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 and exporting wastes out through the cell membrane

The process by which cells divide Cells divide to form two “daughter” cells This is called cell division allows organisms to grow larger while, allowing cells to remain small Before 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 grows Prepares for cell division Divides to form two identical daughter cells These new cells begin the cycle again The 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 G 1 phase is where cell growth occurs G 2 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 interphase Each copy is condensed and separated in mitosis If 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 regulator Several different regulatory proteins regulate different steps of cell division Proteins that respond to events outside the cell are called external regulators Growth 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 growth Can lead to tumors Many 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 division In 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 genes Chromosomes contain many genes These traits remain linked during meiosis It is the chromosomes that assort independently, not the individual genes The rate at which genes are separated due to crossing over is called recombination frequency This can be used to produce a map of the location of genes on the chromosomes