Chapter 12 Gene Expression

From DNA to Protein  Things to remember:  Proteins can be structural (muscles) or functional (enzymes).  Proteins are polymers of amino acids.  Each protein has a specific sequence of amino acids.  The shape of the protein determines the function of the protein.  The DNA code holds the key for the sequence of amino acids for each protein.

From DNA to Protein… DNA transcription RNA translation protein

So, we need RNA, right?  Single stranded – although it can fold back on itself for short double-stranded segments  Ribose (rather than deoxyribose) – this sugar has a hydroxyl group (OH) in the 2’ position rather than just H  Uracil (rather than thymine) – this is a pyrimidine that can form 2 hydrogen bonds  Three types: 1.messenger RNA (mRNA) – carries the code 2.transfer RNA (tRNA) – 20 different ones; folds back on itself into a particular shape which allows it to carry a specific amino acid 3.ribosomal RNA (rRNA) – globular; has catalytic properties during protein synthesis

Transcription means to copy  Transcription comes 1 st: (in the alphabet too!)  mRNA is going to copy the DNA code in the gene  DNA is split – only one strand is read – the template strand  The DNA strand that is not read is the nontemplate strand  Three DNA nucleotides are a triplet. There are 64 possible triplets that code for the 20 different amino acids.  RNA polymerase makes the mRNA by following the rules of base pairing from the sense strand of the DNA, going from 5’ of the RNA to 3’.  Each group of 3 mRNA bases are called a codon.  When RNA polymerase reaches the termination signal, transcription stops and the mRNA can leave the nucleus.

Translation converts the code to a protein  mRNA needs the help of tRNA and rRNA  mRNA binds to a ribosome (Initiation)  3 tRNA anticodons – complementary to the mRNA codons - bring the specified amino acid into position  A condensation reaction occurs to link the amino acids together (Elongation)  The bond between the amino acids is a peptide bond – the chain of amino acids is sometimes called a polypeptide  Translation continues until the end of the mRNA is reached – voila! A protein is born! (Termination)

Translation Codon 1Codon 2Codon 3Codon 4Codon 5Codon 6 Polypeptide Nontemplate strand Transcription DNA Template strand mRNA (complementary copy of template DNA strand)

Second letter UCAG First letter (5’ end) U C A G Third letter (3’ end) UCAGUCAG UCAGUCAG UCAGUCAG UCAGUCAG = Stop Codon = Start Codon

Three representations of a tRNA molecule

Ribosome structure

Ribosomes along two long mRNA molecules. Upper: Emerging nascent polypeptides that emerge from each ribosome during translation. X150,000. Credit: © Kiseleva and Donald Fawcett/Visuals Unlimited 27005

Bacterial vs. Eukaryotic Cells  In bacterial cells, transcription and translation is coupled – translation of the mRNA often begins before the 3’ end of the transcription is complete  mRNA in eukaryotic cells is modified in two ways – adding a 5’ cap and polyadenylation at the 3’ end. These probably help stabilize the mRNA

Inactive DNA segment Active DNA segment RNA polymerase Direction of RNA synthesis Active DNA segment Ribosomes Direction of protein synthesis Poly- ribosome mRNA 0.5 µm (a) (b)

Interrupted coding sequences  Eukaryotic DNA has sections of genes that do not code for a protein – introns.  The coding sections are exons  After the mRNA is transcribed, the introns must be removed and the exons spliced together before translation begins

Retroviruses  Reverse transcriptase makes a DNA molecule from RNA  Found in retroviruses  Example: HIV – 1: the virus that causes AIDS

Mutations  A change in the nucleotide sequence  Once it is in the DNA, replication will cause it to be copied over and over  Base substitution: Simplest type May cause a different amino acid Missense – altered function Nonsense – stops function  Frameshift mutation: Caused by additions or deletions Alters the reading frame for all downstream nucleotides  Mutagens – radiation, certain chemicals, that cause mutations

Mutations