The Code of Life Topic 3 Gene expression

The Code of Life: Topic 3 Gene expression (protein synthesis)

What's in your genes? • Genes are sequences of nucleotides along DNA strands. • Genes (100 s-1000 s of nucleotides long) code for polypeptides. • Your genotype is the actual sequence of DNA that you inherited from your parents. • The expression of those genes results in your phenotype, or how you look.

The big question… How do we go from this DNA to this? An organism

The Central Dogma DNA RNA Polypeptide Protein Universal genetic code of life

RNA • There are 3 major structural differences between RNA and DNA. • There are 3 types of RNA involved in gene expression (protein synthesis): – m. RNA – r. RNA – t. RNA

Types of RNA m. RNA r. RNA t. RNA

Gene expression (protein synthesis) Overview

Transcription: copying DNA's message • DNA is stored safely in the nucleus, but proteins are made in the cytoplasm. • RNA carries the instructions for proteins out of the nucleus to the cytoplasm. • The first step is transcription.

Stages of transcription • Three stages: – Initiation – Elongation – Termination • Important notes: – RNA polymerase does all the work – The end product is called a primary RNA transcript – The primary transcript is then modified before leaving the nucleus.

Practice transcribing… If your DNA strand says this… TACAGGTCAGACTTC What will your complementary RNA strand say? AUGUCCAGUCUGAAG

Before the RNA transcript can leave the nucleus, it has to be processed. Step 1: Cap and tail 5’ cap (GTP): 3’ poly-A tail: • Provides stability to the m. RNA • Point of attachment for a small • Appears to control the subunit of the ribosome during movement of m. RNA across the translation. nuclear membrane.

Step 2: RNA splicing 1. removes introns 2. joins exons, creating an m. RNA molecule with a continuous coding sequence.

After transcription comes translation (protein synthesis) Transcription Translation

Review: Types of RNA m. RNA r. RNA t. RNA

Translation • During translation, the code carried in m. RNA is "translated" into amino acids. • There are 4 nucleotides and 20 amino acids. • Clearly, the nucleotides must be combined somehow in 20 different ways. • m. RNA is "read" as codons, combinations of 3 nucleotides. • One codon, AUG, always signals that start of a gene sequence. • Three codons (UAA, UAG, and UGA) are stop signals, ending the formation of a polypeptide. Take a moment to look at your chart. What observations can you make about this code?

Translating practice • What amino acid does each of the following codons correspond to? AAA Lysine • Try these with the GCA Alanine wheel: UGU Cysteine CCC Proline CAG Glutamine AGU Serine UCA GUC Serine Valine

t. RNA: a closer look

Translation: Initiation • The 5' cap of m. RNA attaches to a a small ribosome subunit. • The initiator t. RNA has the anticodon for the start codon (AUG) on m. RNA. • The initiator t. RNA always carries the amino acid methionine (MET). • After the initiator t. RNA hydrogen bonds to the m. RNA, a large ribosomal subunit also attaches.

Translation: Elongation • Amino acids are added on sequentially when the appropriate t. RNA matches with the next m. RNA codon. • Each new t. RNA bonds its anticodon to the complementary codon on the m. RNA. • The amino acid from the old t. RNA gets passed to the new amino acid on the new t. RNA. They form a peptide bond.

Translation: Termination • Once the stop codon of a sequence is reached, the whole complex comes apart and there is now a new polypeptide.

Summary

Imagine the first line is the message in DNA and the rest of the lines are the same message carried in RNA. What is going on? Pinpoint SPECIFIC errors. How do those errors affect the message? DNA message thesunwashotbuttheoldmandidnotgethishat m. RNA message the sun was hot but the old man did not get his hat versions (as codons) the sun was hot but the ole man did not get his hat the sun was hot but the old man did not get his cat the sun was hot but the old ma. did not get his hat thd esu nwa sho tbu tth eol dma ndi dno tge thi sha t

Mutations • Mutation - any change in the DNA sequence • Causes of mutations: – Errors in DNA replication (permanent error; 1 in every 100, 000 bases) – Mutagens • UV light • Radiation • Chemicals • There are 2 major categories of mutations: – Point mutations – Frameshift mutations

A base pair substitution is when the wrong nucleotide is substituted for the correct one. These mutations may be: silent (no effect, usually last nucleotide in a codon) missense (results in a different amino acid, effect depends on the properties of the new amino) nonsense (premature stop codon) m. RNA transcribed from mutated DNA Point mutations are base pair substitutions. m. RNA transcribed from normal DNA

m. RNA transcribed from normal DNA m. RNA transcribed from mutated DNA • Frameshift mutation – Insertion – Deletion • In this case the entire sequence after the insertion or deletion is shifted by a whole nucleotide. • After the mutation, none of the amino acids are correct**. • Result: non-functioning polypeptide. ** RARE case of three nucleotides being deleted.

http: //www-mic. ucdavis. edu/sklab/genetic%20 recomb. htm