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

What's in your genes? Genes are sequences of nucleotides along DNA strands. Genes (100s-1000s 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.

How do we go from thisto this? The big question… DNAAn 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): –mRNA –rRNA –tRNA

Types of RNA mRNArRNAtRNA

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

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

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

After transcription comes translation (protein synthesis) TranscriptionTranslation

Review: Types of RNA mRNArRNAtRNA

Translation During translation, the code carried in mRNA is "translated" into amino acids. There are 4 nucleotides and 20 amino acids. Clearly, the nucleotides must be combined somehow in 20 different ways. mRNA 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 GCA UGU CAG Lysine Alanine Cysteine Glutamine Try these with the wheel: CCC AGU UCA GUC Proline Serine Valine

tRNA: a closer look

Translation: Initiation The 5' cap of mRNA attaches to a a small ribosome subunit. The initiator tRNA has the anticodon for the start codon (AUG) on mRNA. The initiator tRNA always carries the amino acid methionine (MET). After the initiator tRNA hydrogen bonds to the mRNA, a large ribosomal subunit also attaches.

Translation: Elongation Amino acids are added on sequentially when the appropriate tRNA matches with the next mRNA codon. Each new tRNA bonds its anticodon to the complementary codon on the mRNA. The amino acid from the old tRNA gets passed to the new amino acid on the new tRNA. 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? thesunwashotbuttheoldmandidnotgethishat 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 DNA message mRNA message versions (as codons)

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

Point mutations are base pair substitutions. 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) mRNA transcribed from normal DNA mRNA 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. mRNA transcribed from normal DNA mRNA transcribed from mutated DNA