Advanced BiologyAdvanced Biology

Transcription and TranslationTranscription and Translation

RNA differs from DNA RNA differs from DNA

12.1

DNA DNA Protein Protein

An organisms DNA codes for specific traits by An organisms DNA codes for specific traits by dictating the synthesis of proteinsdictating the synthesis of proteins

The process by which DNA directs protein The process by which DNA directs protein synthesis, synthesis, gene expressiongene expression includes two includes two stages, called transcription and translationstages, called transcription and translation

DNA DNA RNA RNA proteinprotein

12.2

Eukaryotes vs. ProkaryotesEukaryotes vs. Prokaryotes

12.3

CodonsCodons

12.4

CodonsCodons

12.4

Codon Reading FrameCodon Reading FrameAUG is always the AUG is always the “start” codon so all “start” codon so all polypeptides begin with polypeptides begin with Methionine when they Methionine when they are synthesizedare synthesized

Having a consistent start Having a consistent start codon is necessary so codon is necessary so that the reading frame is that the reading frame is always the same.always the same.

12.5

TranscriptionTranscription

The synthesis of RNA from The synthesis of RNA from DNADNA

Promoters and TerminatorsPromoters and Terminators

Are sequences of DNAAre sequences of DNA

Promoters signal the initiation of RNA Promoters signal the initiation of RNA synthesis synthesis (TATA)(TATA)

Terminators signal the end of RNA synthesis Terminators signal the end of RNA synthesis (AAUAAA in eukaryotes)(AAUAAA in eukaryotes)

12.6

Three StepsThree Steps

12.7

Alteration of mRNA EndsAlteration of mRNA Ends

Each end of a pre-mRNA molecule is Each end of a pre-mRNA molecule is modified in a particular waymodified in a particular way– The 5The 5 end receives a modified nucleotide end receives a modified nucleotide

capcap– The 3The 3 end gets a poly-A tail end gets a poly-A tail

Figure 17.9

A modified guanine nucleotideadded to the 5 end

50 to 250 adenine nucleotidesadded to the 3 end

Protein-coding segment Polyadenylation signal

Poly-A tail3 UTRStop codonStart codon5 Cap 5 UTR

AAUAAA AAA…AAA

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATIONRibosome

Polypeptide

G P P P

5 3

12.8

Split Genes and RNA SplicingSplit Genes and RNA Splicing

RNA splicingRNA splicing– Removes introns and joins exons Removes introns and joins exons

Figure 17.10

TRANSCRIPTION

RNA PROCESSING

DNA

Pre-mRNA

mRNA

TRANSLATION

Ribosome

Polypeptide

5 CapExon

Intron

1

5

30 31Exon Intron

104 105 146Exon

3Poly-A tail

Poly-A tail

Introns cut out andexons spliced together

Codingsegment

5 Cap1 146

3 UTR3 UTR

Pre-mRNA

mRNA

12.8

Proteins often have a modular Proteins often have a modular architecturearchitecture

Consisting of discrete structural and functional Consisting of discrete structural and functional regions called domainsregions called domains

In many cases In many cases different exons code for the ifferent exons code for the different domains in a proteindifferent domains in a protein

GeneDNA

Exon 1 Intron Exon 2 Intron Exon 3

Transcription

RNA processing

Translation

Domain 3

Domain 1

Domain 2

Polypeptide12.8

TranslationTranslation

The RNA directed synthesis The RNA directed synthesis of a polypeptideof a polypeptide

Translation - Translation - an overviewan overview

tRNAtRNA

12.9

RibosomesRibosomes

12.10

InitiationInitiation

12.11

12.11

ElongationElongation

TerminationTermination

12.11

PolyribosomesPolyribosomes

12.12

Post TranslationPost Translation

Protein structure is determined by amino Protein structure is determined by amino acid sequence and modificationsacid sequence and modifications

Modifications includeModifications include– The attachment of certain sugars, lipids, or The attachment of certain sugars, lipids, or

phosphate groupsphosphate groups– Joining different subunits of the protein to Joining different subunits of the protein to

create the quaternary structurecreate the quaternary structure

12.13

Point MutationsPoint MutationsThe change of a single nucleotide in the DNA’s template strand

12.14

Two TypesTwo Types

SubstitutionsSubstitutions– Replacing one or more base with othersReplacing one or more base with others

Insertions or DeletionsInsertions or Deletions– Adding or removing one or more baseAdding or removing one or more base

12.14