the cell cycle in eukaryotes is cyclins

Cyclins is an internal regulatorSeveral different regulatory proteins regulate

different steps of cell divisionProteins that respond to events outside the cell

are called external regulatorsGrowth factors are important external

regulators during embryonic development and wound healing

Micro-experiments lead to the understanding of proteins that

regulate cell division

A protein called p53 halts the cell cycle until all chromosomes have been properly replicated

Cancer

A disorder in which cells lose the ability to control growth

Cancer cells do not respond to the signals that regulate growth

This can result in un-controlled growthCan lead to tumorsMany different cancers have different

causes but…All have lost control over the cell cycle

Meiosis, the production of sex cells

Genes are located on the chromosomes in the cell nucleus

All cells have two sets of genes, a single copy from each parent

These are homologous (the same) and the pair make a diploid cell

Reproductive cells are called gametes and contain only one set of genes; so they are haploid

Meiosis is the process of reduction division that cuts the number of chromosomes in half

It does this by separating the homologous chromosomes in a diploid cell.

Meiosis has two distinct divisions

Meiosis 1 is similar to mitosis, this example follows two chromosomes

The big difference between meiosis 1 and mitosis is that during prophase 1 of meiosis, the replicated homologous chromosomes pair up to form a tetrad

There are 4 chromatids in a tetrad that contain one identical copy of each of the original pairs of chromosomes from the diploid cell

In anaphase 1 of meiosis, the homologous chromosomes separate and two new cells are formed

If the original cell was heterozygous this can create two unique cells

Crossing over can also make the new cells unique as the chromosomes exchange alleles

while rubbing together

Crossing over is a process that occurs when chromosomes of the tetrad rub into each other while

being separated (Occurs during Prophase 1)

The two new cells formed in meiosis I now have a set of chromosomes that are different from each other. The next step is meiosis II

Here, replication does not occur before the next cell divisionIn anaphase II, the paired chromatids will separate, this reduces

the number of chromosomes in the new gamete cells by half

The cells will go through telophase and cytokenesis to produce four haploid cells called gametes

The reproductive gamete cells produced in males

are called sperm

The reproductive gamete cells produced in females are

called eggs

These will be the reproductive cells

Meiosis results in the formation of 4 different haploid cells

Linked genesChromosomes contain many genesThese traits remain linked during meiosisIt is the chromosomes that assort independently,

not the individual genesThe rate at which genes are separated due to

crossing over is called recombination frequencyThis can be used to produce a map of the location

of genes on the chromosomes