DNA and the genetic codeDNA and the genetic code

DNA is found in the DNA is found in the chromosomes in the chromosomes in the nucleus in eukaryotic cells nucleus in eukaryotic cells or in the cytoplasm in or in the cytoplasm in prokaryotic cells. prokaryotic cells.

Each chromosome contains one molecule of DNA bound to proteins.Each chromosome contains one molecule of DNA bound to proteins.

DNA and RNA are called NUCLEIC ACIDS. DNA and RNA are called NUCLEIC ACIDS. DNA is only found in the nucleus.DNA is only found in the nucleus. RNA is made in the nucleus but functions in the RNA is made in the nucleus but functions in the

cytoplasmcytoplasm

They carry the genetic code which They carry the genetic code which determines the synthesis of proteins determines the synthesis of proteins since they contain the code which since they contain the code which determines the order of the amino acids determines the order of the amino acids in each protein that a cell makes. in each protein that a cell makes.

They are chemically polymers called They are chemically polymers called POLYNUCLEOTIDES because they are POLYNUCLEOTIDES because they are composed of many NUCLEOTIDES.composed of many NUCLEOTIDES.

DNA Structure

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Structure of DNAStructure of DNA DNA DNA = = DDeoxyriboeoxyribonnucleic ucleic AAcidcid It is a POLYNUCLEOTIDE It is a POLYNUCLEOTIDE i.e. a polymer of ‘building blocks’ called i.e. a polymer of ‘building blocks’ called

NUCLEOTIDES.NUCLEOTIDES. A nucleotide is a complex molecule composed ofA nucleotide is a complex molecule composed of A phosphate groupA phosphate group A pentose sugarA pentose sugar An organic, nitrogen-containing baseAn organic, nitrogen-containing base

Structure of a nucleotideStructure of a nucleotide

phosphate group

deoxyribose(pentose sugar)

Organic nitrogen-containing base

(often referred to as a

nitrogenous base or simply a base

This one is thymine)

BasesBases

In DNA there are FOUR different In DNA there are FOUR different types of basetypes of base

Adenine = AAdenine = A Thymine = TThymine = T Cytosine = CCytosine = C Guanine = GGuanine = G

Nucleotides join together by condensation reactions between the phosphate group of one nucleotide with the hydroxyl group of the next

sugar molecule to form a polynucleotide strand.

The sequence of the different bases determines the primary structureA polynucleotide strand can be composed of millions of nucleotides!.

Note: alternate sugar – phosphate groups make up the ‘backbone’ of the strand and the bases project from this

Bond between a phosphate group and a sugar molecule formed by a condensation

reaction (so water is eliminated)

Structure of the DNA Structure of the DNA moleculemolecule

DNA is DNA is actually a actually a very large very large molecule molecule made from made from TWO TWO polynucleotidpolynucleotide strands e strands joined joined togethertogether

The strands are held The strands are held together by hydrogen together by hydrogen bonds between the bonds between the bases of the two anti-bases of the two anti-parallel strandsparallel strands

The The arrangement of arrangement of the nucleotides the nucleotides cause the cause the molecule to fold molecule to fold to form a spiral to form a spiral and the two and the two strands twist strands twist around each around each otherother

Hence DNA is Hence DNA is referred to as referred to as ‘the double ‘the double helix’helix’

The bases on one strand The bases on one strand can only pair up in a can only pair up in a particular and specific way particular and specific way with the bases on the other with the bases on the other strandstrandAdenine + Thymine A + TAdenine + Thymine A + TCytosine + Guanine G + CCytosine + Guanine G + C

The bases are The bases are complementary to each complementary to each otherotherThis is the base pair rule This is the base pair rule and the bases showand the bases show ‘ ‘base complementaritybase complementarity

The base pairing ruleThe base pairing rule

The sequence of bases in one strand of The sequence of bases in one strand of DNA is DNA is

A T C C C T G A G G T C A G T A T C C C T G A G G T C A G T

What is the complementary base What is the complementary base sequence on the corresponding strand?sequence on the corresponding strand?

A T C C C T G A G G T C A G T A T C C C T G A G G T C A G T T A G G G A C T C C A G T C AT A G G G A C T C C A G T C A

How the genetic code How the genetic code works.works.

DNA contains information in the form of a chemical code to control the growth, development and activities of cells: this is called the genetic code.

Each DNA molecule (which makes up a chromosome) contains the genetic code for a large number of proteins.

The region of a DNA molecule which codes for the synthesis of one particular protein is called a gene.

Therefore one DNA molecule – and hence a chromosome – consists of many genes.

Key concept:Key concept:

It is a It is a triplet codetriplet code. . A sequence of 3 bases codes for one A sequence of 3 bases codes for one

amino acid and is called a amino acid and is called a codon.codon. One codonOne codon codes for codes for one amino acid one amino acid

only (e. g. AAA = phenylalanine) i.e. it is only (e. g. AAA = phenylalanine) i.e. it is specific.specific.

