DNA The Genetic Code of Life The Discovery

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 mice died • Mice given harmless strain (R) all mice live • Mice given heat killed (S) all mice live • Mice given both the harmless and heat killed all mice die

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

• 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 pattern of the atoms.

• 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

What does DNA actually do? 1. It provides the genetic code for an organism. 2. It has a way to be copied into every new cell (called replication) 3. The code is specific instructions on how to make proteins (called protein synthesis)

1. The Process of Replication • DNA is very long. (human DNA laid end to end is 6 feet 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. • This process makes sure each new cell has a complete set of instructions.

• 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 across from each of the old sides. The two resulting strands are identical to the original. • Each new DNA is made of one new side and one old side. This is called semi-conservative • 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 (t. RNA) 2. ribosomal (r. RNA) 3. messenger (m. RNA)

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 m. RNA. It occurs in the nucleus. Steps: • DNA separates and unwinds at the gene to be copied. • m. RNA forms across from one side of the open DNA. • Copying continues until a termination code is reached.

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

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

2. Translation: reading the code to make a protein • Steps: • m. RNA enters a ribosome • It is read 3 letters at a time (called one codon) • After reading the code a t. RNA 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 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 shape of the protein which will change how it functions. • Mutations may be harmless, beneficial or harmful.

Types of mutations: • 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-pushes all the bases forward one place • 2. Deletion-taking out one base-pushes all the bases back one place

• 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 spontaneous or caused by mutagens (things in the environment) like: • Radiation (sun, x rays) • Chemicals (pesticides, drugs) • Pollution