Information of Life

HBIO2 > Genetics

Nucleic Acids - The Key to Life

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
Differ from proteins as it has phosphorus and NO sulphur
Made up of several chains of nucleotides
DNA and RNA are types of nucleic acids

Sugar-phosphate backbone (ensures stability of the molecule)
- Pentose sugar
  - Deoxyribose in DNA
  - Ribose in RNA
- Phosphate group
Organic bases
- Purines (double rings of C and N - bigger)
  - Adenine
  - Guanine
- Pyrimidines (single ring of C and N - smaller)
  - Thymine in DNA only
  - Uracil in RNA only
  - Cytosine

Made up of 2 separate chains of nucleotides hold together by base pairing
- Connected by weak hydrogen bonds
- Can easily be opened for replication
- Adenine-Thymine has 2 H-bonds
- Cytosine-Guanine has 3 H-bonds
DNA normally twist into a helix (coil) / forms a double helix
- Makes the molecule compact (store a lot in small space)
- Protects from damage as base pairs are facing inwards
Both chains of DNA are
- Directional → according to the attachment between sugars and phosphate group
- Antiparallel → essential for gene coding and replication

Semi-conservative replication: each DNA strand acts as a template for the formation of a new strand
Happens during interphase S of the cell cycle
Unwinding
- Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds
- Strands are separated a little at a time (not all at once!)
- This creates a replication fork which moves along the 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
- Enzyme DNA polymerase catalyses the joining of the separate nucleotides
- New strand is completed "all in one go"
For the 3' to 5' strand
- DNA polymerase produces short sections of strand
- 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

Ribose instead of deoxyribose
Single chain (shorter than DNA)
- Can pass through nucleus into cytoplasm
Base difference
- Uracil instead of thymine
- Adenine, guanine, and cytosine are the same
Messenger RNA (mRNA) carries the code from DNA that will be translated into an amino acid sequence
Transfer RNA (tRNA) transfers amino acids to their correct position on the mRNA strand

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 / some amino acids are coded by more than one codon (degenerate code)
Same triplet code will give the same amino acid in 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
- START: AUG
- STOP: UAA, UAG, UGA

Cell metabolism: reactions inside cells
Metabolic pathway: sequence of chemical reactions
Alleles: different forms of the same gene
Gene: 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

Humans have 46 chromosomes
- 22 of them are paired up as homologous chromosomes
- Females have an additional homologous pair of sex chromosomes (XX)
- Males have an X and Y sex chromosome
Pair of homologous chromosomes
- One of the pair is inherited from the mother, one from the father
- Gene is a small section of DNA that codes for a specific characteristic
  - Hair colour,
  - Eye colour, ...
- Found on both pairs of chromosomes at the same locus (position)
- A gene can have different alleles (forms)
  - Brown eyes, blue eyes, ...
  - Black hair, brown hair, ...
- This influences the phenotype
Multiple alleles
- Human ABO blood group is controlled by the gene called immunoglobulin I
- The immunoglobulin gene has 3 alleles IA, IB, I0
- These alleles code for antigen A, B, neither A/B, respectively
- Only 2 alleles can be present → IAIB is codominant, I0 is recessive

Revision from Unit 1 Section 3.1.3(c)
Autosomal recessive disorder
Mutation of the CFTR gene on chromosome 7
Deletion of 3 bases / allele is missing 3 nucleotides
Those nucleotides code for the amino acid phenylalanine
Phenylalanine is missing from the CFTR protein
Faulty CFTR protein cannot control the opening of chloride channels in the cell membrane

Autosomal recessive disorder
Gene mutation in DNA/gene coding for the enzyme phenylalanine hydroxylase
Phenylalanine hydroxylase is not produced
Amino acid phenylalanine cannot be converted to the amino acid tyrosine
Tyrosine
- Necessary to produce the pigment melanin
- Patients are fair-haired, fair skinned and blue eyed (phenotype)
Phenylalanine
- Accumulates in the blood and causes irreversible brain damage
- Found in most food that contains proteins
- Treated by avoiding food that contains phenylalanine (diet low in protein)
- Levels in blood are regularly measured by GP
All babies are screened shortly after birth to prevent learning difficulties

Normal:

Defect (PKU):