DNA

What Do Genes Look Like? Review: Traits are physical characteristics Genes give the instructions for traits Genes are on chromosomes in the nucleus

The Pieces of the Puzzle Chromosomes are made of protein and DNA DNA stands for deoxyribonucleic acid DNA stores the instructions for making proteins on sections of making proteins on sections of DNA called genes DNA called genes Most of your traits depend on the kinds of proteins your cells the kinds of proteins your cells make make

Nucleotides—The Subunits of DNA “How can something so simple hold the key to an organism’s characteristics?” DNA is made of 4 smaller pieces that are called nucleotides. A nucleotide has a sugar, a phosphate, and a base. There are 4 different bases that each have a different shape –Adenine, thymine, guanine, and cytosine

Chargaff’s Rules 1950s Biochemist named Erwin Chargaff He found that in DNA the amount of adenine = thymine and the amount of cytosine = guanine

A Picture of DNA A chemist named Rosalind Franklin took a picture of what DNA looked like She used X-ray diffraction to take this picture She shot X-rays at DNA, they bounced back and made the picture Rosalind Franklin saw a spiral shape

Eureka! James Watson and Francis Crick built a model of DNA DNA is a twisted ladder that is called a double helix

DNA Structure The sides of the ladder are made of sugar, phosphate, sugar, phosphate, sugar, phosphate… The rungs of the ladder are made of a pair of nucleotide bases Remember, the bases always pair up cytosine-guanine and adenine-thymine This explains Chargaff’s rules that A = T and C = G

Making Copies of DNA Because adenine always pairs with thymine and cytosine always pairs with guanine, one side of a DNA molecule is complementary to the other side. Example:AACTGGCTA

Making Copies of DNA Remember mitosis and meiosis –chromosomes had to copy themselves. Chromosomes are made of protein and DNA, so how does DNA copy itself? When a DNA molecule gets ready to copy itself, it will split down the middle like a zipper. A protein called an enzyme helps unzip the DNA

Making Copies of DNA The bases on each side of the unzipped molecule are used as a pattern to build a new molecule. Two identical molecules of DNA are created. ml ml

From Trait to Gene So what exactly makes up a gene? A string of bases The bases can be put in any order so there are billions and billions of combinations. Each combination of bases is a code for a trait All living organisms have DNA and it works the same way in all of them

Putting it all Together All the cells in your body have nuclei  All the nuclei have chromosomes inside (body cells have 46, sex cells have 23)  Every chromosome is made of protein and a whoooooooole lot of DNA wrapped up  Every molecule of DNA has two sides that are twisted together  Each molecule of DNA contains thousands of genes coded in the bases

How DNA Works The bases of DNA are a code that tells your cells what to do How does the code work? When your cell reads a piece of DNA, it reads it like you would read a book: in one direction, from beginning to end

Genes and Proteins The bases (A,T,C,G) are the alphabet of the code (like the dots and dashes in Morse code) Groups of 3 bases code for an amino acid Amino acids are the pieces that make up proteins The order of the bases in the DNA determines the order of the amino acids in the protein that the cell will make So a gene is really just instructions for making a protein

Why Proteins? Proteins are found all throughout cells Proteins act as chemical messengers Proteins determine your traits like hair color, height, etc Humans have about 100,000 genes that each spell out instructions for proteins Your body has about 50,000 different kinds of proteins Proteins make up skin, hair, claws, antlers

The Making of a Protein Steps to making a protein: 1. Copy the section of DNA that has the code for the protein into a molecule of RNA –This happens inside the nucleus –RNA only has one strand instead of two –RNA has a different sugar than DNA. This sugar is called ribose –RNA has 3 of the same bases as DNA: A,G, and C, but it has uracil (U) instead of thymine (T) 2. The RNA copy of DNA goes out of the nucleus into the cytoplasm. 3. In the cytoplasm, the copy of the DNA goes through a ribosome which is a protein factory

The Making of a Protein The Making of a Protein 4. The RNA slides through the ribosome 3 bases at a time 3 bases = codon 3 bases = codon 5. Transfer molecules called tRNA read the RNA by picking up the correct amino acid and leaving it at the ribosome 6. The amino acids are hooked together to make a protein.

Amino Acid Codes Bases (The Code) Amino Acid (The Translation) TTTPhenylalanine TTALeucine TCTSerine TATTyrosine CTTLeucine CGAArginine ATGMethionine

Proteins Chains of amino acids acids Required for your body to work (and body to work (and required for all required for all living things) living things) Cells can turn genes on or off. Muscle cells use different genes and make different proteins than brain cells.

Changes in Genes Mutant Molecules: Not just in superheros! A mutation is a change in the order of bases in DNA There three kinds of mutations

Types of Mutations

Mistakes Happen Repair enzymes are normally able to fix any mistakes that are made in copying the DNA If the mistake does not get repaired, one of 3 things will happen: 1. An improvement 2. No change 3. A harmful change If the mutation is in the sex cells then it can be passed on to the offspring

How Can DNA Become Damaged (Mutated)? Random errors when DNA is being copied Some chemicals called mutagens can cause mutations Examples of mutagens: radiation from X- rays and ultraviolet rays, asbestos (found in the insulation of old homes), cigarette smoke

An Example of a Substitution Mutation Sickle cell anemia is a disease that is caused by a substitution mutation Sickled red blood cells have the wrong shape, they can get stuck in blood vessels and they don’t carry oxygen as well as regularly shaped blood cells

Some Examples of Genetic Diseases Color blindness Cystic fibrosis Down syndrome Muscular dystrophy Hemophelia Sickle cell disease