Genetic Variation

• How many amino acids occur regularly in proteins?

  20.
• What is a triplet code?

  A set of three bases that code for a single amino acid (DNA).
• What are the four DNA bases?

  Adenine

  Thymine

  Cytosine

  Guanine
• Which DNA bases are complementary to one another?

  Adenine & Thymine

  Cytosine & Guanine
• What is a degenerate code?


  A genetic code in which the number of triplet codes exceeds the number of amino acids. This means that some amino acids are coded by more than one codon.

  Most amino acids are coded for by between 2 and 6 triplet codes.
• Triplet codes can only be read in one direction.

  The bases in a triplet code can only be read once.

  The code is non-overlapping.
• Every polypeptide starts with the same triplet code.

  The polypeptide finishes when it reaches a stop codon.
• What is the start codon?

  AUG
• The code is universal - it is the same for almost all organisms on earth.
• Define gene.

  A short section of DNA that codes for a polypeptide and a functional RNA. The code is a specific sequence of bases.
• Define locus.

  The location of a particular gene on a chromosome. This will be the exact same chromosome on the exact same place for every human, but a different version.
• Define allele.

  An alternative form of gene, there are usually two different forms but there can be more.
• Define chromosome.

  The form of DNA found in eukaryotic cells.

  There are 46 chromosomes (23 pairs).

  They are in a thread-like structure made of tightly coiled DNA.
• Each allele has a different base sequence and so codes for a different sequence of amino acids, therefore making a different polypeptide.
• Chromosomes only become visible during cell division.

  The chromosomes are usually structured as a single chromatid with a central centromere.

  They are usually shown as the X shape, as during replication they join with the sister chromatid to form the X shaped chromosome.
• What are homologous pairs?

  Pairs of chromosomes that have the same genes at the same loci, but possibly different alleles.
• Eukaryotic DNA is:

  • Linear in shape
  • Longer than prokaryotic DNA
  • Coiled around proteins called histones which forms the chromosome

  Mitochondria and chloroplasts have DNA that is more like that of prokaryotic cells (shorter and circular).
• Prokaryotic DNA is:

  • Circular in shape
  • Shorter than eukaryotic DNA
  • Not associated with histones
  • Supercoils in order to fit in the cell
• Coding sequences of bases in DNA are called exons.

  Non-coding sequences of bases in DNA are called introns.
• What is the position of a gene on a chromosome called?

  Locus.
• Define genome.

  The complete set of genes in a cell.
• Define proteome. 


  The full range of proteins that a cell is able to produce.
• RNA is a single-stranded polymer. It is a nucleotide chain with a ribose sugar.

  DNA has a deoxyribose sugar instead.
• What are the four RNA bases?

  Adenine

  Uracil

  Guanine

  Cytosine
• Which RNA bases are complementary to one another?

  Adenine & Uracil

  Cytosine & Guanine
• Which bases are purine?

  Adenine and Guanine.
• Which bases are pyrimidine?

  Cytosine, Thymine, and Uracil.
• What does mRNA stand for?

  mRNA stands for messenger RNA.
• What does tRNA stand for?

  tRNA stands for transfer RNA.
• mRNA is formed during transcription in the nucleus.

  It is a copy of a gene on the DNA strand.
• tRNA is located in the ribosomes.

  It is involved in the translation stages of protein synthesis.
• mRNA is formed of thousands of nucleotides to form a single stranded helix.

  It's linear in shape.

  No amino acid binding point.

  No base pairing so no hydrogen bonding as it is only single stranded.

  Codes for a polypeptide chain using triplet codons.

  Unique to the gene it codes for - many kinds made.
• tRNA is formed of around 80 nucleotides.

  It is shaped by a single strand that is folded to form a clover leaf shape.

  Has an amino acid binding point.

  Has base pairing and each tRNA is specific to an amino acid.

  Has an anticodon, which is complementary to the DNA codon.

  There are only 64 tRNA molecules - the number of codons for 20 amino acids.
• What are the two main stages of protein synthesis?

  Transcription and translation.
• In transcription, the gene that codes for a specific polypeptide is copied to form mRNA.

  This happens in the nucleus.
• During translation, mRNA joins with a ribosome and tRNA brings specific amino acids to build the polypeptide chain.

  This happens in the cytoplasm.
• TRANSCRIPTION:

  1. DNA helicase attaches to section of DNA, breaking the hydrogen bonds between the two strands and unwinding the helix.
  2. An mRNA copy of one DNA strand is made, called the template strand.
  3. A mRNA strand is synthesised and free RNA bases are attracted to their complementary DNA counterparts, forming hydrogen bonds.
• TRANSCRIPTION:

  4. RNA polymerase catalyses the formation of phosphodiester bonds between RNA molecules to create a new strand of mRNA.

  5. The DNA molecule reforms once RNA has moved past and RNA polymerase detaches from the DNA when it reaches a stop signal.

  6. mRNA leaves the nucleus through nuclear pores.
• Transcription

  DNA helicase attaches to section of DNA, breaking the hydrogen bonds between the two strands and unwinding the helix.<br>An mRNA copy of one DNA strand is made, called the template strand.<br>A mRNA strand is synthesised and free RNA bases are attracted to their complementary DNA counterparts, forming hydrogen bonds.<br>RNA polymerase catalyses the formation of phosphodiester bonds between RNA molecules to create a new strand of mRNA.<br>The DNA molecule reforms once RNA has moved past and RNA polymerase detaches from the DNA when it reaches a stop signal.<br>mRNA leaves the nucleus through nuclear pores.