From DNA to Proteins Chapter 13

Byssus: Marvelous Mussel Adhesive Mussels live in the surf zone and are subjected to continual pounding by waves Mussel binds itself to rocks with threads coated with the protein bysuss

Same two steps produce ALL proteins: 1) DNA is transcribed to form RNA –Occurs in the nucleus –RNA moves into cytoplasm 2) RNA is translated to form polypeptide chains, which fold to form proteins Steps from DNA to Proteins

Three Classes of RNAs Messenger RNA –Carries protein-building instruction Ribosomal RNA –Major component of ribosomes Transfer RNA –Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA phosphate group sugar (ribose) uracil (base)

Base Pairing During Transcription A new RNA strand can be put together on a DNA region according to base- pairing rules As in DNA, C pairs with G Uracil (U) pairs with adenine (A)

Transcription & DNA Replication Like DNA replication –Nucleotides added in 5’ to 3’ direction Unlike DNA replication –Only small stretch is template –RNA polymerase catalyzes nucleotide addition –Product is a single strand of RNA

Promoter A base sequence in the DNA that signals the start of a gene For transcription to occur, RNA polymerase must first bind to a promoter

Gene Transcription transcribed DNA winds up again DNA to be transcribed unwinds mRNA transcript RNA polymerase

Adding Nucleotides growing RNA transcript 5’ 3’ 5’ 3’ direction of transcription

Transcript Modification unit of transcription in a DNA strand exonintron mature mRNA transcript poly-A tail 5’ 3’ snipped out exon intron cap transcription into pre-mRNA 3’5’

Genetic Code Set of 64 base triplets Codons –Nucleotide bases read in blocks of three 61 specify amino acids 3 stop translation

Code Is Redundant Twenty kinds of amino acids are specified by 61 codons Most amino acids can be specified by more than one codon Six codons specify leucine –UUA, UUG, CUU, CUC, CUA, CUG

tRNA Structure codon in mRNA anticodon in tRNA amino acid OH tRNA molecule’s attachment site for amino acid

Ribosomes tunnel small ribosomal subunitlarge ribosomal subunitintact ribosome

Three Stages of Translation Initiation Elongation Termination

Initiation Initiator tRNA binds to small ribosomal subunit Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon Large ribosomal subunit joins complex

Binding Sites on Large Subunit binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA)

Elongation mRNA passes through ribosomal subunits tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA Peptide bonds form between the amino acids and the polypeptide chain grows

Elongation

Termination A stop codon in the mRNA moves onto the ribosomal binding site No tRNA has a corresponding anticodon Proteins called release factors bind to the ribosome mRNA and polypeptide are released

Polysome A cluster of many ribosomes translating one mRNA transcript Transcript threads through the multiple ribosomes like the thread of bead necklace Allows rapid synthesis of proteins

What Happens to the New Polypeptides? Some just enter the cytoplasm Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

Overview Transcription Translation mRNA rRNAtRNA Mature mRNA transcripts ribosomal subunits mature tRNA

Gene Mutations Base-Pair Substitutions Insertions Deletions

Effect of Base-Pair Substitution original base triplet in a DNA strand As DNA is replicated, proofreading enzymes detect the mistake and make a substitution for it: a base substitution within the triplet (red) One DNA molecule carries the original, unmutated sequence The other DNA molecule carries a gene mutation POSSIBLE OUTCOMES: OR

Frameshift Mutations Insertion –Extra base added into gene region Deletion –Base removed from gene region Both shift the reading frame Result in many wrong amino acids

Frameshift Mutation ARGININEGLYCINETYROSINETRYPTOPHANASPARAGINE ARGININEGLYCINELEUCINEGLUTAMATELEUCINE mRNA PARENTAL DNA amino acid sequence altered mRNA BASE INSERTION altered amino acid sequence

Transposons DNA segments that move spontaneously about the genome When they insert into a gene region, they usually inactivate that gene

Mutation Rates Each gene has a characteristic mutation rate Average rate for eukaryotes is between and per gene per generation Only mutations that arise in germ cells can be passed on to next generation