第十三章 從DNA到蛋白質 前言

Central dogma transcription translation 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

種類:

如何由DNA轉錄RNA RNA的種類 轉錄作用的特性 轉錄產物RNA的潤飾

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

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

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

Transcript Modification 轉錄產物RNA的潤飾 Transcript Modification unit of transcription in a DNA strand 3’ exon intron exon intron exon 5’ transcription into pre-mRNA poly-A tail cap 5’ 3’ snipped out snipped out 5’ 3’ mature mRNA transcript

mRNA的解碼 何謂遺傳密碼? tRNA與 rRNA的結構與功能

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的結構與功能

rRNA的結構與功能 large ribosomal subunit intact ribosome small ribosomal subunit

mRNA如何被轉譯 轉譯作用的階段 新合成多胜鏈的可能遭遇

Three Stages of Translation Initiation Elongation Termination

ELONGATION INITIATION binding site for mRNA intact ribosome P (first binding site for tRNA) A (second binding site for tRNA) INITIATION mRNA transcript Fig. 13.10a, p. 206-7

Fig. 13.10b, p. 206-7

Fig. 13.10c, p. 206-7

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 A (second binding site for tRNA) P (first 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

突變與蛋白質合成的關係 常見的基因突變與來源 基因突變的原因 蛋白質上突變的證據

Base-Pair Substitutions Gene Mutations Base-Pair Substitutions Insertions Deletions

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

Frameshift Mutations Insertion Deletion Both shift the reading frame 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

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 10-4 and 10-6 per gene per generation Only mutations that arise in germ cells can be passed on to next generation