Biological Molecules - Nucleotides and Nucleic acids

• Why are nucleic acids important?

  storage and transfer of genetic information to make proteins
• What are nucleotides?

  + nitrogen-containing organic molecules

  + form basis of nucleic acids DNA and RNA
• How are nucleotides involved in metabolic processes?

  + phosphorylated nucleotides contain more than one phosphate group

  + involved in biochemical pathways e.g. ATP is energy rich end product of respiration

  + components of coenzyme in photosynthesis and respiration biochemical pathways
• What is a phosphate group?

  inorganic molecule, negatively charged
• How are nitrogen bases involved in making nucleotides?

  different nitrogen bases making up five different molecules
• How are polynucleotides formed?

  + carbon 3 bonded to the phosphate

  + nucleotides can polymerise to from polynucleotides

  + form basis of nucleic acids DNA and RNA
• How is DNA structured?

  + long chain of nucleotide monomers

  + polymer called a polynucleotide

  + DNA molecular forms when 2 polynucleotide strands come together

  + hydrogen bonds between bases give a stable structure

  + DNA contains 1:1 ratio of pyrimidine: purine bases
• How is DNA packaged?

  + DNA packaged as chromosomes in nucleus in eukaryotic cells

  + around 2m of DNA in cells, needs to be coiled and folded to fit

  + eukaryotic DNA associated with histone proteins to form chromatin -> substance chromosomes made from

  + DNA loose in cytoplasm in prokaryotic cells

  + 2 DNA strands have 3' and 5' terminals at opposite ends - antiparallel
• Why is a salt solution used when extracting DNA?

  the salt solution neutralises the charge of the DNA's sugar-phosphate backbone, and makes the DNA less soluble in water
• Why is the test tube put into the water baths for 10 minutes when extracting DNA?

  water bath speeds up the action of detergent and the protease and allows time for the nuclear membrane to break down
• Why does DNA precipitate into ethanol when adding ice-cold ethanol?

  DNA is insoluble in ethanol as DNA is polar and ethanol is non-polar
• What holds the DNA helix together?

  weak hydrogen bonds between complementary nitrogenous base pairs
• Why is it important that one purine binds with one pyrimidine?

  one purine always binds with one pyrimidine so that the nucleotide pair is always the same distance across
• Why are not all of the genes in a cell switched on?

  some proteins are not needed by all cells
• What does DNA replication result in?

  results in each chromosome (one molecule of DNA) having an identical copy of itself
• What is conservative DNA replication?

  + a completely new double stranded copy of DNA is made

  + original or parental DNA staying the same ( being conserved)
• What is semi-conservative DNA replication?

  + involves separation of DNA strands

  + followed by replication of each strand separately to produce DNA

  + half parental and half new

  + one strand acts as a template for the new DNA strand
• Describe the experimental evidence for the semi-conservative DNA replication.

  + Meselson and Stahl

  + grow bacteria in nitrogen - 15 (heavy) medium

  + transfer some bacteria to nitrogen - 14 (light) medium; bacterial growth continues

  + take samples after 0 minutes, 20 minutes (after one round of replication and 40 minutes ( two rounds of replication)
• What are the actual steps of semi-conservative DNA replication?

  + hydrogen bonds between bases are broken

  + free nucleotides are present in nucleus

  + free nucleotides pair up with complementary exposed bases

  + new strand linked together

  + two DNA molecules and each contains one old strand and one new one
• Which enzyme catalyses the untwisting of the DNA double helix?

  gyrase
• How is DNA unzipped?


  + by breaking the H bonds between the base pairs

  + exposing DNA bases

  + catalysed by DNA helicase enzyme
• What does DNA polymerase catalyse?

  formation of phosphodiester bonds between phosphate group of one nucleotide and sugar group of next to make the sugar-phosphate backbone
• In which direction does DNA polymerase read and work?

  + only read in 3' to 5' on the parental/ template strand (leading strand) so strand that is unwound

  + 3' to 5' can be continuously replicated
• What are Okazaki fragments and how are they formed?


  + sections of DNA

  + strand that is unwound 5' to 3' (lagging strang) has to wait until a section of the strand has been unwound, and then work back along the strand
• What does DNA ligase do?

  joins the Okazaki fragments together
• What is the genetic code?

  + sequence of bases along an organism's DNA

  + contains thousands of sections called genes

  + each gene codes for a specific polypeptide
• How many bases approximately are in one gene?

  thousands ( around 10-100 000 bases)
• What does it mean that the genetic code is universal?

  same sequence of bases codes for the same amino acid in all organisms
• What is a triplet of bases in DNA called?

  triplet
• What does degenerate mean?

  each amino acid is coded for by more than one triplet (except methionine and tryptophan)
• How does mRNA differ from DNA?

  + single stranded, not double stranded

  + ribose instead of deoxyribose

  + uracil instead of thumine

  + shorter
• What are triplets of RNA bases called?

  a codon
• What does non-overlapping genetic code mean?

  + each base is only part of one triplet/ codon

  + each triplet/ codon codes for one amino acid
• What protein does AUG code for in most living organisms?

  AUG codes for methionine and starts off most proteins as a start codon
• What is UGA and AUA in most living organisms and what are the exceptions?

  + UGA is a stop codon and AUA codes for isoleucine

  + in mitochondria, AUA codes for methionine and UGA codes for trytophan

  + UGA also codes for tryptophan in some bacteria which links to the endosymbiotic theory
• What is the coding and non-coding strand during transcription?

  + coding = sense and runs in 5' to 3' direction that makes 'sense'

  + template strand = antisense stand and runs in 3' to 5' direction as this direction does not make 'sense'
• What are amino acids in the cytoplasm attached to?

  + tRNA molecules

  + each tRNA is specific for one amino acid
• What happens once the mRNA strand moves to the cytoplasm?

  mRNA combines with ribosome
• What happens during translation?

  + tRNA moelcules attach to ribosome

  + anticodons on tRNA molecules pair up with appropriate codons on mRNA

  + amino acids transported by tRNA link together, and RNA molecules return to cytoplasm

  + ribosome moves along mRNA and amino acids continue to join together until all codons are translated and polypeptide completed
• Which three main types of biological processes require energy?

  + synthesis - building of large complex molecules from smaller molecules

  + transport - pumping molecules or ions across membrane by active transport

  + movement - protein muscle fibres in muscle cell require energy to contract