Transcript Pre-mRNA

From DNA to Protein
(DNA로부터 단백질까지)
Chapter 15
Cell Cycle
RNA, Protein
Mitosis, Cytokinesis
Lamin
H1
Abl
Cyc B/A
CDK1
DNA, RNA,
Protein
Cyc A
CDK2
G2 M
3-4 h 1 h
S
6-8 h
Cyc D’s G0
CDK4,6
G1
6-12 h
RNA, Protein
Cyc E
CDK2
p53
pRb
Eric Niederhoffer
SIU-SOM
진핵세포 유전자의 전사 과정 그림.
그림 15.1.해양 홍합 Mytilus와 서식지 [족사 (byssus; 점착성 물질) 분비].
15.1 The Connection between
DNA, RNA, and Protein
 Proteins are specified by genes
 The pathway from gene to polypeptide involves
transcription and translation
 Genetic code is written in three-letter words
using a four-letter alphabet
Do Genes Encode Proteins?
 1896: Garrod studied alkaptonuria disease
(알켑톤뇨증)
• Caused by an “inborn error of metabolism”
 1940s: Beadle and Tatum studied Neurospora
crassa orange bread mold (붉은빵곰팡이)
• Used X-rays to produce nutritional mutants
(auxotrophs)(영양요구성 돌연변이)
• Each mutant had defective gene for enzyme
needed to synthesize a particular nutrient
Do Genes Encode Proteins?
 One gene–one enzyme hypothesis
• Direct relationship between genes and enzymes
 One gene–one polypeptide hypothesis
• Not all proteins are enzymes
• Functional proteins sometimes contain one or
more subunits (polypeptides)
• Different genes encode each distinct subunit
그림 15.2-1. 유전자와 효소의 관계.
그림 15.2-2.
Genes to Proteins
 Transcription
• Nucleotide sequence in DNA is copied into a
complementary sequence in an RNA molecule
• Template strand of DNA is used to create
messenger RNA (mRNA)
 Translation
• Sequence of nucleotides in mRNA molecule
specifies amino acid sequence in polypeptide
• Ribosome assembles the amino acid sequence
그림 15.3. 원핵세포(a)와 진핵세포(b)에서의 전사와 번역.
Genetic Code
 Information
• 4 nucleotide bases in DNA or RNA sequences
• DNA: A,T,G,C
RNA: A,U,G,C
• 20 different amino acids in polypeptides
 Code
• One-letter words: only 4 combinations
• Two-letter words: only 16 combinations
• Three-letter words: 64 combinations
Genetic Code
 DNA
• Three-letter code: triplet (3자암호)
 RNA
• Three-letter code: codon (코돈)
그림 15.4. 유전자와 mRNA의 암호 그리고
폴리펩티드의 아미노산 서열과의 관계.
Features of the Genetic Code (1)
 Sense codons
• 61 codons specify amino acids
• Most amino acids specified by several codons
(degeneracy or redundancy)
• Ex: CCU, CCC, CCA, CCG all specify proline
 Start (시작) codon or initiator (개시) codon
• First amino acid recognized during translation
• Specifies amino acid methionine
Features of the Genetic Code (2)
 Stop (정지) codons or termination (종결)
codons
• End of a polypeptide-encoding mRNA sequence
• UAA, UAG, UGA
 Commaless
• Nucleic acid codes are sequential
• No commas or spaces between codons
• Start codon AUG establishes the reading frame
(번역틀)
그림 15.5. 유전암호.
Genetic Code is Universal
 Same codons specify the same amino acids in
all living organisms and viruses
• Only a few minor exceptions
 Genetic code was established very early in the
evolution of life and has remained unchanged
15.2 Transcription: DNA-Directed RNA
Synthesis
 RNA polymerases work like DNA polymerases,
but require no primer
 Specific sequences of nucleotides in DNA
indicate where transcription of a gene begins
and ends
Transcription
 Information coded in DNA is transferred to a
complementary RNA copy
 Similar to DNA replication, except:
•
•
•
•
•
Only one DNA strand used as a template
Only transcribes one gene at a time
RNA polymerases used to form RNA strands
Resulting RNA molecules are single strands
Uracil replaces thymine
RNA Polymerases
 No primers needed to start complementary copy
 RNA is made in the 5´→ 3´ direction
• DNA template strand is 3´→ 5´
그림 15.6. 전반적인
전사과정.
