Biology DNA & the Language of Life

Genes are Made of DNA Fredrick Griffith (1928) studied pneumonia strains (one was harmless while the other was pathogenic, or disease-causing) –Made non-harmful strains harmful (transformed) –Discovered the ‘transforming agent’ was genetic material Oswald Avery (1944) confirmed that the genetic material was DNA

Alfred Hershey & Martha Chase performed the ‘blender experiments’ using a virus (specifically a bacteriophage, or virus that infects a bacteria), bacteria, and radioactively labeled probes. –They concluded that DNA was the genetic material.

Structure of DNA Structure determines function. DNA is made up of nucleotides. A nucleotide is made up of a 3-carbon sugar called a deoxyribose, a phosphate group (PO 3 ) and a nitrogenous base.

There are 4 nitrogenous bases in DNA: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). A & G are purines and are larger than T & C which are pyrimidines.

Edwin Chargaff (1950’s) discovered that in different species there is always an equal number of A’s and T’s and an equal number of C’s and G’s. –These findings are Chargaff’s Rules. Adenine matches with Thymine and Cytosine matches with Guanine.

A purine matches with a pyrimidine. (a large base matches with a small base, making DNA have a uniform width throughout). Maurice Wilkins and Rosalind Franklin (1952) used x-rays to photo DNA & discovered that DNA has a spiral shape. –Unfortunately, this x-ray was taken by another scientist, James Watson who realized what the x-ray revealed.

James Watson and Francis Crick (1953) published their results (and received the Nobel Prize). –They received credit for discovering the shape of DNA. DNA is double stranded & a double helix (or twisted ladder). This double helix is formed by 2 strands of nucleotides. Each strand is composed of nucleotides: the sugar and phosphate bind to each other to form the “backbone” of the ladder while the nitrogenous bases form the “rungs”. Each rung is formed with 1 purine bonded with 1 pyrimidine (A-T or C-G). psy339evolutionarypsychologyroots/watson- crick-dna.jpg

DNA Replication: This is DNA copying itself. This occurs during S phase (synthesis) of Interphase. What happens? DNA unwinds (unzips): enzymes open the base pairs and hold the double helix apart. Each DNA strand acts as a template for DNA replication for a new complimentary strand

Nucleotides join the original strand 1 at a time DNA polymerase is the enzyme responsible for the base-pair matching This is called semi- conservative replication because the new DNA that results has 1 old strand of DNA and 1 new strand of DNA. Watson & Crick hypothesized this as well.

Gene  Protein George Beadle & Edward Tatum hypothesized ‘one gene-one enzyme’ (based on their work w/ bread mold Neurospora crassa). From this hypothesis, it has been discovered that many genes code for polypeptides (but they are specific): one gene-one polypeptide (part of a protein)

From Genotype to Phenotype Protein Synthesis is the production of a protein. –This is taking the organism’s genotype (genetic makeup) and translating into the phenotype (the physical traits). DNA is made up of nucleotides (bases). –The bases make up a gene. –Genes code for the sequence of amino acids (a.a.). –A.A. code for proteins. –Therefore, DNA is a template for making proteins. An expressed gene is a gene that codes for a protein that is synthesized. (exon) There are 2 types of nucleic acids: DNA and RNA. RNA= ribonucleic acid. RNA differs from DNA

RNA Is single stranded Contains uracil (U) instead of thyamine (T) Has a ribose instead of a deoxyribose There are 3 types of RNA: mRNA is messenger RNA tRNA is transfer RNA rRNA is ribosomal RNA

(Contains rRNA) tRNA Types of RNA:

Protein Synthesis occurs in 2 stages: 1.Transcription: DNA is copied RNA is synthesized or transcribed This occurs within the nucleus

What happens during transcription? RNA polymerase (enzyme that is responsible for RNA synthesis) separates the DNA double helix & matches RNA bases with 1 DNA template only. This is specifically making mRNA. Instead of T, U is inserted. (U binds with A, C binds with G) Only 1 RNA strand is made.

In prokaryotes, mRNA goes directly to the ribosome (there is no nucleus). In eukaryotes, mRNA is spliced. –Splicing is when noncoding regions called introns (junk DNA) are removed and coding regions called exons are sealed together. Both introns and exons are copied during transcription. After splicing, mRNA leaves the nucleus and finds a ribosome.

2. Translation: This is when nucleic acids are changed, or translated, into the language of proteins (amino acids). This involves mRNA, rRNA, and tRNA. It occurs on a ribosome (either free or attached to the ER, depending on where the protein is going).

Translation: net/BiologyPages/T/Translation.gif

What happens during translation? mRNA attaches to a ribosome. A tRNA molecule “transfers” or brings over an a.a. forming an amino acid chain; With each additional a.a. the chain grows longer. tRNA brings the correct a.a. over based on the complementary codons & anticodons (base sequences)

a codon is a base sequence on the mRNA strand. This codes for a specific a.a. An anticodon is 3 bases found on tRNA that match the codon. Example: Codon= GCU on mRNA (codes for alanine, an a.a.) Anticodon= CGA on tRNA

The Triplet Code tRNA wil ultimately be translated into amino acids. 20 amino acids & 64 codons 3 stop codons (UAA, UGA, UAG) & 1 start codon (AUG) which also codes for methinionine (met)

Mutations: Changes in Chromosomes Proteins have various functions: they may act within the cell OR serve a purpose outside of the cell. They may be activates or repressors (turning genes on or off). A mutation is a random change in the DNA (sequence of nucleotides). –This can be chromosomal mutations (which involve entire chromosomes) or gene mutations (which involve individual genes). A mutagen is an environment factor that causes a DNA mutation, like radiation and chemicals. A carcinogen is a cancer causing agent (this is also a mutagen). –These can be tars in cigarette smoke, UV radiation, and other chemicals.