From DNA to Proteins Chapter 13 Central

From DNA to Proteins Chapter 13

Central Dogma • DNA • RNA • Protein

Steps from DNA to Proteins 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

Three Classes of RNAs • Messenger RNA (m. RNA) – Carries protein-building instruction • Ribosomal RNA (r. RNA) – Major component of ribosomes • Transfer RNA (t. RNA) – Delivers amino acids to ribosomes

DNA vs RNA • 1) Sugar (ribose) differs – DNA sugar is deoxyribose (missing O) – RNA sugar is ribose • 2) Possible bases differ – DNA uses bases A, C, G, T – RNA uses bases A, C, G, U • 3) DNA stable as double-stranded structure RNA stable as single-stranded structure

A Nucleotide Subunit of RNA

Base Pairing during Transcription • A new RNA strand can be put together on a DNA region according to basepairing rules • As in DNA, C-G, but A-T does not exist • Uracil (U) pairs with adenine (A)

Transcription & DNA Replication • Like DNA replication – Nucleotides added to growing RNA strand in the 5’ to 3’ direction • Unlike DNA replication – Small in length compared to DNA – RNA polymerase makes RNA from DNA – Product is a single strand of RNA

Base Pairing Compared

Promoter - place where RNA polymerase binds • 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 region

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

Adding Nucleotides direction of transcription 3´ 5´ 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 (20) kinds of amino acids are specified by 61 codons • Most amino acids can be specified by more than one codon • i. e. six codons specify leucine – UUA, UUG, CUU, CUC, CUA, CUG

Near-Universal Genetic Code AUG AAA CGA UGA

t. RNA Structure - brings 20 different amino acids to ribosome There at least 20 different populations of t. RNAs inside the cytoplasm of the cell.

Ribosomes - structures where proteins are made

Three Stages of Translation Initiation Elongation Termination

Initiation • Initiator t. RNA binds to small ribosomal subunit • Small subunit/t. RNA complex attaches to m. RNA and moves along it to an AUG “start” codon • Large ribosomal subunit joins complex

Binding Sites on Large Subunit binding site for m. RNA P (first binding site for t. RNA) A (second binding site for t. RNA)

Elongation • m. RNA passes through ribosomal subunits • t. RNAs deliver amino acids to the ribosomal binding site in the order specified by the m. RNA • 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 m. RNA 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 lot of ribosomes • A cluster of many ribosomes translating one m. RNA transcript • Transcript threads through the multiple ribosomes like thread of bead necklace • Why? - Allows rapid synthesis of proteins

What Happens to the New Polypeptides (proteins)? • 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 (small) Insertions (small or large) Deletions (small or large)

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 - example of substitution & deletion

Transposons - naughty things! • 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 10 -4 and 10 -6 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 • Genetic defects to genes that repair mutations leave mutations behind in large numbers.