Genes, DNA, RNA

Nucleic acids carry the genetic code that determines the order of amino acids in proteins
Genetic material stores information, can be replicated, and undergoes mutations
Differs from proteins as it has phosphorus and NO sulphur

DNA Deoxyribonucleic Acid

Nucleotides are smaller units of long chains of nucleic acids. Each nucleotide has
- A pentose sugar (deoxyribose in DNA, ribose in RNA)
- A phosphate group
- An organic base which fall into 2 groups,
  - Purines (double rings of C and N - bigger)
    - Adenine or Guanine
  - Pyrimidines (single ring of C and N - smaller)
    - Thymine or Cytosine
  - Base pairing by weak hydrogen bonds
    - Adenine-Thymine 2 H- bonds
    - Cytosine-Guanine 3 H- bonds
Chains are directional according to the attachment between sugars and phosphate group
They are antiparallel which is essential for gene coding and replication
DNA molecule has 2 separate chains of nucleotides hold together by base pairing / DNA normally twist into a helix (coil) / forms a double helix

DNA codes for assembly of amino acids / forms a polypeptide chain (proteins - enzymes)
The code is read in a sequence of three bases called
- Triplets on DNA e.g. CAC TCA
- Codons on mRNA e.g. GUG AGU
- Anticodons on tRNA e.g. CAC UCA
- (must be complementary to the codon of mRNA)
Each triplet codes for one amino acid / single amino acid may have up to 6 different triplets for it due to the redundancy of the code / code is degenerate. Some amino acids are coded by more than one codon
Same triplet code will give the same amino acid in virtually all organisms, universal code
We have 64 possible combinations of the 4 bases in triplets, 43
No base of one triplet contributes to part of the code next to it, non-overlapping
Few triplets code for START and STOP sequences for polypeptide chain formation
eg START AUG and STOP UAA UAG UGA

Happens during Interphase 'S'
Separate the strands, a little at a time to form a replication fork
Events:
- Unwinding / Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds
- Semi-conservative replication / each strand acts as a template for the formation of a new strand
- Free DNA molecules join up to exposed bases by complementary base pairing
  - Adenine with Thymine (A=T 2 -H bonding)
  - Cytosine with Guanine (CΞG 3 -H bonding)
- For the new 5' to 3' strand the enzyme DNA polymerase catalyses the joining of the separate nucleotides
"All in one go" → completed new strand
For the 3' to 5' strand DNA polymerase produces short sections of strand but these sections have to be joined by DNA ligase to make the completed new strand. Specific base pairing ensures that two * identical copies of the original DNA have been formed

DNA in nucleus unzips - bonds break
Single template strand of DNA used for mRNA (triplet on DNA = codon for amino acid on mRNA)
Enzyme RNA polymerase joins nucleotides together
Free RNA nucleotides are assembled according to the DNA triplets (A-U / C-G / T-A)
mRNA bases are equivalent to the non-template DNA strand
Start and stop codons are included
Introns (Non-coding) and exons (coding) DNA sequences are present in the primary mRNA transcript. Introns are removed before the mRNA is translated so that exons are only present in the mature mRNA * transcript
[EXAM] Total number of bases in the DNA sense strand and total number of bases in the mRNA are different
mRNA moves into cytoplasm and becomes associated with ribosomes

Translation is the synthesis of a polypeptide chain from amino acids by using codon sequences on mRNA
tRNA with anticodon carries amino acid to mRNA associated with ribosome
"Anticodon - codon" complementary base pairing occurs
Peptide chain is transferred from resident tRNA to incoming tRNA
tRNA departs and will soon pick up another amino acid

Pool of amino acids / building blocks from which the polypeptides are constructed
ATP and enzymes are needed
Complementary bases are hydrogen-bonded to one another
Structure involved in translation
Messenger RNA (mRNA)
- Carries the code from the DNA that will be translated into an amino acid sequence
Transfer RNA (tRNA)
- Transfer amino acids to their correct position on mRNA strand
Ribosomes
- Provide the environment for tRNA attachment and amino acid linkage

Reactions in cells is referred to as cell metabolism
A sequence of chemical reactions is called a metabolic pathway
Different forms of the same gene are alleles
A gene is the length of DNA that carries the code for a protein (enzyme)
- Enzyme effect the cell's metabolism
- Visible changes are described with the phenotype
The phenotype is influenced by the metabolic pathway
Therefore
- DNA controls enzyme production
- Enzymes control metabolic pathways
- Metabolic pathways influence the phenotype of an organism

Mutation causes the deletion of 3 bases in DNA. One amino acid (phenylalanine) is not coded for in the Cystic Fibrosis Transmembrane Regulator CFTR protein
Faulty CFTR protein cannot control the opening of chloride channels in the cell membrane
Results in production of thick sticky mucus, especially in lungs, pancreas and liver
Organs cannot function normally and infection rate increases

Normal: IMG 3-12-5 Defect (PKU): IMG 3-12-5