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 DNADNA stands for deoxyribonucleic acidDNA stores the instructions for

making proteins on sections of DNA called genes

Most of your traits depend on the kinds of proteins your cells 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 Rules1950s

Biochemist named Erwin Chargaff

He found that in DNA the amount of adenine = thymine and the amount of cytosine = guanine

A Picture of DNAA chemist named Rosalind Franklin took a picture of what DNA looked likeShe used X-ray diffraction to take this pictureShe shot X-rays at DNA, they bounced back and made the pictureRosalind 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 basesRemember, the bases always pair up cytosine-guanine and adenine-thymineThis explains Chargaff’s rules that A = T and C = G

Making Copies of DNABecause 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.

http://www.johnkyrk.com/DNAreplication.html

From Trait to Gene

So what exactly makes up a gene? A string of basesThe bases can be put in any order so there are billions and billions of combinations. Each combination of bases is a code for a traitAll 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 nucleiAll 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 upEvery molecule of DNA has two sides that are twisted togetherEach 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 acidAmino acids are the pieces that make up proteinsThe order of the bases in the DNA determines the order of the amino acids in the protein that the cell will makeSo 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

4. The RNA slides through the ribosome 3 bases at a time 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)

TTT Phenylalanine

TTA Leucine

TCT Serine

TAT Tyrosine

CTT Leucine

CGA Arginine

ATG Methionine

Proteins

Chains of amino acids

Required for your body to work (and required for all living things)

Cells can turn genes on or off. Muscle cells use different genes and make different proteins than brain cells.

Changes in GenesMutant 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 HappenRepair enzymes are normally able to fix any mistakes that are made in copying the DNAIf the mistake does not get repaired, one of 3 things will happen:1. An improvement2. No change3. A harmful changeIf 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