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

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 cytoplasm RNA is made in the nucleus but functions in the cytoplasm They carry the genetic code which determines the synthesis of proteins since they contain the code which determines the order of the amino acids in each protein that a cell makes. They carry the genetic code which determines the synthesis of proteins since they contain the code which determines the order of the amino acids in each protein that a cell makes. They are chemically polymers called POLYNUCLEOTIDES because they are composed of many NUCLEOTIDES. They are chemically polymers called POLYNUCLEOTIDES because they are composed of many NUCLEOTIDES.

DNA Structure

Structure of DNA DNA DNA = Deoxyribonucleic Acid = Deoxyribonucleic Acid It is a POLYNUCLEOTIDE It is a POLYNUCLEOTIDE i.e. a polymer of ‘building blocks’ called NUCLEOTIDES. i.e. a polymer of ‘building blocks’ called NUCLEOTIDES. A nucleotide is a complex molecule composed of A nucleotide is a complex molecule composed of A phosphate group A phosphate group A pentose sugar A pentose sugar An organic, nitrogen-containing base An organic, nitrogen-containing base

Structure 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)

Bases In DNA there are FOUR different types of base In DNA there are FOUR different types of base Adenine = A Adenine = A Thymine = T Thymine = T Cytosine = C Cytosine = C Guanine = G Guanine = 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 structure A 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 molecule DNA is actually a very large molecule made from TWO polynucleotide strands joined together DNA is actually a very large molecule made from TWO polynucleotide strands joined together

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

The arrangement of the nucleotides cause the molecule to fold to form a spiral and the two strands twist around each other The arrangement of the nucleotides cause the molecule to fold to form a spiral and the two strands twist around each other Hence DNA is referred to as ‘the double helix’ Hence DNA is referred to as ‘the double helix’

The bases on one strand can only pair up in a particular and specific way with the bases on the other strand Adenine + Thymine A + T Cytosine + Guanine G + C The bases are complementary to each other This is the base pair rule and the bases show ‘base complementarity ‘base complementarity The base pairing rule

The sequence of bases in one strand of DNA is The sequence of bases in one strand of 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 sequence on the corresponding strand? What is the complementary base 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 A T A G G G A C T C C A G T C A

How the genetic code 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: It is a triplet code. It is a triplet code. A sequence of 3 bases codes for one amino acid and is called a codon. A sequence of 3 bases codes for one amino acid and is called a codon. One codon codes for one amino acid only (e. g. AAA = phenylalanine) i.e. it is specific. One codon codes for one amino acid only (e. g. AAA = phenylalanine) i.e. it is specific. The code is universal: the same triplet codes for the same amino acid in all organisms. The code is universal: the same triplet codes for the same amino acid in all organisms.

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 proteins only 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 acid If two bases coded for one type of amino acid there would be 4 2 = 16 amino acids in proteins there would be 4 2 = 16 amino acids in proteins – but there are 20 – but 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 4 3 = 64 amino acids in proteins there could 4 3 = 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 one type of amino acid because several codons can be used to code for one type of amino acid

The degenerate code there are actually 64 possible codons - most amino acids are coded for by more than one codon there are actually 64 possible codons - most amino acids are coded for by more than one codon There is one ‘start’ codon (AUG) There is one ‘start’ codon (AUG) There are 3 codons which do not code for any amino acid and are called ‘stop’ codons. There are 3 codons which do not code for any amino acid and are called ‘stop’ codons.

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

The code is non- overlapping The code is non- overlapping The codons are ‘read’ individually and in sequence (just like reading the fat cat sat…) The codons are ‘read’ individually and in sequence (just like reading the fat cat sat…) i.e. CTACTC is only read as two codons, CTA and CTC (rather than CTA, TAC, ACT etc if the code was overlapping). i.e. CTACTC is only read as two codons, CTA and CTC (rather than CTA, TAC, ACT etc if the code was overlapping).

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

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

How the genetic code works. DNA  transcription  mRNA  translation  protein too big to leave nucleus 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

tein%20Synthesis%20-%20long.html Intro/Overview/Transcription

Transcription. DNA in nucleus unwinds in region of the gene to be copied (controlled by enzymes) DNA in nucleus unwinds in region of the gene to be copied (controlled by enzymes) DNA strands separate along the hydrogen bonds. DNA strands separate along the hydrogen bonds.