Organization of a Gene
 Promoter
• Control sequence initiates transcription
 Transcription unit (전사단위)
• Portion of gene that is copied into RNA
 Terminator (종결자)
• Signals the end of transcription of a gene
그림 15.7. 유전자의
일반적인 구조와 RNA
전사
Termination
 Termination of transcription differs in eukaryotes
15.3 Production of mRNAs in Eukaryotes
 Eukaryotic protein-coding genes are transcribed
into precursor-mRNAs (pre-mRNA; 전구체
mRNA) that are modified in the nucleus
 Introns are removed during pre-mRNA
processing to produce the translatable mRNA
 Introns contribute to protein variability (다양성)
Messenger RNA
 Prokaryotes
• Coding region flanked by 5´ and 3´ untranslated
regions
 Eukaryotes
• Coding region flanked by 5´ and 3´ untranslated
regions (as in prokaryotes)
• Additional noncoding elements
Pre-mRNA (1)
 Precursor-mRNA (pre-mRNA)
• Must be processed in nucleus to produce
translatable mRNA
 5´ cap
• Reversed guanine-containing nucleotide
• Site where ribosome attaches to mRNA
 Poly(A) tail
• 50 to 250 adenine nucleotides added to 3´ end
• Protects mRNA from RNA-digesting enzymes
Pre-mRNA (2)
 Introns
• Non-protein-coding sequences in the pre-mRNA
• Must be removed before translation
 Exons
• Amino acid coding sequences in pre-mRNA
• Joined together sequentially in final mRNA
그림 15.8. 진핵세포 단백질
암호화 유전자, 전구체 mRNA
그리고 가공된 mRNA의 관계
mRNA Splicing (1)
 Introns in pre-mRNAs removed
 Spliceosome (이어맞추기복합체?)
• Pre-mRNA
• Small ribonucleoprotein particles
(snRNP)(‘snurps’, 스너프스)
• Small nuclear RNA (snRNA) + several proteins
mRNA Splicing (2)
 snRNPs
•
•
•
•
Bind to introns
Loop introns out of the pre-mRNA,
Clip (자르다) the intron at each exon boundary
Join adjacent exons together
그림 15.9. spliceosome에서의
인트론의 제거와 엑손의 연결
Why are Introns Present?
 Alternative splicing (선택적 이어맞추기)
• Different versions of mRNA can be produced
 Exon shuffling (엑손 뒤섞기)
• Generates new proteins
Alternative Splicing
 Exons joined in different combinations to
produce different mRNAs from the same gene
 Different mRNA versions translated into different
proteins with different functions
 More information can be stored in the DNA
그림 15.10. 평활근과 가로무늬근에서의 알파
트로포미오신 mRNA의 alternative splicing
Exon Shuffling (엑손 뒤섞기)
 Intron-exon junctions often occur between major
functional regions in encoded protein
 Exon shuffling mixes protein regions or domains
into novel combinations
• Allows evolution of new proteins more quickly and
efficiently than random mutations
15.4 Translation: mRNA-Directed
Polypeptide Synthesis
 tRNAs are small, highly specialized RNAs that
bring amino acids to the ribosome
 Ribosomes are rRNA-protein complexes that
work as automated protein assembly machines
 Translation initiation brings the ribosomal
subunits, an mRNA, and the first aminoacyltRNA together
15.4 (cont.)
 Polypeptide chains grow during the elongation
stage of translation
 Termination releases a completed polypeptide
from the ribosome
 Multiple ribosomes simultaneously translate a
single mRNA
15.4 (cont.)