The code is The code is universaluniversal: the same triplet : the same triplet codes for the same amino acid in all codes for the same amino acid in all organisms.organisms.

Why a triplet code?Why a triplet code? If one base coded for one type of amino acid, If one base coded for one type of amino acid, only 4 amino acids could be found in proteinsonly 4 amino acids could be found in proteins – – but there are 20.but there are 20. If two bases coded for one type of amino acidIf two bases coded for one type of amino acid there would be 4there would be 422 = 16 amino acids in proteins = 16 amino acids in proteins – – but there are 20but there are 20 If 3 bases coded for one type of amino acid If 3 bases coded for one type of amino acid there could 4there could 433 = 64 amino acids in proteins = 64 amino acids in proteins – – but there are only 20!but there are only 20! but it is not a problem but it is not a problem because several codons can be used to code for because several codons can be used to code for

one type of amino acidone type of amino acid

The degenerate codeThe degenerate code

there are actually 64 possible codons there are actually 64 possible codons - most amino acids are coded for by - most amino acids are coded for by more than one codonmore than one codon

There is one ‘There is one ‘start’start’ codon (AUG) codon (AUG) There are 3 codons which do not There are 3 codons which do not

code for any amino acid and are code for any amino acid and are called ‘called ‘stopstop’ codons.’ codons.

This is actually the genetic code in RNA, which is how the genetic dictionary is usually shown.

The code is The code is non-non-overlappingoverlapping

The codons are The codons are ‘read’ individually ‘read’ individually and in sequence (just and in sequence (just like reading like reading the fat the fat cat satcat sat…) …)

i.e. CTACTC is only i.e. CTACTC is only read as two codons, read as two codons, CTA and CTC (rather CTA and CTC (rather than CTA, TAC, ACT than CTA, TAC, ACT etc if the code was etc if the code was overlapping).overlapping).

As an example the hormone insulin, As an example the hormone insulin, which is made from 51 amino acids, which is made from 51 amino acids, is coded for by a gene with 52 is coded for by a gene with 52 codons – 51 codons for the amino codons – 51 codons for the amino acids (including the start codon) + 1 acids (including the start codon) + 1 stop codon.stop codon.

Fact: DNA also contains some sequences Fact: DNA also contains some sequences of bases in its structure, which do not code of bases in its structure, which do not code for anything at all: these are called for anything at all: these are called non-non-coding DNAcoding DNA. Many genes in the DNA . Many genes in the DNA contain more codons than there are amino contain more codons than there are amino acids in the protein they code – the non-acids in the protein they code – the non-coding base sequences are called coding base sequences are called intronsintrons – but these are omitted in the mRNA which – but these are omitted in the mRNA which takes the code out of the nucleus. In fact takes the code out of the nucleus. In fact > 80% of the DNA is non-coding and is > 80% of the DNA is non-coding and is sometimes referred to as ‘junk DNA’ sometimes referred to as ‘junk DNA’ because it as no known function.because it as no known function.

How the genetic code works.How the genetic code works.

DNADNA transcriptiontranscription mRNA mRNA translationtranslation proteinprotein

too big too big to to leave leave nucleunucleuss

sequence of codons in DNA gene copied as sequence of codons in mRNA

small, single stranded molecule which leaves nucleus, attaches to ribosome

sequence of codons in mRNA translated into sequence of amino acids

amino acids joined together to form the protein

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Intro/Overview/Transcription

Transcription.Transcription. DNA in nucleus DNA in nucleus

unwinds in region of unwinds in region of the gene to be copied the gene to be copied (controlled by (controlled by enzymes)enzymes)

DNA strands separate DNA strands separate along the hydrogen along the hydrogen bonds.bonds.

RNA nucleotides pair up RNA nucleotides pair up with their with their complimentary bases (A complimentary bases (A + U, C + G); this only + U, C + G); this only happens on one strand happens on one strand (called the ‘template (called the ‘template strand’) which is strand’) which is complimentary to the complimentary to the coding strand).coding strand).

RNA polymeraseRNA polymerase joins joins the nucleotides the nucleotides together to form a together to form a single complimentary single complimentary polynucleotide strand polynucleotide strand (copy of just one gene) (copy of just one gene) with is mRNA. with is mRNA.

The strand which is The strand which is transcribed is called the transcribed is called the TEMPLATE STRAND TEMPLATE STRAND

NB: A pairs with U in RNA

Comparison of DNA and Comparison of DNA and RNARNA

DNADNA PolynucleotidePolynucleotide Very large moleculeVery large molecule Long livedLong lived Double strandedDouble stranded Sugar is Sugar is

deoxyribosedeoxyribose Has Has bases bases A + C + GA + C + G

HasHas T T

RNARNA PolynucleotidePolynucleotide Much smallerMuch smaller Short livedShort lived Single strandedSingle stranded Sugar is Sugar is riboseribose Has bases Has bases A + C + GA + C + G HasHas U U

The mRNA diffuses out of the nucleus The mRNA diffuses out of the nucleus

through the nuclear pores through the nuclear pores

into the cytoplasm.into the cytoplasm.

