bio nucleic acids minus protein synth + ATP

structure of a nucleotide

phosphate bonded to a pentose sugar (phosphodiester, covalent bond) bonded (glycosidic, covalent) to a base (ATCGU)

DNA bases

purines: adenine & guanine

pyrimidines: thymine/uracil & cytosine

DNA structure

two antiparallel strands forming a double helix structure made up of complementary bases hydrogen bonded together - A double bonded to T, C triple bonded to G. watson & crick put this together

RNA structure

uracil replaces thymine, single stranded, deoxyribose replaces ribose sugars, RNA is smaller

DNA replication

• double helix unwound by DNA helicase (breaks the hydrogen bonds between bases)

• DNA binding proteins keep bases apart

• free nucleotides with correct bases slot into place opposite the exposed complementary bases (AT CG), and hydrogen bonds form

• DNA polymerase joins nucleotides continuously in the 5’ to 3’ direction due to it being complementary to this end

• in the 3’→5’ end the DNA is formed in Okazaki fragments which are joined by DNA ligase

• two strands of DNA are formed each containing 1 strand of the original and 1 new strand.

random errors (mutations) can occur during this process via spontaneous errors, radiation and cellular processes like hypermutation. odds of the former are reduced via DNA proofreading through DNA polymerase.

alternate theorised methods for DNA replication

conservative: old strand remains the same, new strand is all new

dispersive: old and new parts exist in both strands

proof of semiconservative replication

meselson & stahl grew bacteria in ¹⁵N and ¹⁴N solutions, isolated the DNA, centrifuged them and recorded the position of the bands. a sample of the ¹⁵N bacteria were then added to the ¹⁴N solution, and the resultant generations had their DNA extracted and centrifuged to see the ratio of ¹⁴N light DNA to ¹⁵N heavy DNA change.

the ratio of DNA where n is the generation number, hybrid:light, is 1:1+2n. after the parent generation no pure heavy DNA exists as all strands have formed new light DNA strands. after gen 1, light strands form new light pairings forming pure light DNA.

DNA extraction from a strawberry/fruit

• detergents: break down cell membranes (lipids in the phospholipid bilayer are soluble in these)

• cold ethanol: DNA is insoluble in this and condenses allowing for it to be seen / collected

• salt: makes water solution hypertonic, causing plasmolysis and so easier breakdown

• protease / hot water: denatures proteins that would break down DNA

features of the genetic code

• triplet code: 3 bases code for 1 amino acid (codon)

• specific: same code always refers to the same amino acid

• universal: same code applies to all species

• nonoverlapping: each 3 bases refer to one amino acid, then the next three are the next etc.

• punctuated (no definition listed)

• degenerate: only 20 amino acids exist, 64 combinations exist. multiple codes give the same amino acid