ocr a level biology nucleic acids and nucleotides

nucleic acid definition

large polymers formed from many nucleotides

nucleotides definition

monomers consisting of the elements carbon, hydrogen, oxygen, nitrogen and phosphorus

nucleotide structure

a pentose sugar, phosphate group and nitrogenous base

polynucleotide definition

nucleotide monomers bonded together in a chain

producing polynucleotides

condensation reaction between nucleotide monomers

phosphodiester bond

the covalent bond between two nucleotide monomers (at the hydroxyl group of one and the third carbon of another)

sugar phosphate backbone

the long chain of phosphodiester bonds in a polynucleotide

four nitrogenous bases

adenine, guanine, cytosine, thymine

two types of nitrogenous bases

purines and pyrimidines

purine definition

larger base containing double carbon ring structures, e.g adenine and guanine

pyrimidine definition

smaller base containing a single carbon ring structure, e.g cytosine and thymine

nitrogenous bases are said to be

complementary

adenine is complementary with

thymine

cytosine is complementary with

guanine

guanine is complementary with

cytosine

thymine is complementary with

adenine

DNA structure

consists of two strands of polynucleotides held together by hydrogen bonding between the complementary bases coiling the molecule into a double helix

antiparallel

DNA polynucleotide strands run in opposite directions (this allows the base sequence to be transcribed)

gene definition

section of DNA that contains a complete sequence of codons coding for an amino acid sequence of a protein

RNA structure

(similar structure to DNA) a single stranded polynucleotide transcribed from DNA with the nitrogenous base uracil (U) in place of thymine

RNA function

(as DNA can't leave the nucleus) RNA carries transcribed genetic material from DNA to sites of protein synthesis

DNA replication purpose

assures that newly divided cells have a complete set of identical genetic information

semi-conservation replication

1. double helix structure is unwound by enzyme DNA helices by breaking the hydrogen bonds between the complementary bases

2. new free DNA nucleotides pair with their adjacent complementary base, forming new hydrogen bonds

3. enzyme,e DNA polymerase catalyses the formation of phosphodiester bonds between the nucleotides

4. this produces two molecules of DNA, each composed of old and new nucleotides (making it semi-conservative)

reasoning for semi-conservative replication

maintains an original strand of DNA in the nucleus so it can act as a template

evidence for semi-conservative replication

bacteria was grown in nitrogen 15 isotope before moving to nitrogen 14, the nitrogen 14 was used with the nitrogen 15 already in the bacteria's DNA to synthesise new DNA molecules, over generations layers of N14N14 formed above the N15N14 layer showing semi-conservative replication

genetic code definition

the base sequence coding for primary structure of proteins

codon definition

a series of three nucleotides that determine a single amino acid (also known as a triplet)

anticodon definition

the series of three nucleotides complementary to a codon

genetic code as universal

all organisms use this same code although the individual base sequences will differ

genetic code as non-overlapping

each codon is read as discreet units

genetic code as degenerate

there are four nitrogenous bases to code for 20 amino acids, meaning there must be 64 combinations (4x4x4) with start and stop codons to indicate the specific base sequence needed.

as there is 64 combinations for only 20 amino acids, many amino acids can be coded for by more than one codon

protein synthesis overview

the transcription of a gene into RNA molecules to be translated into a specific amino acid sequence

transcription

1. section of DNA containing the specific gene is unwound by DNA helicase at the start codon

2. the sense strand of DNA contains the code for the protein and the antisense strand is complementary to the sense strand, the antisense strand acts as a template so the base sequence is the same as the sense strand

3. enzyme RNA polymerase forms phosphodiester bonds between the transcribed nucleotides forming mRNA

4.once transcription ends at the stop codon, the mRNA molecule detaches from the DNA template (allowing DNA double helix to reform) and mRNA leaves the nucleus through a nuclear pore and travels to a ribosome

ribosome structure

composed of two subunits, a larger one and a small one produced by ribosomal (r)RNA

rRna function

produces ribosomes, maintains the structural stability of protein synthesis sequence and catalyses the reaction

three processes of translation

initiation, elongation and termination

translation initiation

1. mRNA binds to the small subunit

2. a tRNA molecule with the complementary anticodon binds to the mRNA start codon, carrying the amino acid methionine

3. the large subunits binds, delivering a second amino acid (the large subunit has a P site which binds to the growing peptide chain and an A site which binds to the incoming tRNA)

4. the second amino acid binds to the correct adjacent codon and a peptide bond forms between the two adjacent amino acids

5. the covalent bond between methionine and its tRNA molecule is hydrolysed and the tRNA molecule detaches and the ribosome moves along the mRNA to the next adjacent codon

translation elongation

6. a new tRNA molecule carrying another amino acid binds to the adjacent codon

7. a peptide bond forms between the second and third amino acid and another tRNA molecules detaches and the ribosome moves along another codon

8. this process continues until the end codon, producing a polypeptide chain of amino acids bonded together by peptide bonds

translation termination

9. once the ribosome reaches the stop codon, a releasing factor is coded for (by the stop codon) instead of another tRNA molecule carrying another amino acid

10. the bond between the final amino acid and tRNA molecule is hydrolysed, releasing the completed synthesised polypeptide

11. the components of the translation complex are disassembled

protein synthesis after translation

1. polypeptide folds into its secondary and tertiary structure

2. it undergoes further modification in the golgi apparatus

3. completed polypeptide is transported in a vesicle to the cell surface membrane

ATP function

universal energy currency in all living cells

ATP stands for

adenosine triphosphate

ATP structure

nitrogenous base, ribose sugar, 3 phosphate groups

ATP releases energy by

releasing one of the phosphate groups in a hydrolysis reaction

ATP turns into

ADP (adenosine diphosphate)

phosphorylation

the addition of the phosphate group to ADP to form ATP again, another example of a condensation reaction

kinase

enzyme that catalyses phosphorylation reaction