DNA Replication and the Genetic Code Flashcards
What is DNA replication?
- two strands of DNA double helix separate and each strand serves as template for new DNA molecule
- complementary base pairing means the two new strands are identical to the original
What is semi-conservative replication?
- two new molecules of DNA are produced. Each one consists of one old strand and one new strand
What is the process of DNA replication?
- an enzyme, DNA helicase, causes the two strands of DNA to separate
- Meanwhile, free nucleotides that have been activated are attracted to their complementary bases
- Once the activated nucleotides are lined up, they are joined by DNA polymerase.
- All the nucleotides are joined to form complete polynucleotide chain.
What is the role of DNA helicase?
- travels along DNA backbone, catalysing the reaction that breaks the hydrogen bonds between the bases.
- separates/unwinds the two strands of DNA
What is the role of DNA polymerase?
- catalyses the formation of phosphodiester bonds between nucleotides
What is continuous replication?
- DNA polymerase moves along template strand in same direction. It can bind to 3’ so travels in direction of 3’ to 5’.
- as DNA unwinds in one direction, DNA polymerase has to replicate each of the template strands in opposite directions
- the strand that is unzipped from 3’ end can be continuously replicated as strands unzip.
- called leading strand and said to undergo continuous replication
What is discontinuous replication?
- other strand unzipped from 5’ end, so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.
- results in DNA being produced in sections (Okazaki fragments) which are then joined by DNA ligase
- this is called the lagging strand and undergoes discontinuous replication
What is the difference between continuous and discontinuous replication?
- continuous replication: DNA polymerase binds to end of strand and free DNA nucleotides are added without any breaks
- discontinuous replication: DNA polymerase can’t bind to end of the strand so free DNA nucleotides added in sections.
Why does DNA polymerase not catalyse the joining of the Okazaki fragments into a single strand but a different enzyme is used?
- enzymes are substrate specific
- DNA polymerase catalyses joining of nucleotides
- nucleotides have different shape to Okazaki fragments
What is a mutation?
- change in genetic material which may affect phenotype of an organism.
- leads to change in sequence of bases
What is the genetic code?
- sequences of bases in DNA that are ‘instructions’ for sequences of amino acids in the production of proteins
What is a gene?
- a section of DNA that contains complete sequences of bases to code for a protein
Why is the genetic code ‘universal’?
- all organisms use same code, although sequences of bases coding for each protein may be different
What is a codon?
- a 3 base sequence of DNA or RNA that codes for 1 amino acid
Why is the genetic code described as non-overlapping?
- single codon signals start of sequence so DNA is read from base 1 snd not 2 or 3 and also stops at last base.
How is the genetic code degenerate?
- 20 different amino acids
- more codons than amino acids
- many amino acids can be coded for by more than 1 codon
What is meant by the triplet code?
- triplet code is particular sequence of 3 bases that codes for specific amino acid
How may a genetic mutation result in an enzyme becoming non-functional?
- mutation in DNA changes triplet code
- means there are different amino acids in the protein/enzyme that the DNA codes for
- may change structure of active site so substrate can’t bind making enzyme non-functional
Why are there more likely to be more differences, overall, between base sequences of DNA than between amino acid sequences of proteins?
- triplet code is degenerative
- 64 different codons but only 20 amino acids
- therefore amino acid can be coded by more than one codon, so more opportunity for differences in DNA sequence than amino acid sequence
