chapter 6: chromosomes and cell reproduction. section 1: chromosomes how many cells do you think are...
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Chapter 6:Chromosomes and Cell Reproduction
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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
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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
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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.
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Binary Fission
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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
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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.
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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)
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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)
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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.
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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
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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.
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• 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.
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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.
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Chapter 7: Meiosisand Sexual Reproduction
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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
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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)
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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.
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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.
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