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DNA:

DNA:

  • DNA is a type of nucleic acid called deoxyribonucleic acid.
  • It is a long chain molecule made up of nucleotides.
  • One nucleotide is made up of:
  • A 5 carbon sugar
  • A phosphate group
  • An organic base
  • Nucleotides link together by condensation reactions between the sugar of one and the phosphate group of the other.
  • Each nucleotide in DNA has 1 of 4 different bases: Adenine, Guanine, Cytosine, or Thymine.
  • Two long polynucleotide strands, running in opposite directions, are held together by hydrogen bonds between the bases.
  • This ladder-like structure, with alternating sugar and phosphate molecules forming the uprights and pairs of bases forming the rungs, is then twisted in a helix.
  • The bases pair in a particular way, based on their shape and chemical structure:
  • A & T pair forming 2 hydrogen bonds
  • C & G pair forming 3 hydrogen bonds

 

  • RNA (ribonucleic acid) is made up a single strand of nucleotides. In these, the sugar is called ribose and the bases are adenine, guanine, cytosine, and uracil.
  • There are 3 types of RNA:
  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)

 

  • Protein Synthesis occurs in two stages
  • Transcription:
  • Takes place in nucleus
  • A complementary copy of the gene is made using RNA

 

  • Gene opens up. Hydrogen bonds break between bases.
  • RNA nucleotides attracted to complementary bases and form hydrogen bonds.
  • RNA nucleotides joined together by RNA Polymerase.
  • Complementary RNA copy of gene now made. It is called mRNA (messenger RNA).
  • mRNA molecule leaves nucleus through nuclear pore

 

  • Translation:
  • Occurs on the ribosomes of the rough endoplasmic reticulum
  • The beginning of the sequence is always marked with the start codon AUG which codes for the amino acid methionine
  • A transfer RNA molecule (tRNA) with 3 bases exposed (an anticodon) pairs with a specific codon on the mRNA
  • Attached to the tRNA molecule is a specific amino acid
  • The amino acids, arranged in the order dictated by the mRNA codons, are joined with peptide bonds to form a polypeptide
  • A stop codon signals the last amino acid in the polypeptide chain

 

Base triplets in DNA

Transcription (in the nucleus)

Codons in mRNA

Translation (on the ribosomes)

Amino acid sequence in polypeptide chain

 

  • The genetic code in the DNA making up the chromosomes acts as a code for protein synthesis.
  • It dictates the amino acids required to make the protein and the order in which they should be bonded together.
  • 3 bases code for 1 amino acid and these base triplets are non-overlapping.
  • The code is degenerate: there is more than 1 triplet for each amino acid.
  • A gene is a sequence of bases on a DNA molecule (a short section of a chromosome) coding for a sequence of amino acids in a polypeptide chain.

 

  • DNA copying or replication must occur before a cell divides to ensure that daughter cells receive a copy of the genetic code.
  • DNA double helix unwinds
  • Hydrogen bonds between the base pairs break
  • Free DNA nucleotides line up along side each strand
  • Hydrogen bonds form between complementary bases
  • DNA polymerase links adjacent nucleotides
  • 2 identical DNA double helices are formed by this semi-conservative replication
  • Original DNA, all heavy ∴ DNA band at bottom of centrifuge
  • 1st generation DNA, ½ old, ½ new ∴ DNA band in middle of centrifuge
  • 2nd generation DNA, some ½ and ½ (forms one band at top) & some all new ∴ second band in the middle of centrifuge.

 

  • Sometimes, the DNA replication does not work perfectly – an incorrect base may slip into place. This is called a gene mutation.
  • If this occurs in a sperm or ovum, which ultimately forms a zygote, every cell in the new organism will carry the mutation.
  • If the mutation occurs in non-coding DNA, it will have no effect.
  • In a gene, it will cause an error in the mRNA and an incorrect amino acid may be included in the polypeptide chain causing a genetic disorder e.g. sickle cell anaemia.
  • A number of different mutations can affect the gene coding for the cystic fibrosis transmembrane regulatory (CFTR) protein channels, which allow chloride ions to pass through the membrane.
  • The most common mutation is a deletion of 3 nucleotides.
  • The altered protein may not open, or may reduce the flow of chloride ions through the channel.

 

  • Human cells contain 23 pairs of homologous chromosomes. At a particular position/locus on each of the pair is found a gene for a particular characteristic.
  • Different forms of the same gene are called alleles. If a cell contains two copies of an allele, their genotype is described as homozygous. Different alleles at a locus result in a heterozygous
  • The characteristic resulting from the genotype is the organism’s phenotype.
  • A recessive allele (represented by a small case letter) is only expressed in the homozygous condition.
  • A dominant allele (represented by the same letter in the upper case) will be expressed in the phenotype in either the homozygous or heterozygous condition.
  • In humans, recessive mutations of single genes result in:
  • Cystic fibrosis: mucus that is too viscous.

It affects…

  • Lungs:
  • The amount of water in the mucus produced must be regulated:
  • Too runny and it floods the airway
  • Too viscous (sticky) and it can’t be cleared by the cilia
  • This is controlled by the transport of sodium and chloride ions across the epithelial cells.
  • Water follows the ions because of osmosis.
  • Summary:
  • The CFTR channel is non-functional, so chloride ions cannot pass out of the cell towards the lumen.
  • The sodium ion channels are open and sodium ions are continually absorbed from the mucus.
  • Water is drawn out of the mucus by osmosis and it becomes much too viscous.
  • The cilia cannot move the viscous mucus – it builds up in the airway and becomes infected.
  • Because of low oxygen levels in the mucus, anaerobic bacteria thrive.
  • White blood cells invade the mucus, then die and release DNA making it even more viscous.
  • Mucus blocks the bronchioles, reducing the number of ventilated alveoli. This reduces the efficiency of gas exchange.

 

  • Digestive System:
  • The viscous mucus blocks the pancreatic duct.
  • Enzymes are not released into the small intestine and food is therefore not digested effectively. Undigested food cannot be absorbed and energy is lost in the faeces (mal-absorption syndrome).

 

  • Reproductive System:
  • In females, a mucus plug blocks the cervix
  • In males, the vas deferens leading from the testes is either blocked or missing
  • Thalassemia: abnormal haemoglobin formation.
  • Albinism: lack of pigment production.