The Language of Life Chetan Vaity March 2007 The Cell


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The Language of Life

  • Chetan Vaity
  • March 2007

The Cell

  • A eukaryotic cell
  • Nucleus
  • Chromosomes
    • Very long, continuous piece of DNA
  • Endoplasmic Reticulum
    • Ribosomes

DNA

  • All living things on the planet contain DNA
  • They store information
    • Information to do what?
  • Genes

Nucleobases

  • In genetics, these are simply called bases
  • Adenine, Thymine, , Guanine, Cytosine or A, T, G, C
  • Nucleotides pair in a specific way - called the Base-Pair Rule

RNA

  • DNA remains in the nucleus, but in order for it to get its instructions translated into proteins, it must send its message to the ribosomes, where proteins are made. The chemical used to carry this message is Messenger RNA
  • RNA is similar to DNA except:
    • has on strand instead of two strands.
    • has uracil instead of thymine

Transcription

  • The codons of a gene are copied into messenger RNA
  • Transcription proceeds in a particular direction
  • A section of DNA double helix is uncoiled and only one of the DNA strands serves as a template
  • Note that the new RNA is identical to non coding DNA with the exception of uracil where thymine was located in DNA

Amino Acids

  • The general formula is NH2CHRCOOH
  • These are molecules where the amino and carboxylate groups are attached to the same carbon atom
  • The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon.
    • R = H (Hydrogen atom) : Glycine
    • R = CH3 (Methyl group) : Alanine
    • R = CH2C8H6N : Tryptophan

More about Amino Acids

  • Humans can produce 10 of the 20 amino acids. The others must be supplied in the food.
  • Failure to obtain enough of even 1 of the 10 essential amino acids, those that we cannot make, results in degradation of the body's proteins—muscle and so forth—to obtain the one amino acid that is needed.
  • Unlike fat and starch, the human body does not store excess amino acids for later use—the amino acids must be in the food every day.

The Genetic Code

  • Since there are 4 bases in 3-letter combinations, there are 64 possible codons (4^3 combinations). These encode the twenty standard amino acids.
  • Most amino acids, therefore, have more than one possible codon.
  • There are also three ‘STOP' codons signifying the end of the coding region
    • These are TAA, TGA and TAG

Genetic Code and Translation into proteins

  • mRNA copy is decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA to transfer-RNA (tRNA), which carries amino acids.
  • In proteins, amino acids are joined together in a chain by peptide bonds between their amino and carboxylate groups.

Proteins are a chain of amino acids

  • Peptide bond

Protein Synthesis

  • The overall picture

Example of a protein in action

  • Haemoglobin
    • Oxygen transport in red blood cells
  • Sickle-cell anemia is caused by a point mutation
    • Replace the amino acid glutamic acid with valine at the sixth position of the β chain.
    • Causes distortion of red blood cells and a tendency for them to lose their elasticity.
  • The link with Malaria
    • The malaria virus attacks the red blood cells
    • This mutation is prevalent in Africa
    • If one inherits this gene from one of you parents, one is almost immune to malaria


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