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: Meiosisand 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 VariationRandom 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.