Chapter 6: Chromosomes and Cell Reproduction Section 1: Chromosomes • How many cells do you think are produced by the human body everyday? • 2 trillion cells, that’s 25 million cells every second • Why do cells divide? • Cells need to grow, develop and repair themselves • When a cell divides, the DNA must be copied before the genetic information is distributed Sexual vs. Asexual Reproduction • Organisms reproduce two different ways: – Sexual reproduction requires two parents (one male and one female) to make a genetically similar offspring. • Gametes are an organism’s reproductive cells – Males have sperm – Females have eggs – Asexual reproduction requires only one parent to make a genetically identical offspring Prokaryotic Cell Reproduction • Bacteria have a single circular strand of DNA that “floats” around the cell; the DNA is not contained within a nucleus • Prokaryotes reproduce by a type of cell division called binary fission. • Binary fission is a form of asexual reproduction; a single parent passes exact copies of all of its DNA to its offspring. Binary Fission Eukaryotic Cell Reproduction • In eukaryotes, DNA is organized into units called genes • Genes are small segments of DNA • A single molecule of DNA has thousands of genes lined up next to each other Chromosomes • When a cell prepares to divide, the DNA coils up into a structure called a chromosome. • Each chromosome has two strands; each strand is an exact copy of the other. • Each individual strand is called a chromatid. • The two chromatids are connected by a point called a centromere. Chromosome Structure • Homologous chromosomes are those that are identical in structure • Most humans have 23 pairs of chromosomes (46 total) • There are two types of cells: somatic (body cells) and gametes (sex cells) Chromosome Number • When egg and sperm fuse together during fertilization, a single cell is formed called a zygote. • The egg and sperm each have 23 individual chromosomes. • When fertilized, the zygote has 46 chromosomes (23+23) Sex Chromosomes • In humans and other organisms, the two sex chromosomes are referred to as the X and Y chromosomes. • Males are XY and females are XX. • Males determine the sex of new offspring because females only have an X to contribute to the zygote. Mutations • Mutations are changes in an organism’s chromosome structure. • There are four type of mutations. Sketch each mutation in your notes using page 124 in text. – deletion – duplication – inversion – translocation Section 2: The Cell Cycle • Cell cycle – a repeating sequence of events that allow a cell to grow and divide. • How do cells know when to divide? • Just as traffic lights control the flow of traffic, cells have a system that controls the phases of the cell cycle. • Cells have a number of “red light-green light” switches that regulate information traveling through the cell. • Cells can’t divide unless they pass all checkpoints with green lights. • Yellow or red lights would slow or stop cell division. Cancer • Sometimes cells have mutated chromosomes that lead to cancer. These cancer cells can change all the checkpoints to green lights and they coast through the cell cycle reproducing rapidly. • All cancers are different, but if scientists can figure out what changes all the checkpoints to green lights we could cure cancer. Chapter 7: Meiosis and Sexual Reproduction Formation of Gametes • To make sex cells (egg and sperm), many living things undergo a process called meiosis. • Meiosis is a form of cell division that halves the number of chromosomes. • Meiosis involves two divisions: – Meiosis I – Meiosis II Reasons for Genetic Variation Random Fertilization: • Except for twins, no two people are exactly alike. This is because there are so many millions of sperm or eggs within a given individual that have a chance to be fertilized. • About 223 or 8 million different sperm or egg exist inside of one living individual. • Because fertilization of an egg by a sperm is random, the number of possible outcomes is 64 trillion (8 x 8) Crossing Over • Chromosomes have the ability to cross over during the early stages of meiosis I. • Crossing over – when sections of a chromatid on one homologous chromosome are broken or exchanged with a section of the other chromatid on the corresponding chromosome. Gamete Formation • Males produce sperm through a process called spermatogenesis. • Females produce eggs through a process called oogenesis. • After undergoing meiosis I and II, 4 sperm are produced but only 1 egg survives.