CELL DIVISION

Types of Cell Division Mitosis – makes new body cells (in eukaryotes) 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)? –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.

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.

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.

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 (mutate DNA) Old Age Viruses –Can mutate DNA causing some cancers –Example: Human papillomavirus (HPV) associated with cervical cancer in women Poor diets

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

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 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). It increases genetic diversity in the offspring.