Gene Expression: From Gene to Protein Chapter 17

YOU MUST KNOW How RNA and DNA are similar and different, and how this defines their roles The differences between replication, transcription, and translation, and the role of DNA and RNA in each process How eukaryotic cells modify RNA after transcription How genetic material is translated into polypeptides How mutations can change the amino acid sequence of a protein and be able to predict how a mutation can result in changes in gene expression

Polymer of nucleotides DNA vs. RNA DNA RNA Polymer of nuleotides Polymer of nucleotides Deoxyribose sugar Ribose sugar Double stranded Single stranded Bases = A, T, G, C Bases = A, U, G, C

Genes – sections of DNA that code for proteins Transcription Genes – sections of DNA that code for proteins Only 1 strand is transcribed Sense strand – NOT transcribed - carries same sequence as resulting mRNA (but with Ts instead of Us) Anti-sense strand – Transcribed - complimentary to sense strand and to mRNA

3 segments of the gene 1. Promoter – short sequence of bases that is not transcribed, but indicates where transcription should start a. RNA Polymerase – unzips DNA and attaches at promoter, creating a transcription bubble b. Usually indicated by TATA box

2. Transcription unit – section of the DNA that codes for RNA a 2. Transcription unit – section of the DNA that codes for RNA a. Can code for mRNA for a polypeptide or rRNA or tRNA

3. Terminator – sequence of nucleotides that cause the mRNA to detach from the DNA

3 Stages of Transcription Initiation a. In prokaryotes, RNA polymerase simply binds to promoter b. Eukaryotes – use RNA polymerase II, requires transcription factors (proteins to help it bind to promoter) – together they form the transcription initiation complex

a. Free-floating RNA nucleotides pair up with complimentary DNA Elongation a. Free-floating RNA nucleotides pair up with complimentary DNA b. New nucleotides always add on to the 3’ end (build 5’ 3’) c. RNA Polymerase bonds RNA nucleotides together

Termination At termination sequence, RNA is released, polymerase detaches, sense and antisense strand rejoin

Post-transcription modification 1. Addition of 5’ cap and poly-A tail – protect mRNA from degradation by enzymes and help mRNA attach to ribosome

RNA splicing – removal of introns and fusing of exons by spliceosomes (complex of proteins and snurps) a. Snurps (small nuclear RNA/snRNA) – RNA that works like an enzyme to catalyze the reactions b. Alternative splicing means 1 gene can code for more than one polypeptide (20,000 genes, 100,000 proteins in humans)

Translation Types of RNA involved RNA Type Function mRNA – messenger Complimentary to a gene, carries message (genetic code) from DNA in nucleus to ribosomes in cytoplasm rRNA – ribosomal Component of the ribosomes (along with proteins) tRNA – transfer Brings amino acids to the ribosome to build the polypeptide

Forms a t-shape because part of it are complimentary with itself tRNA Forms a t-shape because part of it are complimentary with itself Sequence of anticodon determines which amino acid is attached

Structure of the ribosome Consists of a large and a small subunit, each made of rRNA (2/3) and proteins (1/3) rRNA constructed in nucleolus and exits nucleus through nuclear pores mRNA will fit in between large and small subunits

3 sites A – holds tRNA carrying next amino acid P – holds tRNA carrying polypeptide E – site where deactivated tRNA exits ribosome

The Genetic Code Bases are read 3 at a time DNA – triplets mRNA – codons tRNA - anticodons Each codon codes for a specific amino acid, but there can be more than one codon for an amino acid Universal – same in all species

Initiation Small subunit attaches to mRNA and moves along it until it finds start codon (AUG) Activated tRNA hydrogen bonds to codon – anti-codon is UAC, amino acid is methionine

Large subunit joins small subunit/mRNA to form translation initiation complex Initiation factors (proteins that help the process ) join the complex – this requires GTP (energy!)

Elongation tRNAs bring in amino acids 1 at a time Elongation factors – proteins that assist tRNA binding at A site Bond holding amino acid to tRNA in P site breaks and peptide bond forms between that amino acid and the amino acid on the tRNA in the A site

Translocation – tRNA in P site, now deactivated, moves to E site and tRNA in A site moves to P site Deactivated tRNA leaves ribosome (goes to be reactivated)

Ribosome moves along mRNA in the 5’ 3’ direction, new activated tRNAs move into the A site, translocation happens repeatedly

Stop codon on mRNA moves into A site Termination Stop codon on mRNA moves into A site Release factor – protein fills the A site – has no amino acid attached Catalyzes hydrolysis between tRNA in the P site and its amino acid (which is holding the polypeptide chain) Polypeptide is released and ribosome subunits break apart

Polypeptide to Protein Polypeptides fold and associate with other molecules to gain secondary, tertiary, and quarternary structure Signal peptide – sequence of about 20 amino acids and the leading end, helps direct proteins to their destination

Point Mutations – alteration of 1 nucleotide Nucleotide – pair substitution – one nucleotide replaced with a different one Silent mutations – mutation still codes for same amino acid Missense mutation – mutation codes for a different amino acid Nonsense mutation – change amino acid to a stop

Insertions and deletions – bases added or taken out Unless 3 bases are added or deleted, it results in a frameshift mutation-shifts the reading frame for every codon after the mutation

Substances or forces that interact with DNA and cause mutations Mutagens Substances or forces that interact with DNA and cause mutations X-rays, radiation, chemicals