BIOL 3301 - Genetics Ch14A - Translation I Ribosomes Students

8/18/2019 BIOL 3301 - Genetics Ch14A - Translation I Ribosomes Students

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Translation

• Translation of mRNA is the biological

polymerization of amino acids into polypeptide

chains using mRNA as a template – making proteins

• The sequence of amino acids in polypeptide is

determined by the sequence of ribonucleotides in

mRNA

• Each trinucleotide (codon codes for one amino

acid – genetic code


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• !f the genetic code consisted of a single nucleotide ore"en pairs of nucleotides per amino acid# there $ould not

be enough combinations (% and &' respecti"ely to docefor all ) amino acids*

• Triplets of nucleotide bases are the smallest units ofuniform length that can code for all the amino acids*

• !n the triplet code# three consecuti"e bases specify anamino acid# creating %+, ('% possible code $ords*

• The genetic instructions for a polypeptide chain are$ritten in -NA as a series of three.nucleotide $ords*

Genetic Code

Copyright© 2002 Pearson Education, Inc., publishing as Benjamin Cummings


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/enetic 0ode• The genetic code1

– !s $ritten in linear form

– Each group of three nucleotides specifies one amino acid –codon

– !s unambigious – one codon for one amino acid – !s degenerate . gi"en amino acid can be specified by more than

one codon

– 0ontains start AT/ in -NA .2 A3/ in mRNA and stop signals

– !s commaless

– !s nono"erlapping

– !s uni"ersal


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/enetic 0ode

• &4')s . 5arshall Nirenberg determined the first

match# that 333 coded for the amino acid

phenylalanine*


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Amino AcidsAlanine Ala A

Arginine Arg RAspartic Acid Asp D

Asparagine Asn N

Cysteine Cys C

Glutamic Acid Glu E

Glutamine Gln Q

Glycine Gly GHistidine His H

Isoleucine Ile I

Leucine Leu L

Lysine Lys K

Methionine Met M

henylalanine he !

roline ro

"erine "er "

#hreonine #hr #

#ryptophan #rp $

#yrosine #yr %

&aline &al &


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Types of RNA

• 5essenger RNA – mRNA

• Transfer RNA – tRNA

• Ribosomal RNA – rRNAa* mRNA carries the message that directs the

synthesis of proteins

b* tRNA carries amino acids during protein

synthesis

c* rRNA molecules are components of theribosomes


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Types of RNA

• 6mall nuclear RNA (snRNA – structuralcomponents of nucleosomes

• 5icro RNA (miRNA – short ).nucleotide single stranded# block e7pressionof complementary mRNAs

• All fi"e types are produced by transcription• The final products of tRNA# rRNA# snRNA

and miRNA genes are RNA molecules


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Ribosomal 6tructure

• 6ynthesis of polypeptides occurs onribosomes

• 8acterial ribosome is about 9) nmdiameter and consists of t$o subunits# largeand small

• 8oth subunits consist of one or moremolecules of rRNA and array of ribosomal proteins ( appro7imately 9)19)


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Ribosomes

• :lay t$o acti"e roles in proteinsynthesis1

– 0oordinate protein synthesis by placingmRNA# aminoacyl.tRNA and associated protein factors in their corresponding

positions – 0atalyze at least some of the reactions in

translation


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rRNA /enes

• E.coli1 se"en copies of a single sequence that

encodes all three components (,6# &'6# 96*

!nitial transcript is ,)6 RNA is clea"edenzymatically into three

• -rosophila1 &) copies per haploid genome# each

is transcribed into %96 mRNA# processed into ;6#

&;6# and 96 RNAs

• rRNA genes or r-NA represent moderately

repetiti"e -NA# present in clusters


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rRNA /enes

• Each r-NA cluster in eukaryotes consists of

tandem repeats# $ith each unit separated by

a non.coding spacer -NA – form nucleolus• !n humans# nucleoli are located on chr

&,#&%# &9# *

• 96 rRNA in humans is not part of a largertranscript# located separately on chr &


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Transfer RNA

• 0entral question1 ho$ triplet codons of

mRNA direct specific amino acids into their

correct positions in the polypeptide<• =======================$as disco"ered


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tRNA 6tructure

• RNA molecules >9.4) nucleotides long

• Transcribed as a large precursor clea"ed

into %6 tRNA

• 0lo"erleaf model of tRNA


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tRNA 6tructure


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0harging tRNA

• 8efore translation can proceed# the tRNAs must be

chemically linked to their respecti"e amino acids

• This acti"ation process is called charging oraminoacylation

• !s directed by aminoacyl tRNA synthetases

•There are at least , different tRNAs due to$obbling

• ?igh fidelity – recognize only one amino acid


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Translation –general

• Translation occurs on ribosomes $ith tRNAs ser"ing as

adaptiors bet$een the mRNA template and amino acids

• Adaptor functions of tRNA1

– ,@ end binds the amino acid

– the anticodon of the tRNA recognize the codons on the mRNA

• mRNA strand ready for translation is composed of leader#

reading frame# trailer sequences

• mRNA is read 9B to ,B*

• The codons in the mRNA specify the amino acids that are

incorporated by the tRNA into the protein being synthesized


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mRNA Ready Cor Translation<


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Translation –general

• The ribosome has three sites – The : (or peptidyl site#

– the A (or aminoacyl site

– the E (or e7it site*

• A3/ is al$ays the first codon for translation* A3/codes for methionine

• The amino acid methionine is al$ays the first aminoacid incorporated

• :roteins are synthesized from amino terminus tocarbo7yl terminus


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Dpen Reading Crames (DRCs

• /enes that code for proteins comprise open

reading frames (DRCs consisting of a series of

codons that specify the amino acid sequence of the protein that the gene codes for*

• The DRC begins $ith an initiation codon . usually

(but not al$ays AT/ . and ends $ith a

termination codon1 TAA# TA/ or T/A


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The sequence of nucleotides in mRNA can be read in three REA-!N/

CRA5E6

!n order to synthesize the correct protein# a ribosome has to kno$ $hich

reading frame to use and $here# on mRNA# to start translation


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DRCs

6i7 reading frames

BIOL 3301 - Genetics Ch14A - Translation I Ribosomes Students

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