DNA and Genetics What is DNA? How do changes in the sequence of DNA affect traits? Lesson 3 Reading Guide

The Structure of DNA Chromosomes are made of proteins and deoxyribonucleic acid, or DNA —an organism’s genetic material. A gene is a segment of DNA on a chromosome. Genes provide directions for cells to assemble chromosomes Lesson 3

The Structure of DNA (cont.) DNA is shaped like a twisted ladder which is referred to as a double helix. Lesson 3

The Structure of DNA (cont.) DNA is made of nucleotides, which are molecules made of a nitrogen base, a sugar, and a phosphate group Sugar-phosphate groups form the sides of the DNA ladder. The 4 nitrogen bases, adenine (A), cytosine (C), thymine (T), and guanine (G), bond and form the rungs of the ladder. Lesson 3

The Structure of DNA (cont.) A and T always bond together, and C and G always bonds together. Lesson 3

Replication produces two identical strands of DNA Replication is the process of copying a DNA molecule to make another DNA molecule. Replication produces two identical strands of DNA Lesson 3

Mutations Most of the time replication works properly, but occasionally there is a mistake in the copying process. A change in the nucleotide sequence of a gene is called a mutation. mutation from Latin mutare, means “to change” Lesson 3

Mutations (cont.) The 46 human chromosomes contain between 20,000 and 25,000 genes that are copied during replication. Sometimes mistakes happen during replication; most mistakes are corrected by the cell, but occasionally they cause a mutation. Mutations can also be triggered by environmental factors such as exposure to X-rays, ultraviolet light, radioactive materials, and some kinds of chemicals. Lesson 3

There are several types of mutations There are several types of mutations. Three types of mutations are substitution, insertion, and deletion mutations. Lesson 3

THE BIG CAT ATE THE FAT RAT Deletions One or more bases are deleted CCGTAG becomes CCGAG Mutations (cont.) In a deletion mutation, one or more nitrogen bases is left out of the DNA sequence THE BIG CAT ATE THE FAT RAT THE CAT ATE THE FAT RAT

Mutations (cont.) Insertions One or more bases are added CCGTAG becomes CCGTGAG In an insertion mutation, one or more nitrogen bases is added to the DNA. THE BIG CAT ATE THE FAT RAT THE BIG CAT ATE THE FAT RED RAT

Mutations (cont.) Substitution One base is replaced by another CCGTAG becomes CCGAAG In a substitution mutation, one nitrogen base is replaced by a different nitrogen base. THE BIG CAT ATE THE FAT RAT THE BIG CAT ATE THE FAT BAT

Mutations (cont.) The effects of a mutation depend on where in the DNA sequence the mutation happens and the type of mutation. Because mutations can change proteins, they can cause traits to change as well. Some mutations in human DNA cause genetic disorders. Lesson 3

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Mutations (cont.) However, not all mutations have negative effects. Some mutations have no effect at all and don’t change any traits Other mutations might cause a trait to change in a way that even benefits the organism. Lesson 3