CELL DIVISION

Types of Cell Division Mitosis – makes new body cells (in eukaryotes) Binary Fission – makes new cells (in prokaryotes) Meiosis – makes new sex cells (in eukaryotes)

WHY DIVIDE? Why do multicellular organisms do mitosis? –To repair or heal –To grow and develop –To replace cells that die Why do unicellular organisms do mitosis (eukaryotes) or binary fission (prokaryotes)? –To reproduce asexually (cloning) Why do multicellular organisms for meiosis? –To reproduce sexually

Important part of Cell Division Nucleus - contains genetic information (DNA) Chromosomes – strands of DNA When a cell divides, the new cells made must have the correct number of chromosomes. Most of the time, DNA looks like a lump of spaghetti, and is called chromatin.

Cell Cycle

Interphase –G 0 – cell at rest; is not preparing for division –G 1 – cell gets ready for division –S – chromosomes copy themselves –G 2 – cell checks for mistakes and prepares for division Mitosis – cell divides

Most of the time cells are not dividing. Several proteins are involved in starting division. Some cells can divide in as little as 20 minutes.

Cells typically stop dividing when: – Nutrient levels drop (no food) –They come into contact with other cells (no room) –Receive chemical signals to stop.

INTERPHASE Cells spend most of their life in interphase. Normal cell activities occur. To prepare for cell division, the chromosomes make copies of themselves. In other words, the DNA replicates. Recall, DNA polymerase… etc.

MITOSIS: CELL DIVISION FOR EUKARYOTES Four Stages 1.Prophase 2.Metaphase 3.Anaphase 4.Telophase

Some animations…. Cells Alive animation John Kyrk animation McGraw animation

PROPHASE Chromatin condenses and becomes visible as chromosomes. The nucleus and its various parts disappear. Spindle fibers are made and attach to the chromosomes.

Centrioles separate and move to opposite ends (poles) of the cell. Each copy of the chromosome is called a chromatid. The two chromatids are attached at the centromere.

Chromosome centromere chromatids

METAPHASE The chromosomes line up at the middle of the cell (equator).

ANAPHASE The joined chromatids separate and move to opposite ends of the cell. In plant cells, the cell plate begins to form.

TELOPHASE The chromatids arrive at the opposite poles of the cell. Spindle fibers disappear. New nuclei form in the two new cells. Cytokinesis - pinching in of cell membrane (formation of cleavage) in order to form two new cells

END RESULTS OF MITOSIS Two new “daughter” cells are made with each containing the same number and kinds of chromosomes as the original (parent) cell. They have the same genetic code.

BINARY FISSION: CELL DIVISION FOR PROKARYOTES (BACTERIA) Bacteria only have one chromosome so they do not have to go through the steps of mitosis and instead do binary fission. The single chromosome attaches to the cell membrane. The DNA replicates making 2 identical chromosomes – one for each new cell.

The cell elongates and eventually separates into 2 new cells. Each cell gets one chromosome.

CANCER Caused by uncontrollable mitosis. Cells either fail to stop dividing or get signals to continue dividing. An increased number of cells makes a tumor. Tumors can be benign (non-cancerous) or malignant (cancerous). Cancer cells spread (metastasis) throughout body, unlike normal cells!

Some Causes of Cancer Genetics Radiation Carcinogens Old Age Viruses –Can mutate DNA causing some cancers –Example: Human papillomavirus (HPV) associated with cervical cancer in women Poor diets

Carcinogens Cancer causing agents –Mutate DNA turning a normal cell into a cancer cell –Examples: cigarette smoke, pollution Can cause increased amounts of free radicals

Free Radicals Free radicals form when a weak bond splits between two molecules causing one of those molecules to lose an electron. The free radical then attacks a different molecule to steal its electron, which causes more free radicals to form. Free radicals can attack DNA!

Free Radicals and antioxidants Free radicals can occur in your body naturally. Carcinogens can make free radicals in the body. Antioxidants may battle free radicals by bonding with them before the free radicals can do any harm.

