DNA, RNA & protein synthesis

• What is a gene?

  Small sequence of DNA bases that codes for either a polypeptide or functional RNA (RNA molecules other than mRNA, which perform tasks during protein synthesis e.g, tRNA & ribosomal RNA).
• How does DNA in prokaryotic chromosomes differ from DNA in eukaryotic chromosomes?

  Shorter, circular & isn't wound around histone proteins (condenses to fit in cell by supercoiling instead).
• Structure of eukaryotic chromosomes:

  Thread-like structures made up of a long molecule of DNA wound around histone proteins. DNA & proteins coiled up to form a chromosome. (Histones help support DNA).
• Why is the genetic code universal?

  Same specific base triplets code for same amino acids in all living things- evidence of evolution (code might have been preserved from a common ancestor of all living organisms).
• What is a cell's genome?

  Complete set of genes in a cell.
• What is a cell's proteome?

  Full range of proteins that cell is able to produce.
• What are exons?

  Coding sections of DNA. (Codes for polypeptide or functional RNA).
• What are alleles?

  Forms of a gene. Order of bases slightly different, so code for slightly different versions of same polypeptide.
• What are homologous pairs of chromosomes?

  Pairs of matching chromosomes- same size & same genes (could have different alleles). Alleles coding for same characteristic found at same fixed position (locus) on each chromosome.
• How does a gene code for the production of a polypeptide?


  Order of bases in gene determine order of amino acids in polypeptide (primary structure). Each amino acid is coded for by a sequence of 3 bases in a gene, called a codon.
• Why is the genetic code called 'non-overlapping' ?

  Each base triplet read in sequence separate from triplet before & after it. Triplets don't share bases- code is non-overlapping.
• What is the genetic code?

  Sequence of base triplets (codons) in mRNA which code for specific amino acids.
• Why is the genetic code 'degenerate' ?

  More possible combinations of triplets than there are amino acids (20 amino acids, 64 possible triplets). Some amino acids coded for by more than 1 base triplet.
• How is mRNA edited?

  In eukaryotes introns & exons both copied into pre-mRNA. Splicing- introns removed & exons joined together, forming mRNA strands. Takes place in nucleus. In prokaryotes, mRNA produced directly from DNA no splicing needed as there aren't introns in prokaryotic DNA.
• What is a triplet/codon ?


  Sequence of 3 bases in a gene that codes for an amino acid.
• What is protein synthesis?

  Production of proteins from info within a cell's DNA. 2 stages: transcription (DNA code copied into mRNA) & translation (mRNA joins with ribosome & code it carries is used to synthesise a protein).
• What are introns?

  Non-coding sections of DNA. Only in eukaryotes DNA-removed during protein synthesis (splicing). Non-coding multiple repeats = regions of multiple repeats outside of genes- don't code for amino acids.
• Where does transcription take place in eukaryotic & prokaryotic cells?

  Eukaryotic= nucleus & prokaryotes= cytoplasm.
• What is tRNA?

  Transfer RNA-involved in translation. Carries amino acids used to make proteins to the ribosomes. Single polynucleotide strand folded into clover shape. Hydrogen bonds between base pairs. Every tRNA molecule has a specific sequence of 3 bases at 1 end called an anticodon. Amino acid binding site at other end.
• What is mRNA?

  Messenger RNA-made during transcription. Carries genetic code from DNA to ribosomes. Single polynucleotide strand. Groups of 3 adjacent bases (codons).
• 1st step in transcription:

  RNA polymerase attaches to DNA double helix at beginning of a gene. Hydrogen bonds between 2 DNA strands in gene break, separating strands. DNA molecule uncoils exposing some bases. 1 strand used as template to make mRNA copy. (DNA helicase used in eukaryotes).
• What happens in transcription?

  mRNA copy of a gene is made from DNA.
• What happens in the 2nd step in transcription?

  RNA polymerase lines up free RNA nucleotides alongside exposed bases on template strand. Free bases attracted to exposed bases. mRNA strand complementary copy of DNA template strand (T replaced by Uracil). Once RNA nucleotides paired with specific bases on DNA strand, they're joined together by RNA polymerase, forming mRNA strand.
• What happens in the 3rd step in transcription?

  RNA polymerase moves down DNA strand, assembling mRNA strand. Hydrogen bonds between uncoiled strands of DNA re-form once RNA polymerase has passed by & the strands coil back into double-helix.
• What happens in the 4th step in transcription?

  RNA polymerase reaches stop signal & stops making mRNA & detaches from the DNA. In eukaryotes- mRNA moves out of nucleus through nuclear pore & attaches to ribosome in cytoplasm.
• Where does translation occur?

  In the ribosomes in cytoplasm. Amino acids joined to make polypeptide chain following sequence of codons carried by mRNA.
• Explain translation:


  mRNA attaches to ribosome & tRNA carry amino acids to it. ATP provides energy needed for bond between amino acid & tRNA to form. tRNA (carrying amino acid) with an anticodon complementary to first codon on mRNA attaches to mRNA via complementary base pairing. 2nd tRNA attaches to next codon on mRNA in same way. 2 amino acids attached to tRNA molecules joined by peptide bond. First tRNA moves away leaving its amino acid. 3rd tRNA binds to next mRNA codon-its amino acid binds to first 2 & 2nd tRNA moves away. Process continues producing amino acid chain until stop signal.
• Describe how phosphodiester bond formed between 2 nucleotides within DNA molecule:

  Condensation reaction between phosphate & deoxyribose catalysed by DNA polymerase.