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Intro/Overview/translation

Translation.Translation.

a small organelle made of rRNA and a small organelle made of rRNA and proteins found in large numbers in proteins found in large numbers in the cytoplasm or attached to an the cytoplasm or attached to an internal membrane system called the internal membrane system called the rough endoplasmic reticulumrough endoplasmic reticulum

RER

The mRNA The mRNA molecule molecule attaches to a attaches to a ribosomeribosome

Amino acids are brought Amino acids are brought from the cytoplasm to from the cytoplasm to the ribosomes attached the ribosomes attached to a specific tRNA to a specific tRNA moleculemolecule

tRNA is a small single tRNA is a small single stranded polynucleotide stranded polynucleotide which has a specific which has a specific sequence of 3 bases sequence of 3 bases called the called the anticodon anticodon which is complimentary which is complimentary to the codon for the to the codon for the specific amino acid specific amino acid which the tRNA carries. which the tRNA carries.

e.g. the anticodon UUU is complimentary to e.g. the anticodon UUU is complimentary to the codon AAA which is specific for the amino the codon AAA which is specific for the amino acid phenylalanineacid phenylalanine

mRNA binds to the ribosomethe first two codons are positioned on the ribosome

tRNA molecules with complimentary anticodons bind with the first two mRNA codons

this gets the first two amino acids of the protein in the right place in its primary structure

a peptide bond forms between these two amino acids (controlled by enzymes, requires energy from respiration)

a peptide bond forms between these two amino acids (controlled by enzymes, requires energy from respiration)

the third mRNA codon is now on the ribosome

the tRNA with a complimentary anticodon binds to it, bringing the third amino acid into position

a peptide bond forms between the second and third amino acids

the mRNA moves along by one codon at a time and the process continues to add amino acids to increase the length of the growing polypeptide chain

the process continues until the STOP codon is in position on the ribosome

there is no complimentary tRNA for this codon so translation ceases and the completed polypeptide chain is released into the RER

Transported to Golgi body where it folds into its specific tertiary structure to

form the functional protein.

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Transcription and translation animation

SummarySummary(this is what examiners expect)(this is what examiners expect)

TranscriptionTranscription

DNA unwinds at geneDNA unwinds at gene hydrogen bonds brokenhydrogen bonds broken RNA nucletides line up on single RNA nucletides line up on single

template strand followng base template strand followng base complementaritycomplementarity

RNA polymerase joins them togetherRNA polymerase joins them together to form a single strand of mRNA to form a single strand of mRNA

which diffuses out of the nucleuswhich diffuses out of the nucleus

SummarySummary (this is what examiners expect)(this is what examiners expect)

TranslationTranslation mRNA attaches to ribosomemRNA attaches to ribosome tRNA picks up specific amino acidtRNA picks up specific amino acid tRNA binds to mRNA by matching codon tRNA binds to mRNA by matching codon

and anticodonand anticodon gets right amino acids in the correct placesgets right amino acids in the correct places peptide bonds formedpeptide bonds formed process continues until stop codonprocess continues until stop codon polypeptide releasedpolypeptide released polypetide folds into tertiary structurepolypetide folds into tertiary structure

TEST YOURSELFTEST YOURSELF

On which type of RNA would you findOn which type of RNA would you find A) a codonA) a codon Ans: mRNAAns: mRNA B) an anticodonB) an anticodon Ans: tRNAAns: tRNA

What would be the sequence of bases on a length of What would be the sequence of bases on a length of RNA built using the following DNA template strand?RNA built using the following DNA template strand?

T A C A T G G A T T C C G A T T A C A T G G A T T C C G A T Ans: Ans: A U G U A C C U A A G G C U A A U G U A C C U A A G G C U A

The sequence of bases AGT from a triplet The sequence of bases AGT from a triplet code on the sense strand. :What is code on the sense strand. :What is i) its triplet code on the antisense (= i) its triplet code on the antisense (= template) strandtemplate) strandAns:Ans: TCA TCAii) Its codonii) Its codonAnsAns: AGU: AGUiii) Its anticodoniii) Its anticodonAns: Ans: UCAUCA

How many tRNA molecules would be involved in How many tRNA molecules would be involved in the synthesis of the protein coded for by this the synthesis of the protein coded for by this section of DNA?section of DNA?

T A C A T G G A T T C C G A T T A C A T G G A T T C C G A T

Ans: Ans: 5 5

What are the tRNA anticodons, assuming you What are the tRNA anticodons, assuming you read the section of DNA from left to right?read the section of DNA from left to right?

T A C A T G G A T T C C G A T T A C A T G G A T T C C G A T

Ans: Ans: UAC; AUG; GAU; UCC; GAUUAC; AUG; GAU; UCC; GAU