BiologyDNA & 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 http://www.google.com/imgres

• 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. http://www.google.com/imgres

Structure of DNAStructure determines function.• DNA is made up of nucleotides.• A nucleotide is made up of a 3-carbon

sugar called a deoxyribose, a phosphate group (PO3) and a nitrogenous base.

http://www.google.com/imgres

• 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.

http://www.google.com/imgres

• 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.

http://www.google.com/imgres

• 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.

http://kentsimmons.uwinnipeg.ca/cm1504/Image265.gif

• 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).

http://www.flyfishingdevon.co.uk/salmon/year3/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

http://www.google.com/imgres

• 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.

http://www.google.com/imgres

http://library.thinkquest.org/04apr/00217/images/content/74-Summary-DNA-Replication.jpg

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

http://images1.clinicaltools.com/images/gene/dna_versus_rna_reversed.jpg

RNA • Is single stranded• Contains uracil (U) instead of thyamine

(T) • Has a ribose instead of a deoxyriboseThere are 3 types of RNA:• mRNA is messenger RNA• tRNA is transfer RNA• rRNA is ribosomal RNA

(Contains rRNA)

tRNA

Types of RNA:

http://publications.nigms.nih.gov/thenewgenetics/images/ch1_trans.jpg

Protein Synthesis occurs in 2 stages:

1.Transcription:• DNA is copied• RNA is synthesized or

transcribed• This occurs within the

nucleus

http://biology.unm.edu/ccouncil/Biology_124/Images/transcription.gif

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. http://www.google.com/imgres

• 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.

http://faculty.uca.edu/~johnc/RNA%20splicing.gif

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:

http://users.rcn.com/jkimball.ma.ultranet/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)

http://www.google.com/imgres

• 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

http://www.google.com/imgres

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)

http://repository.uwlax.edu/~Bob/assets/Code.jpg

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.