From DNA to Proteins Chapter 13

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 (mRNA) –Carries protein-building instruction Ribosomal RNA (rRNA) –Major component of ribosomes Transfer RNA (tRNA) –Delivers amino acids to ribosomes

A Nucleotide Subunit of RNA

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

Base Pairing Compared

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 promoter region

Gene Transcription DNA template winding up DNA template unwinding newly forming RNA transcript DNA template at selected transcription site

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

Transcript Modification

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

Near-Universal Genetic Code

tRNA Structure

Ribosomes

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 moves into the area where the chain is being built. It is the signal to release the mRNA transcript from the ribosome. The new polypeptide chain is released from the ribosome. It is free to join the pool of proteins in the cytoplasm or to enter rough ER of the endomembrane system. The two ribosomal subunits now separate also.

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

Gene Mutations Base-Pair Substitutions Insertions Deletions

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

Mutations in Genes

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

Causes of Mutations Exposure to harmful radiation and chemicals in the environment can cause DNA mutations