The Genetic Code of Life DNA The Genetic Code of Life

The Discovery of DNA 1928-Griffith Studied bacteria that caused pneumonia He used two strains of bacteria—one disease causing and one harmless strain. He used the bacteria in 4 ways:

Mice given disease bacteria (S) all died Mice given harmless strain (R)  all live Mice given heat killed (S)  all live Mice given both the harmless and heat killed  all die

The experiment showed that somehow the dead disease bacteria passed on the instructions of how to cause disease to the living harmless bacteria Griffith did not know what the cause of this was. He called the process “Transformation”

1944-Avery, McCarty, Macleod Repeated Griffith’s experiment to see if they could find what caused transformation They destroyed the lipids, proteins, carbohydrates and RNA in the bacteria. Transformation still occurred. They destroyed the DNA in the cell. Transformation did not occur. The experiment showed DNA transmits information from one generation to the next.

1952-Hershey and Chase Studied viruses that infect bacteria (called bacteriophages). They added radioactive tracers to the virus to see which part carried the message into a cell Added radioactive sulfur which attaches to the protein coat around the virus. Added radioactive phosphorus which attaches to DNA

Radioactive materials give off particles that can be traced Radioactive materials give off particles that can be traced. They followed both particles and found all the sulfur was outside the cell, the phosphorus was inside the cell The experiment proved it was DNA that entered cells and not protein. Radioactive P Radioactive S

Structure of DNA It is made up of repeating units called nucleotides P Each nucleotide is in turn made up of a sugar, a phosphate and a base There are 4 possible bases: A = adenine T = thymine G = guanine C = cytosine P B S

Chargaff’s work found that the bases always pair a certain way: A with T and G with C Rosalind Franklin made an x ray picture of DNA to show the spiral pattern.

It was called a “double helix”—it has 2 sides that twist. Watson and Crick—made a 3-D model of DNA. The model showed DNA was like a ladder and was twisted. It was called a “double helix”—it has 2 sides that twist. Sugar and phosphate make up the sides of the ladder Steps are the bases

DNA does 2 jobs in your cells: 1. Replication 2. Protein synthesis

1. The Process of Replication DNA is very long. To fit inside of a nucleus it is wrapped around proteins called histones. This is the chromatin material we see in the nucleus. When a cell gets ready to divide the DNA is first copied in a process called replication.

The DNA opens up down the middle of the ladder with the help of enzymes. This exposes the bases. New bases are brought in that are complementary to the ones already there. A new side forms along the old side. The two resulting strands are identical. Then the DNA is coiled tightly and looped to make the structures known as chromosomes.

2. The Process of Protein Synthesis Proteins are made at the protein factories called ribosomes. DNA cannot leave the nucleus to go to the ribosome to make the protein. DNA needs a helper molecule to do that job for it. The helper molecule is RNA.

The structure of RNA RNA is also made of nucleotides. There are 3 Differences between DNA and RNA: RNA is only one side of a ladder RNA uses a different base substituting Uracil for Thymine RNA uses a different sugar called ribose

There are 3 types of RNA found in cells: 1. transfer (tRNA) 2 There are 3 types of RNA found in cells: 1. transfer (tRNA) 2. ribosomal (rRNA) 3. messenger (mRNA)

There are 2 steps in the making of a protein: The first step is called transcription. Overall, the process of transcription is copying DNA onto mRNA. It occurs in the nucleus. Steps: DNA separates and unwinds at the gene. mRNA forms across from one side of the open DNA. Copying continues until a termination is reached.

The second step is called translation. Two words to learn first: Codon-3 bases on the mRNA each codon is for 1 amino acid Anticodon-3 corresponding bases on the tRNA pick up the correct amino acid

Overall, the process of translation is building a protein out of amino acids from the instructions on the mRNA. It occurs at the ribosome.

2. Translation: reading the code to make a protein Steps: mRNA enters a ribosome It is read 3 letters at a time (called one codon) After reading the code a tRNA with the complementary bases (called the anticodon) picks up the amino acid from the cytoplasm It is brought back to the ribosome and bonded into position. The next codon is read and another amino acid is bonded to the first one. The chain grows until a stop code is read. The amino acid chain is then released.

Mutations A mutation is a change in the base sequence on either the DNA or RNA. The mutation changes the protein. Mutations may be harmless, beneficial or harmful.

Types of gene mutations 1. Point mutations Substitution-one base is changed, changes one amino acid only. 2. Frameshift mutations-changes all amino acids after the mutation 1. Insertion adding in one base 2. Deletion-taking out one base

Chromosomal mutations 1. Deletion-part of a chromosome is broken off and lost 2. Duplication-repeating a section of a chromosome 3. Inversion-rotating the order of genes on the chromosome 4. Translocation-part of one chromosome breaks off and attaches to another one

Mutations can be caused by mutagens (things in the environment) like: Radiation (sun, x rays) Chemicals (pesticides, drugs) Pollution