chapter 8 - DNA, Genes & Protein Synthesis

• what 3 features does the genetic code have?

  1. degenerate
  2. universal
  3. non-overlapping
• what is a start codon?

  the 3 bases at the start of a gene on DNA which is copied onto mRNA& initiates translation
• what is a stop codon?

  the final 3 bases on the end of every gene on DNA which is also copied onto mRNA
• what is the function of a stop codon?

  they mark the end of a polypeptide chain & causes ribosomes to detach & stops translation
• what is the 1st feature of the genetic code?

  • degenerate -> more than one amino acid is coded for by more than one triplet. There are 20 amino acids & 3 bases are needed to code for
• what is the 2nd feature of the genetic code?

  • the same triplet of bases codes for the same amino acid in all organisms ( indirect evidence for evolution)
• what is the 3rd feature of the genetic code?

  non-overlapping -> each base in the sequence is read only once
• how many times is a triplet read in a particular direction?

  once
• specification point (3.4.1) - a gene is a base sequence of DNA which codes for an amino acid sequence of a polypeptide & a functional RNA e.g. tRNA
• spec point - a triplet is a sequence of 3 DNA bases which codes for a specific amino acid
• what are introns & exons?

  introns don't code for proteins whereas exons code for proteins
• how many bases code for an amino acid?

  Three
• what is a locus?

  the location of a particular gene on a chromosome
• what is a genome?

  all of the genetic material containing the full set of DNA found in an organism
• what is a proteome?

  the full range of proteins that can be synthesised from the genome
• what are homologous chromosomes?

  a pair of chromosomes that carry the same gene but have different alleles, 1 from maternal & 1 from paternal
• what are chromosome numbers 1-22 called?

  autosomes
• what is chromosome number 23 called?

  Sex chromosomes
• DNA in eukaryotic cells:

  • tightly wrapped around histone proteins
  • long
  • linear with 2 ends

  DNA in prokaryotic cells:

  • circular with no ends
  • relatively short
  • not bound with histone proteins
• DNA in eukaryotic cells found in:

  • nucleus,
  • mitochondria (inherited from mother)
  • chloroplasts

  DNA in prokaryotic cells:

  • free floating genetic material
  • found in plasmids
• what is the DNA found in mitochondria & chloroplasts similar to?

  DNA in prokaryotic cells
• structure of RNA:

  • polymer of a nucleotide
  • ribose sugar
  • adenine
  • phosphate group
  • nitrogenous bases - A, U, C & G
  • relatively short polynucleotide chain
  • single stranded
• what is the function of RNA?

  to copy and transfer the genetic code from DNA in the nucleus to the ribosomes and some is also combined with proteins to create ribosomes
• what is a codon?

  the sequence of 3 bases in mRNA that codes for a single amino acid
• what are the 2 types of RNA important in protein synthesis?

  mRNA, tRNA
• structure of mRNA:

  • long strand arranged in a single helix
  • single stranded & every codon for a specific amino acid
  • much shorter than DNA
  • bases - A, U, C & G
  • created in the nucleus then leaves to carry the copy of the genetic code of 1 gene to a ribosome in the cytoplasm
• structure of tRNA:

  • found in the cytoplasm
  • single stranded but cloverleaf shaped held in place by hydrogen bonds
  • attaches to one of the 20 amino acids then transfers it to the ribosome to create the polypeptide. Specific amino acids attach to specific tRNA molecules, determined by the 3 bases found on the tRNA which are complementary to 3 bases on mRNA
  • anticodon as it's complementary to the codon on mRNA
• what happens in transcription?

  a complementary mRNA copy of one gene on the DNA is created in the nucleus
• what is the advantage of mRNA being much shorter than DNA?

  it is able to carry the genetic code to the ribosome in the cytoplasm to enable the proteins to be made
• what is step 1 of transcription?

  RNA polymerase breaks hydrogen bonds between DNA bases and only one strand acts as a template
• what is step 2 of transcription?

  free RNA nucleotides align by complementary base pairing
• what is step 3 of transcription?

  in RNA, uracil base pairs with adenine on DNA
• what is step 4 of transcription?

  RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds between adjacent RNA nucleotides
• what is the final step of transcription?

  introns are removed off pre mRNA by splicing to form mRNA
• what happens in translation?

  a polypeptide chain is created by using both the mRNA base sequence & tRNA
• what is the role of tRNA in translation?

  • the anticodon on tRNA binds to complementary codon on mRNA
  • tRNA brings specific amino acids
• what is made in translation?

  a polypeptide chain is created using both the mRNA base sequence & the tRNA
• what are the steps of translation?

  1. mRNA attaches to ribosome
  2. tRNA anticodons binds to complementary mRNA codons
  3. tRNA brings specific amino acid
  4. amino acids joined by peptide bonds with the use of ATP
  5. tRNA is released after amino acid joined to polypeptide
  6. the ribosome moves along the mRNA to form the polypeptide
• describe how a gene is a code for the production of a polypeptide

  1. base sequence
  2. in triplets
  3. determines order of amino acid sequence
• why do not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide?

  1. because the genetic code is degenerate
  2. occurs in introns