RNA nucleotides pair up with their complimentary bases (A + U, C + G); this only happens on one strand (called the ‘template strand’) which is complimentary to the coding strand). RNA nucleotides pair up with their complimentary bases (A + U, C + G); this only happens on one strand (called the ‘template strand’) which is complimentary to the coding strand). RNA polymerase joins the nucleotides together to form a single complimentary polynucleotide strand (copy of just one gene) with is mRNA. RNA polymerase joins the nucleotides together to form a single complimentary polynucleotide strand (copy of just one gene) with is mRNA. The strand which is transcribed is called the TEMPLATE STRAND The strand which is transcribed is called the TEMPLATE STRAND

NB: A pairs with U in RNA

Comparison of DNA and RNA DNA Polynucleotide Polynucleotide Very large molecule Very large molecule Long lived Long lived Double stranded Double stranded Sugar is deoxyribose Has bases A + C + G Sugar is deoxyribose Has bases A + C + G Has T Has T RNA RNA Polynucleotide Polynucleotide Much smaller Much smaller Short lived Short lived Single stranded Single stranded Sugar is ribose Sugar is ribose Has bases A + C + G Has bases A + C + G Has U Has 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.

tein%20Synthesis%20-%20long.html Intro/Overview/translation

Translation. a small organelle made of rRNA and proteins found in large numbers in the cytoplasm or attached to an internal membrane system called the rough endoplasmic reticulum a small organelle made of rRNA and proteins found in large numbers in the cytoplasm or attached to an internal membrane system called the rough endoplasmic reticulum RER The mRNA molecule attaches to a ribosome The mRNA molecule attaches to a ribosome

Amino acids are brought from the cytoplasm to the ribosomes attached to a specific tRNA molecule Amino acids are brought from the cytoplasm to the ribosomes attached to a specific tRNA molecule tRNA is a small single stranded polynucleotide which has a specific sequence of 3 bases called the anticodon which is complimentary to the codon for the specific amino acid which the tRNA carries. tRNA is a small single stranded polynucleotide which has a specific sequence of 3 bases called the anticodon which is complimentary to the codon for the specific amino acid which the tRNA carries. e.g. the anticodon UUU is complimentary to the codon AAA which is specific for the amino acid phenylalanine e.g. the anticodon UUU is complimentary to the codon AAA which is specific for the amino acid phenylalanine

mRNA binds to the ribosome the 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)

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.

Transcription and translation animation

Summary (this is what examiners expect) Transcription DNA unwinds at gene DNA unwinds at gene hydrogen bonds broken hydrogen bonds broken RNA nucletides line up on single template strand followng base complementarity RNA nucletides line up on single template strand followng base complementarity RNA polymerase joins them together RNA polymerase joins them together to form a single strand of mRNA which diffuses out of the nucleus to form a single strand of mRNA which diffuses out of the nucleus

Summary (this is what examiners expect) Translation mRNA attaches to ribosome mRNA attaches to ribosome tRNA picks up specific amino acid tRNA picks up specific amino acid tRNA binds to mRNA by matching codon and anticodon tRNA binds to mRNA by matching codon and anticodon gets right amino acids in the correct places gets right amino acids in the correct places peptide bonds formed peptide bonds formed process continues until stop codon process continues until stop codon polypeptide released polypeptide released polypetide folds into tertiary structure polypetide folds into tertiary structure

TEST YOURSELF

On which type of RNA would you find On which type of RNA would you find A) a codon A) a codon Ans: mRNA Ans: mRNA B) an anticodon B) an anticodon Ans: tRNA Ans: tRNA What would be the sequence of bases on a length of RNA built using the following DNA template strand? What would be the sequence of bases on a length of 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: A U G U A C C U A A G G C U A Ans: A U G U A C C U A A G G C U A

The sequence of bases AGT from a triplet code on the sense strand. :What is i) its triplet code on the antisense (= template) strand Ans: TCA Ans: TCA ii) Its codon Ans: AGU Ans: AGU iii) Its anticodon Ans: UCA Ans: UCA

How many tRNA molecules would be involved in the synthesis of the protein coded for by this section of DNA? How many tRNA molecules would be involved in the synthesis of the protein coded for by this 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: 5 Ans: 5 What are the tRNA anticodons, assuming you read the section of DNA from left to right? What are the tRNA anticodons, assuming you 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: UAC; AUG; GAU; UCC; GAU Ans: UAC; AUG; GAU; UCC; GAU