Lecture 10 DNA Translation and Control

Translation Translation converts the order of the nucleotides of a gene into the order of amino acids in a protein The rules that govern translation are called the genetic code mRNAs are the “blueprint” copies of nuclear genes mRNAs are “read” by a ribosome in three-nucleotide units, termed codons Each three-nucleotide sequence codes for an amino acid or stop signal

The Genetic Code The genetic code is (almost) universal Only a few exceptions have been found

Sites play key roles in translation Ribosomes The protein-making factories of cells They use mRNA to direct the assembly of a protein A ribosome is made up of two subunits Each of which is composed of proteins and rRNA Sites play key roles in translation

Hydrogen bonding causes hairpin loops Transfer RNA Hydrogen bonding causes hairpin loops tRNAs bring amino acids to the ribosome They have two business ends Anticodon which is complementary to the codon on mRNA 3’–OH end to which the amino acid attaches 3-D shape

Making the Protein mRNA binds to the small ribosomal subunit The large subunit joins the complex, forming the complete ribosome mRNA threads through the ribosome producing the polypeptide

How translation works The process continues until a stop codon enters the A site The ribosome complex falls apart and the protein is released Play Protein Synthesis

Architecture of the Gene In eukaryotes, genes are fragmented They are composed of Exons – Sequences that code for amino acids Introns – Sequences that don’t Eukaryotic cells transcribe the entire gene, producing a primary RNA transcript This transcript is then heavily processed to produce the mature mRNA transcript This leaves the nucleus for the cytoplasm

Protect from degradation and facilitate translation Processing eukaryotic mRNA Protect from degradation and facilitate translation Different combinations of exons can generate different polypeptides via alternative splicing Play How Spliceosomes Process RNA

How protein synthesis works in eukaryotes Cytoplasm Nuclear membrane tRNA Amino acid 4. tRNA molecules become attached to specific amino acids with the help of activating enzymes. Amino acids are brought to the ribosome in the order dictated by the mRNA. 6. The polypeptide chain grows until the protetin is completed. Completed polypeptide 7. Phosphorylation or other chemical modifications can alter the activity of a protein after it is translated. 5. tRNAs bring their amino acids in at the A site of the ribosome. Peptide bonds form between amino acids at the P site, and tRNAs exit the ribosome from the E site. 5’ 3’ Ribosome Ribosome moves toward 3’ end DNA RNA polymerase Primary RNA transcript 5’ 3’ 1. In the cell nucleus, RNA polymerase transcribes RNA from DNA 5’ mRNA Cap Nuclear pore Poly-A tail 3. mRNA is transported out of the nucleus. In the cytoplasm, ribosomal subunits bind to the mRNA Small ribosomal subunit Large ribosomal subunit 5’ 3’ Cap Poly-A tail 2. Introns are excised from the RNA transcript, and the remaining exons are spliced together, producing mRNA Introns mRNA Exons Play Control of Gene Expression

Architecture of the Gene Most eukaryotic genes exist in multiple copies Clusters of almost identical sequences called multigene families As few as three and as many as several hundred genes Transposable sequences or transposons are DNA sequences that can move about in the genome They are repeated thousands of times, scattered randomly about the chromosomes

Turning Genes Off and On Genes are typically controlled at the level of transcription In prokaryotes, proteins either block or allow the RNA polymerase access to the promoter Repressors block the promoter Activators make the promoter more accessible Most genes are turned off except when needed

The lac Operon An operon is a segment of DNA that contains a cluster of genes that are transcribed as a unit The lac operon contains Three structural genes Encode enzymes involved in lactose metabolism Two adjacent DNA elements Promoter Site where RNA polymerase binds Operator Site where the lac repressor binds

The lac Operon In the absence of lactose, the lac repressor binds to the operator RNA polymerase cannot access the promoter Therefore, the lac operon is shut down

The lac Operon In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor This induces a change in the shape of the repressor which makes it fall off the operator RNA polymerase can now bind to the promoter Transcription of the lac operon is ON

The lac Operon

The lac Operon What if the cell encounters lactose, and it already has glucose? The bacterial cell actually prefers glucose! The lac operon is also regulated by an activator The activator is a protein called CAP It binds to the CAP-binding site and gives the RNA polymerase more access to the promoter However, a “low glucose” signal molecule has to bind to CAP before CAP can bind to the DNA Play Combination of Switches

Activators and repressors of the lac operon

Enhancers DNA sequences that make the promoters of genes more accessible to many regulatory proteins at the same time Usually located far away from the gene they regulate Common in eukaryotes; rare in prokaryotes

Mutation The genetic material can be altered in two ways Recombination Change in the positioning of the genetic material Mutation Change in the content of the genetic material Bithorax mutant

Mutation Mutation and recombination provide the raw material for evolution Evolution can be viewed as the selection of particular combinations of alleles from a pool of alternatives The rate of evolution is ultimately limited by the rate at which these alternatives are generated Mutations in germ-line tissues can be inherited Mutations in somatic tissues are not inherited They can be passed from one cell to all its descendants

Kinds of Mutation Mutations are caused in one of two ways Errors in DNA replication Mispairing of bases by DNA polymerase Mutagens Agents that damage DNA

Kinds of Mutation The sequence of DNA can be altered in one of two main ways Point mutations Alteration of one or a few bases Base substitutions, insertion or deletion Frame-shift mutations Insertions or deletions that throw off the reading frame

Kinds of Mutation

Kinds of Mutation The position of genes can be altered in one of two main ways Transposition Movement of genes from one part of the genome to another Occurs in both eukaryotes and prokaryotes Chromosomal rearrangements Changes in position and/or number of large segments of chromosomes in eukaryotes

Kinds of Mutation

Kinds of Mutation

Mutation, Smoking and Lung Cancer Agents that cause cancer are called carcinogens These are typically mutagens The hypothesis that chemicals cause cancer was first advanced in the 18th century Many investigations since then have determined that chemicals can cause cancer in both animals and humans For example, tars and other chemicals in cigarette smoke can cause cancer of the lung