DNA Replication and Genes

semi conservative method of replication

• DNA helicase breaks hydrogen bonds between complementary base pairs

• opens up DNA double strand

• Causes a replication fork

• each strand acts as a template for free floating nucleotides

• 2 H bonds between R and T

• 3 H bonds between G and C

• DNA polymerase moves from 5’ to 3’ and lines up new nucleotides along DNA template strands

• DNA ligase catalyses formation of phosphodiester bond between new nucleotides and creates sugar-phosphate backbone of new DNA

lagging strand and leading strand

• DNA strands are antiparallel

• Leading strand is synthesised continuously

• Lagging strand is synthesised in fragments

• DNA Polymerase binds to 5’ section of lagging strand then catalyses the formation of a few phosphodiester bonds up until the 3’ section

• Once more of the DNA unwinds the DNA polymerase jumps back to the next 5’ section of the lagging strand and repeats the process

• Fragments of lagging strand are called okazaki fragments

• DNA ligase joins the okazaki fragments together

Gene

a strand of DNA that codes for a particular protein

Features of genetic code

• triplet code

• universal

• degenerate

• non overlapping

triplet code

the code of three bases that is the basis of genetic info in DNA

universal code

the same DNA triplet codes for the same amino acid in almost all organisms

degenerate code

code containing more information than needed

non-overlapping code

a code where each codon codes for only one thing with no overlap between codons

Transcription

• RNA polymerase binds to DNA

• Hydrogen bonds break and DNA strands seperate

• Antisense strand acts as a template for mRNA synthesis

• Free RNA nucleotides align with exposed DNA bases

• RNA polymerase catalyses the formation of phosphodiester bonds between RNA nucleotides

• RNA is released from DNA once RNA polymerase reaches stop codon

• mRNA exits nucleus via nuclear pores

Translation

• Ribosome attaches to mRNA at start codon

• tRNA with anticodon complimentary to mRNA start codon binds to mRNA and carries amino acid specific to the codon

• Second tRNA with anticodon complimentary to the next codon binds to mRNA and carries specific amino acid

• two amino acids are linked by a peptide bond

• Max of 2 tRNA can attach to mRNA strand so the first tRNA is released from the strand as soon as the amino acids are linked

• the ribosome moves along the mRNA strand and repeats the process until a stop codon is reached

• polypeptide chain is released and folded into a specific shape