Nucleic Acids

Describe the structure of DNA (6)

• polymer with anti-parallel strands made up of nucleotides

• nucleotide containing: phosphate group, pentose sugar, nitrogenous base

• 4 bases: A-T and C-G which are complimentary pairs

• hydrogen bonds in between bases 

• phosphodiester bond between sugar + phosphate to form backbone 

• glycosidic bond between sugar + base

What are nucleic acids

• deoxyribonucleic acid (DNA): acts as the information store

• ribonucleic acid (RNA): reads & translates information

What are the 4 enzymes involved in DNA replication

• DNA gyrase + DNA helicase (scissors)

• DNA polymerase (builds)

• DNA ligase (glues)

Explain why complimentary base-pairing is important in DNA replication

• so DNA is replicated without error, producing the same sequences of nucleotides

• reduces the occurrence of mutations

Describe the process of DNA replication

• original DNA double helix unwinds using DNA gyrase

• DNA helicase causes the 2 strands of DNA to unzip, breaking the hydrogen bonds

• both strands act as templates

• free DNA nucleotides in nucleoplasm bind to the template strands by complementary base pairing

• A--T C---G , weak H bonds reform

• DNA polymerase rejoins the sugar phosphate backbone using phosphodiester bonds via condensation reaction

• each molecule contains one strand of the original DNA and the other strand of the new DNA (SEMI-CONSERVATIVE)

What is the template strand

• the nucleotides only pair with bases on one strand of the DNA molecule

• this is known as the template strand and is used to produce the mRNA molecule

• when A pairs, it’ll pair with uracil (U)

How does RNA polymerase move across the strand

• RNA polymerase moves along the template strand in the 3’ to 5’ direction

• this means the mRNA molecule grows in the 5’ to 3’ direction

What is the coding strand

• non-template strand

• the base sequence of the coding strand will be the SAME as the base sequence as the mRNA apart from T which will be replaced with U

What are okazaki fragments

• the other strand of DNA is replicated discontinuously in the opposite direction with the formation of a series of short DNA segments called okazaki fragments

• this is called the lagging strand

How did Meselson and Stahl use the replication of DNA in E.Coli to prove the semi-conservative theory

• they used isotopes of N15 and N14 (isotopes of N), growing bacteria on the heavy N

• after growth the DNA was spun in centrifuge to distribute the mass

• the bacteria was then grown on a light only containing medium

• the data showed that the DNA contained both N15 and N14 within the DNA and none of the DNA contained only one of the isotopes 

What happens to the DNA strands after each division?

• overtime the 14N was the most present in the sample as there is less heavy 15N

What are purines

• Adenine and Guanine

• contain a double ring structure as there are two carbon-nitrogen rings joined together

What are pyrimidines

• Thymine, Cytosine and Uracil

• contain a single ring structure as there is only one carbon-nitrogen ring

• smaller than purines

What are the differences between DNA and RNA

• RNA is single stranded

• RNA contains uracil instead of thymine (A,U,C,G)

• RNA has a ribose sugar (OH is below the Carbon-2)

• RNA is a shorter polynucleotide chain

What are the 3 types of RNA

• messenger (mRNA)

• ribosomal (rRNA)

• transfer (tRNA)

What is mRNA

• single polynucleotide strand formed in nucleus during transcription

• it is complimentary to the template strand (OG DNA molecule)

• carries genetic code from DNA → cytoplasm → ribsome

• split into codons

What is the genetic code

• sequence of base triplets (codons) which codes for specific amino acids

• each base triplet is read in sequence, separate from the triplet before and after it

What is rRNA

• produced in nucleolus, found in ribosomes

• forms 2 subunits in ribosome to allow mRNA and tRNA to bind

• catalyses the formation of peptide bonds between amino acids as the polypeptide chain forms

What is the exact structure of tRNA

• single polynucleotide strand

• folds over itself to form a clover-leaf shape, held in place by H bonds

• has amino acid attachment site at the top (trio of nucleotide bases)

• has anti-codon at the loop (base triplet which is complimentary to a specific mRNA codon)

What is the role of tRNA

• made in nucleolus

• carries specific amino acids to ribosomes

• the amino acids it carries are bonded together to form a polypeptide

What is the nature of the genetic code

• universal

• degenerate

• non-overlapping

What is meant by a universal genetic code

• the same triplet of DNA bases code for the same amino acids in almost all living organisms

What is meant by a degenerate genetic code

• for all amino acids (except two) there is more than one base triplet which codes for it

• four different bases means there are 64 different codons (4×4×4) including 1 start codon and 3 stop codons

• this reduces the effect of point mutations

What do start and stop codons do

• a start codon is at the beginning of the sequence, if it is in the middle it codes for methioine

• 3 stop codons do not code for any amino acids and signal the end of the sequence

What is meant by a non-overlapping genetic code

• having a single start codon to start the sequence ensures the codons are read ‘in frame’

• it is read starting from a fixed point in groups of three bases

• base triplets do not share their bases

• if a base is added or deleted it causes a frame shift (mutation)

Describe the process of transcription

• DNA gyrase causes a section of the DNA molecule to unwind

• strands unzip by DNA helicase, breaking the hydrogen bonds

• one strand acts as a template

• free RNA nucleotides bind to the template strand by complementary base pairing

• A--U C---G

• RNA polymerase joins adjacent nucleotides forming the sugar phosphate backbone

• mRNA detaches from the DNA and leaves through nuclear pore

Describe the process of translation

• mRNA moves and attaches to a ribosome

• ribosome reads the mRNA in codons

• complementary anti-codon at tRNA binds to the codon via hydrogen bonds

• two tRNAs can be bound at once with an amino acid attached

• peptide bond is formed between amino acids

• first tRNA leaves and the next one binds - repeats until a stop codon