 Newly synthesized polypeptides are processed
and folded into finished form
 Finished proteins contain sorting signals that
direct them to cellular locations
 Base-pair mutations can affect protein structure
and function
Translation Overview
 Assembly of amino acids into polypeptides
 Occurs on ribosomes
 P, A, and E sites on ribosome used for stepwise
addition of amino acids to polypeptide as
directed by mRNA
그림 15.11. 번역의 전체 과정.
tRNAs
 Transfer RNAs (tRNA)
• Bring specific amino acids to ribosome
• Cloverleaf shape
 Bottom end of tRNA contains anticodon
sequence that pairs with codon in mRNAs
그림 15.12. tRNA 구조.
Wobble Hypothesis
 61 different sense codons do not require 61
different tRNAs
• First two nucleotides of anticodon and codon
must match exactly
• Third nucleotide has more flexibility
 Example: tRNA carrying phenylalanine
• Matches codons UUU and UUC
 Example: tRNA carrying glutamine
• Matches codons CAA and CAG
Aminoacylation
 Adds amino acid to tRNA
• Aminoacyl-tRNA (amino acid linked to tRNA)
• Aminoacyl-tRNA synthetases catalyze reaction
AA + ATP → AA-AMP + 2 Pi
AA-AMP + tRNA → AA-tRNA + AMP
그림 15.13. 아미노아실화(charging).
그림 15.14. 리보솜 구조.
Translation Stages
 Initiation (개시)
• Ribosome assembled with mRNA molecule and
initiator methionine-tRNA
 Elongation (신장)
• Amino acids linked to tRNAs added one at a time
to growing polypeptide chain
 Termination (종결)
• New polypeptide released from ribosome
• Ribosomal subunits separate from mRNA
그림 15.15. 진핵세포에서의
번역 개시의 단계들.
Elongation (1)
 Aminoacyl-tRNA matching the next codon
enters A site
 Peptidyl transferase catalyzes formation of
first peptide bond and cleaves tRNA in P site
Elongation (2)
 Ribosome moves along mRNA to next codon
• Empty tRNA moves from P site to E site, then
released
• Newly formed peptidyl-tRNA moves from A site
to P site
• A site empty again
그림 15.16. 번역 신장과정의
단계들.
그림 15.17-1. 번역 종결과정의 단계.
그림 15.17-2.
그림 15.18. 하나의 mRNA를 번역하고
있는 일련의 리보솜으로 구성된 폴리솜.
그림 15.19. 동시에 일어나는 원핵생물의 전사와 번역.
Polypeptide Processing
 Processing reactions convert polypeptides into
finished form
• Removal of one or more amino acids from the
protein chains
• Addition of organic groups
• Folding guided by chaperones
• Alternative pathways to different mature
polypeptides
Sorting Signals
 Proteins are distributed within cells by sorting
signals (분류 신호)
 Signals are coded in DNA, appear when protein
is made
Sorting Signals in ER (1)
 Proteins sorted at rough endoplasmic reticulum
 Signal peptide (signal sequence)
• At beginning of polypeptide chain
 Signal recognition particle (SRP)
• Binds to signal peptide
Sorting Signals in ER (2)
 SRP receptor
• SRP binds to protein receptor in ER membrane
• Growing polypeptide pushed inside ER lumen
 Signal peptidase
• Removes signal sequence
 Translation continues until polypeptide complete
그림 15.20. 단백질을 ER로 보내는 신호.
Mutations
 Changes in genetic material
 Base-pair mutations change DNA triplet
• Results in change in mRNA codon
• May lead to changes in the amino acid sequence
of the encoded polypeptide
그림 15.21. 단백질 암호화 유전자
내의 돌연변이가 폴리펩티드의
아미노산 서열에 미치는 영향.
Sickle-Cell Disease
 Caused by a single missense mutation
그림 15.22. 헤모글로빈의 두 폴리펩티드 중의
하나에서의 유전자 내 과오 돌연변이.