Knowledge-12 DAP

• The genetic code held in DNA is the same in all organisms, providing evidence for evolution.
• Prokaryotic cell:

  • short in length
  • circular DNA
  • not associated with histone proteins
• Eukaryotic cells:

  • long length
  • linear DNA
  • associated with histone proteins
• chloroplasts/ mitochondria:

  • short DNA length
  • circular DNA
  • not associated with histones
• Structure of chromosome:

  in eukaryotic nucleus, a DNA molecule , made up of two polypeptide strands is twisted into a double helix around associated histones. This is called a chromosome.

  1. DNA molecule
  2. DNA is combined with histones
  3. DNA-histone complex is coiled
  4. coils fold to form loops
  5. loops coil and pack together to form the chromosome.
• A gene is a section of DNA located at a fixed location on a particular chromosome, called its locus. A chromosome carries many genes.
• The sequence of bases of a gene carries the coded genetic information that determines:

  • the amino acid sequence of a polypeptide during protein synthesis
  • a functional RNA molecule that is not translated into a protein but has a biological function.
• A complete set of genes in a cell is called the genome. The proteome is the full range of proteins that a cell is able to produce.
• The genetic code is carried as aa sequence of three DNA bases called a triplet or codon. Most triplets code for a specific amino acid.
• Some non-coding sequences signal the end of transcription.
• The genetic code is:

  • universal
  • non-overlapping
  • degenerate ( more than one triplet can code for and amino acid)
• The molecules mRNA and tRNA share some structural similarities:

  • a single polynucleotide chain
  • a pentose sugar , ribose , and the bases
• mRNA:

  • longer chain than tRNA
  • single stranded helix molecule
  • chemically unstable; only present when required for protein synthesis.
• tRNA:

  • short chain (approx, 80 nucleotides)
  • clover leaf shaped ; has amino acid attachment site and anticodon mRNA binding site on lower lobe.
  • chemically stable
• Protein synthesis is a mechanism in the cells whereby:

  1. DNA template is transcribed into messenger RNA molecule in the nucleus.
  2. The mRNA base sequence is then transcribed into an amino acid sequence in association with tRNA on ribosomes in the cytoplasm.
• Transcription is the production of mRNA using DNA. In eukaryotes , pre-mRNA is made as an intermediate step:

  1. DNA helicase breaks hydrogen bonds between bases , causing DNA to unzip and expose bases
  2. The enzyme RNA polymerase moves along DNA template strand and attaches free nucleotides to their complementary bases on DNA.
  3. RNA polymerase makes short strands of pre-mRNA until it comes to terminator sequence.
  4. sections of pre-mRNA are spliced together to remove intron sequences. this forms mRNA
• Translation is the production of polypeptides from the sequence of codons carried by mRNA.
• Translation:

  1. mRNA moves from the nucleus through nuclear pore to the the cytoplasm and the start codon attaches to a ribosome.
  2. a tRNA with a complementary anti-codon moves to the ribosome and pairs with the first mRNA codon.
  3. the ribosome moves along the mRNA to the next codon and again pairs it up with comp tRNA to bring the two amino acid carrying tRNAs together
  4. energy released from ATP is used to form peptide bonds
  5. the ribosome repeats this process till it reaches the stop codon where the polypeptide is complete and released.