3.4.1 DNA, genes and chromosomes
Definitions
● proteome - the full range of proteins in an organism
● genome - the complete set of genes in an organism
● homologous pair - same size + same genes but different alleles
● alleles - different version of a gene
Prokaryotic DNA
- short + circular
- no association with histones
- no introns
- not included in an envelope - free flowing
Eukaryotic DNA
- long + linear
- associated with histones to form chromosomes
- DNA enclosed in a nuclear envelope
- contains introns
A gene is a sequence of DNA bases that code for:
- a specific polypeptide
- functional RNA
Position of the gene is called a locus.
Nature of the genetic code
- 3 nucleotides code for 1 amino acid - allows for 64 different codes.
- Only 20 amino acids so: code is degenerate - multiple codes for 1 amino acid.
- All genes start with the code methionine - if not detected in the polypeptide - deleted.
- Code is non overlapping - each base only used once.
- Universal - same code in all organisms.
Eukaryotic DNA and introns
- much of the DNA is non coding multiple repeats
- only exons code separated by introns
3.4.2 DNA and protein synthesis
mRNA (messenger RNA)
- made through transcription
- template for translation
- sequence of RNA determines the sequence of the amino acids
- linear and short
- unstable - will break down after a few days
- possesses information as codons
, tRNA (transfer RNA)
-carries an amino acid to the binding site
- anticodon bases complementary to mRNA codon
- each tRNA has anticodon specific to one amino acid
- single polynucleotide strand folded into ‘clover’ shape
- held together by hydrogen bonds
First stage of protein synthesis: Transcription
In the nucleus
- DNA double helix unwound - by DNA helicase - H bonds broken
- RNA nucleotides align next to their complementary bases on the template strand
- forming temporary H bonds - thymine is replaced by uracil
- adjacent RNA nucleotides form phosphodiester bonds - condensation reactions
- when stop code is reached RNA polymerase detaches from the locus of the gene
- pre mRNA detaches from the locus and leaves via the nuclear pore
2nd stage: splicing
pre mRNA - introns spliced to only have coding DNA
(prokaryotic DNA does not contain introns)
3rd stage: translation
- mRNA associates with a ribosome
- Ribosomes moves to AUG - the start codon
- tRNA brings the specific amino acid
- Ribosome moves along to the next codon
- Amino acids joined by the peptide bonds in condensation reactions - forming the 1ry
structure of the protein.
Roles of specific molecules in translation
ATP - hydrolysis of ATP allows for condensation reactions between adjacent amino acids to
form a peptide bond.
tRNA - brings amino acid to the ribosome - amino acid that is complementary to the
anticodon.
Ribosome - attaches to mRNA and houses tRNA - allows codon + anticodon complementary
base pairing so that an amino acid may be attached to the polypeptide chain.
Definitions
● proteome - the full range of proteins in an organism
● genome - the complete set of genes in an organism
● homologous pair - same size + same genes but different alleles
● alleles - different version of a gene
Prokaryotic DNA
- short + circular
- no association with histones
- no introns
- not included in an envelope - free flowing
Eukaryotic DNA
- long + linear
- associated with histones to form chromosomes
- DNA enclosed in a nuclear envelope
- contains introns
A gene is a sequence of DNA bases that code for:
- a specific polypeptide
- functional RNA
Position of the gene is called a locus.
Nature of the genetic code
- 3 nucleotides code for 1 amino acid - allows for 64 different codes.
- Only 20 amino acids so: code is degenerate - multiple codes for 1 amino acid.
- All genes start with the code methionine - if not detected in the polypeptide - deleted.
- Code is non overlapping - each base only used once.
- Universal - same code in all organisms.
Eukaryotic DNA and introns
- much of the DNA is non coding multiple repeats
- only exons code separated by introns
3.4.2 DNA and protein synthesis
mRNA (messenger RNA)
- made through transcription
- template for translation
- sequence of RNA determines the sequence of the amino acids
- linear and short
- unstable - will break down after a few days
- possesses information as codons
, tRNA (transfer RNA)
-carries an amino acid to the binding site
- anticodon bases complementary to mRNA codon
- each tRNA has anticodon specific to one amino acid
- single polynucleotide strand folded into ‘clover’ shape
- held together by hydrogen bonds
First stage of protein synthesis: Transcription
In the nucleus
- DNA double helix unwound - by DNA helicase - H bonds broken
- RNA nucleotides align next to their complementary bases on the template strand
- forming temporary H bonds - thymine is replaced by uracil
- adjacent RNA nucleotides form phosphodiester bonds - condensation reactions
- when stop code is reached RNA polymerase detaches from the locus of the gene
- pre mRNA detaches from the locus and leaves via the nuclear pore
2nd stage: splicing
pre mRNA - introns spliced to only have coding DNA
(prokaryotic DNA does not contain introns)
3rd stage: translation
- mRNA associates with a ribosome
- Ribosomes moves to AUG - the start codon
- tRNA brings the specific amino acid
- Ribosome moves along to the next codon
- Amino acids joined by the peptide bonds in condensation reactions - forming the 1ry
structure of the protein.
Roles of specific molecules in translation
ATP - hydrolysis of ATP allows for condensation reactions between adjacent amino acids to
form a peptide bond.
tRNA - brings amino acid to the ribosome - amino acid that is complementary to the
anticodon.
Ribosome - attaches to mRNA and houses tRNA - allows codon + anticodon complementary
base pairing so that an amino acid may be attached to the polypeptide chain.
