Genes and Proteins

DNA• Hershey and Chase Experiments– Showed that viruses inject their DNA into bacteria

and direct bacteria to replicate it for them• DNA not protein is genetic material

• DNA– Double helix– Made of EQUAL amounts of nucleotides: Adenine,

Guanine, Cytosine, Thymine– Each part of helix is complementary to other, run

in opposite directions– 3 prime and 5 prime ends

The Structure of DNA

DNA vs. RNA

DNA• Double helix• Backbone -

Deoxyribose• Thymine binds with

adenine• Stores code for making

RNA and proteins

RNA• Single helix• Backbone - Ribose• Uracil pairs with adenine• Types of RNA

– Messenger RNA (mRNA) – carries info. For a specific protein. Segment is codon

– Ribosomal RNA (rRNA) – combines with proteins to form ribosome

– Transfer RNA (tRNA) – connectors to bind an mRNA codon to a specific RNA

The Genetic Code• Genome – All the genetic material in cells– All different sizes depending on complexity of

organisms• Chromosome – Package of DNA and associated

proteins– You have 23 pairs or 46 total chromosomes

• Gene – sequence of DNA on a chromosome that codes for a specific protein or RNA molecule

Protein Synthesis• Transcription – Copying a gene’s DNA to a

complementary RNA molecule, occurs in nucleus

• Translation – Copying translating an mRNA strand into the language of amino acids

Transcription and Translation

Transcription• Transcription – Copying a gene’s DNA to a

complementary RNA molecule, occurs in nucleus– Just copying the “words”

• Occurs in 3 Steps– Initiation – Enzymes unzip DNA double helix, RNA

polymerase binds to promoter• Promoter – DNA sequence at the beginning of a gene

– Elongation – RNA polymerase, adds nucleotides from 3’ to 5’ end making RNA molecule

– Termination – RNA polymerase gets to termination sequence at end of gene, separates and releases new RNA molecule

Transcription

Preparation for Translation

• Prokaryotic Cells - begin translation as RNA molecule is being transcribed

• Eukaryotic Cells – mRNA can’t cross nuclear membrane– Add nucleotide cap to end of 5’ end of mRNA– Add 100-200 adenines – “poly a-tail”– Helps ribosomes attach to 5’ end of mRNA– Helps prevent degradation of mRNA– Introns removed from RNA, exons spliced together

Translation• Translate DNA/RNA language into a amino acid

language to make protein• Occurs at ribosome– Made of rRNA

• Uses– mRNA – carries codon information– tRNA – binds to mRNA and amino acid

• Functional Unit = Codon – 3 base pair “word” that coincides with an amino acid– Genetic Code

Triplet Codon

Translation

• Occurs in 3 Steps– Initiation – at 5’ end “start” codon (AUG) codes for

methionine, calls large subunit, start polypeptide– Elongation – tRNA brings 2nd amino acid, covalently

bonds with 1st amino acid, release 1st tRNA, get another tRNA and so on to make poly peptide chain

– Termination – “Stop” codon (UGA, UAG, or UAA). NO amino acid corresponds to “stop”, release factors, release last tRNA, ribosomal units separate, polypeptide chain released

Translation

Polypeptide to Protein• Polypeptide chain is NOT a protein• Folding/packaging occurs in endoplasmic reticulum

and/or Golgi apparatus• Chain folds in cytoplasm to get 3-D structure• Errors can occur– Wrong amino acid sequence “messes” up folding

• Cystic fibrosis– Error in folding with correct sequence

• Alzheimer Disease – incorrect folding of amyloid, forms mass in brain

– Error in joining polypeptide chains• Misfire in types or how joined. Hemoglobin

Expressing Proteins

Prokaryotic• No transcription factors• Expresses whole

genome at once• No introns – no splicing

needed

Eukaryotic• Uses transcription

factors• Expresses only certain

needed genes• Must splice to get rid of

introns

Mutations

• Mutation – Change in cell’s DNA sequence• Not always harmful, can lead to genetic variability• Point Mutation – changes 1 or a few base pairs in

a gene– Silent – mutation codes for same protein– Missense – mutation codes for different amino acid,

changing proteins shape (ex. Sickle cell anemia)– Nonsense – mutation codes for “stop” codon instead

of amino acid – makes shorter peptide chain

Mutations: Substitution

Mutations• Frameshift Mutation

– One or more nucleotides are added or subtracted from gene• Adds or deletes nucleotides in number other than multiple of 3

– Disrupts codon reading, alters amino acid sequence

– Insertions – Deletions– Diseases – colorectal cancer, Crohn’s, Cystic Fibrosis

• Expanding Repeats– Number of a 2 or 4 nucleotide sequence increases over several

generations• Symptoms get more and more severe• Huntington’s Disease – makes extra glutamines, makes fibrous clumps in

brain

Mutations: Deletions

Mutations

• Causes– Spontaneous – DNA replication error– Mutagens – external agent that induces mutations

• UV Radiation, x-rays, chemical weapons, nuclear energy, tobacco– During Meiosis– Transposons – jumping pieces of DNA

• Types– Germline – occurs in cells that give rise to sperms and eggs

• Things that run in families– Somatic – occurs in non-sex cells

• DOES NOT get passed on