2.8-2.14 : nucleotides and nucleic acids

describe how polynucleotide strands form

condensation reactions between nucleotides form phosphodiester bonds (sugar-phosphate backbone)

describe the structure of DNA

double helix of 2 deoxyribose polynucleotide strands (so there are 2 sugar phosphate backbones)

H-bonds between complementary base pairs on opposite strands (AT & GC)

describe the structure of RNA

single stranded, made up of ribose mononucleotides, linked through condensation reactions, possible bases are A,C,G and U, C pairs with G,A pairs with U

name the complementary base pairs in DNA

2 H-bonds between adenine (A) + thymine (T)

3 H-bonds between guanine (G) + cytosine (C)

name the complementary base pairs in RNA

2 H-bonds between adenine (A) + uracil (U)

3 H-bonds between guanine (G) + cytosine (C)

why is DNA replication described as semiconservative

strands form original DNA molecule act as templates

new DNA molecule contains one old strand and one new strand

summarise the process of DNA replication

• double helix unwinds and the hydrogen bonds break, catalysed by DNA helicase

• complementary base pairing occurs between the template strand and free nucleotides

• the nucleotides are joined by phosphodiester bonds, catalysed by DNA polymerase

what is the role of the enzyme DNA polymerase in DNA replication

it catalyses the formation of phosphodiester bonds between the free nucleotides to form the new complementary strand of DNA

describe the meselson and stahl experiment procedure

• E.coli bacteria was grown in a growth medium containing an isotope of nitrogen (15N)

• the bacteria incorporated the isotope into the nucleotides

• the bacteria were allowed to divide and samples were taken after each division

• the samples were centrifuged using a salt with a density gradient

describe the result from the meselson and stahl experiment

generation 0 - all DNA contaisn only 15N nucleotides

generation 1 - DNA produces a band showing an intermediate density which is due to the new DNA molecules produced containing one 15N strand and one 14N strand

generation 2 - an intermediate band is also seen but there is now also a band showing a density of just 14N, proving that replication has to occur semi-conservatively

what is meant by triplet code

a triplet code is a section of DNA where groups of 3 nucleotides code for one amino acid

describe the structure of messenger RNA (mRNA)

• long ribose polynucleotide with sugar phosphate backbone

• nitrogenous bases : A,U,G,C

• single stranded & linear (no H bonds between complementary base pairs)

• codon sequence is complementary to exons of 1 gene from 1 DNA strand

state the function of mRNA

transfers genetic code from DNA in nucleus to ribosomes for translation into a specific polypeptide

what is the role of RNA polymerase in transcription

it catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides to form the mRNA strand

outline the process of transcription

• RNA polymerase binds to promoter region on a gene

• section of DNA uncoils into 2 strands with exposed bases, antisense strand acts as template

• free nucleotides are attracted to their complementary bases

• RNA polymerase joins adjacent nucleotides to form phosphodiester bonds

describe the structure of transfer RNA (tRNA)

• single strand folded into clover shape (some paired bases)

• anticodon on one end, amino acid binding site on the other

state the function of transfer RNA (tRNA)

carries amino acids to specific codons on mRNA for translation

what does translation produce and where does it occur

produces proteins

occurs in cytoplasm on ribosomes

outline the process of translation

• mRNA travels to a ribosome

• the mRNA is passed through the ribosome 3 bases (one codon) at a time

• tRNA molecules with complementary anticodons to the mRNA codons hydrogen bond

• the ribosome catalyses the formation of peptide bonds between the amino acids on the tRNA molecules

• tRNA molecules leave, the ribosome moves along the mRNA and the cycle repeats until the polypeptide has been synthesised (this process requires ATP)

list 3 types of mutations

• insertion

• deletion

• substitution

give 3 effects of mutations

harmful - can cause cellular damage and disease through the production of non functional proteins

neutral - the mutation causes no noticeable change to the function of the protein produced

beneficial - mutations which can prove beneficial to organisms and may produce proteins that are better adapted to their functions, this can give organisms a selective advantage

what is cancer

a non communicable disease which is characterised by uncontrolled cell division, it can be caused by mutations in regulatory genes