DNA!! (and RNA as well)

Structure and function of DNA - 7 points

Structure and function of DNA - 7 points

• Double helix shape with the sugar-phosphate backbone on the outside - protects the nitrogenous bases on the inside

• The two strands are complementary and are joined together by weak hydrogen bonds - the two strands can separate easily to synthesise mRNA in transcription and for (semi-conservative) DNA replication

• Many weak hydrogen bonds between bases (and higher proportion of G-C pairings) - the double helix is stable

• Very stable structure - mutations rarely happen and genetic information can be passed down from generation to generation

• Extremely large molecule - carries an immense amount of genetic information

• Specific sequence of base pairs - codes for many specific proteins

• Compact and helical - can be stored in the nucleus - a small space

Structure of DNA

DNA is a double-stranded polymer, coiled into a double helix. Each strand is made up of monomers called nucleotides, joined together by phosphodiester bonds. Each nucleotide comprises a phosphate group, bonded to a pentose sugar (deoxyribose), bonded to a nitrogenous base. The two strands are joined together by hydrogen bonds between complementary bases - adenine joins to thymine with two hydrogen bonds and cytosine to guanine with three.

Reaction that joins nucleotides

Condensation

Bond between nucleotides

Phosphodiester

Bond between phosphate group and pentose sugar

Ester

Bond between a pentose sugar and nitrogenous base

Glycosidic

Why is DNA anti-parallel

The two strands in DNA run in opposite directions - on one strand, the 5 prime end of the deoxyribose sugar is above the 3 prime end, and on the other strand the 3 prime end is above the 5 prime end

Purpose of RNA

mRNA - transfers genetic information from DNA to the ribosomes for protein synthesis. tRNA - transfers amino acids to the ribosomes to be joined together in the correct order (determined by the codons on the mRNA) into a polypeptide by the formation of peptide bonds

Structure of tRNA

• Single strand of RNA

• Folded into a clover-leaf shape

• Hydrogen bonds between complementary base pairs

• Binding site for an amino acid

• Anticodon to bind with hydrogen bonds to a codon on mRNA

Purpose of semi-conservative replication of DNA

Ensures genetic continuity between generations of cells because it ensures that daughter cells produced in mitosis inherit all the genes from their parent cell

Process of DNA replication

• Semi-conservative replication

• DNA helicase moves along the molecule of DNA, causing the molecule to unwind and breaking the hydrogen bonds between the complementary bases on the two strands so that the strands separate

• Each strand acts as a template: nucleotides with bases complementary to the exposed bases join to the exposed bases on each strand with hydrogen bonds (A to T with two and C to G with three)

• DNA polymerase moves along the new strand of DNA and joins the nucleotides together by catalysing the formation of phosphodiester bonds between them