Examples of Antioxidants Vitamins A, C, and E Beta carotene Lycopene (red pigment in tomatoes)

Fruits –Ex. berries, pomegranate, grapes, orange, plum, pineapples, kiwi, grapefruit Vegetables –Ex. cabbage, artichokes, Brussel sprouts, spinach, red beets Beans/ Legumes –Ex. soybeans, red kidney beans, coco beans Foods with High Antioxidants

More Foods with High Antioxidants Dry fruits –Ex. apricots, prunes, and dates Nuts and seeds –Ex. pecans, walnuts, hazelnuts, sunflower seeds Cereal –Ex. barley, millet, oats, corn

Fight cancer! Chemotherapy (anti-cancer drugs) kills cancer cells, but unfortunately it also damages healthy cells. This is why people get sick when they undergo chemotherapy. Radiation – kills cancer cells Vaccines – help prevent some cancers that are caused by viruses (ex. Gardasil against HPV) Surgery – removal of cancerous tumor Newer, experimental treatments include gene therapy, antiangiogenesis therapy, and hypothermia Don’t smoke, eat healthy, and exercise!

MEIOSIS: Making sex cells

CHROMOSOME NUMBER The number of chromosomes varies from one kind of organism to another. Humans have 46 chromosomes & fruit flies have 8. Some plants have hundreds.

Karyotype – a picture of chromosomes that shows the number and the size of chromosomes that a cell has Amniocentesis and Chorionic Villi Sampling– tests done on pregnant women in order to look at the chromosomes of the developing baby

Chromosomes of animal cells are usually paired. WHY? Humans have 23 pairs, fruit flies have 4 pairs. Haploid - the # of pairs or n Diploid - the total number of chromosomes or 2n For humans: n=23 and 2n=46

In diploid cells, each chromosome pair has the same basic structure. Each chromosome in a pair carries a message for the same trait, although not necessarily the same message. For example, both chromosomes in a pair have a gene for eye color, but one may have a blue gene and the other may have a brown gene. It is the combination of the two chromosomes or genes that determines the eye color of the person.

SEXUAL REPRODUCTION Fertilization - the combination of chromosomes (genetic information) from two sex cells (combining sperm and egg) Each sex cell gives half of the total chromosomes.

Sex cells are also called gametes. –Female sex cell = egg or ovum –Male sex cell = sperm Gametes are haploid (half the chromosomes).

Gamete Production Gametes are produced in gonads. Eggs are made in ovaries. –Females begin making their eggs before they are born! –They stop making eggs during menopause. Sperm are made in testes. –Males make sperm from approximately puberty until death.

After fertilization, the developing organism is called a zygote and is diploid. After fertilization the zygote grows bigger by doing MITOSIS. A zygote becomes an embryo and later a fetus.

MEIOSIS Meiosis converts a diploid cell into 4 haploid gametes. It increases genetic diversity in the offspring. Meiosis is divided into two main parts labeled I and II.

MEIOSIS I PROPHASE I –Homologous (similar) chromosomes find their partner (a step unique to meiosis) –This means the same-sized chromosomes find each other.

METAPHASE I »The chromosomes line up in the middle of the cell with their partners.

ANAPHASE I –The pairs separate and the individual chromosomes move to separate poles.

TELOPHASE I –Each of the daughter cells is now haploid, but each chromosome still has two chromatids.

MEIOSIS II Meiosis II is similar to mitosis. However, there is no interphase and almost no prophase.

METAPHASE II –The individual chromosomes line up in the middle of the two new cells.

ANAPHASE II –The chromatids separate and move to opposite ends of the two cells.

TELOPHASE II –Both new cells complete division making four new cells.

MEIOSIS END RESULT Meiosis makes 4 haploid, sex cells. These 4 cells are sex cells (sperm and egg). Each cell has half the number of chromosomes as the parent cell (haploid).