Dna genes and chromosomes

• Dna transfers genetic info from the nucleus to the ribosomes. 
• Ribosomes are made up of proteins and RNA
• Monomers for DNA and RNA is nucleotide. Nucleotides are joined together through condensation reactions. A phosphodiester bond will form between the sugar of 1 nucleotide and the phosphate of another nucleotide.
• There are 4 bases in a nucleotide. Adenine, guanine, cytosine and thymine.

  Ribose is the sugar in RNA. In RNA uracil replaces thymine.
• The differences in DNA:

  prokaryotic cells: DNA is shorter, circular, no proteins and no chromosomes

  Eukaryotic cells: DNA is longer, linear, associated with histones and form chromosomes

  Mitochondria and chloroplasts: DNA is shorter circular and has no proteins
• Chromosomes are only visible during cell division. The double stranded DNA is wound around histones ( proteins). A DNA- histone complex coils. The coils fold to form loops. The loops then coil and pack to form a chromosome.
• Locus


  The position of each gene on a chromosome
• Humans have 23 pairs of chromosomes
• Homologous pairs


  Carry the same genes, one from the mother and one from the father
• Diploid number


  The total number of chromosomes in a cell's nucleus
• Homologous pairs carry the same genes, one from the mother and one from the father
• Alleles are alternative forms of the same genes. E.g Gene for eye colour, alleles can be for blue, green and brown eyes.
• The genetic code:

  There are 3 bases for an amino acid. This is called the triplet code.

  There are 20 different amino acids.

  Some amino acids have more than one triplet code (between 2-6). These are known as degenerate codes.

  Some amino acids are coded for by only one triplet.

  A triplet is always read in one direction.
• DNA sequences always start with the amino acid methionine.

  3 triplets that don't code for an amino acid end the polypeptide chain like a full stop.

  The code is non overlapping, so each base is only read once.

  Universal code means each triplet codes for the same amino acid in all organisms
• Exons- base sequences that code for amino acids.

  Introns- non coding sequences
• The genetic code:

  A gene is a sequence of bases in DNA that codes for the sequence of amino acids in a polypeptide.

  The 'language' of a gene only has 4 letters: A,T,C,G
• There are two major processes involved in protein synthesis:

  Transcription And translation
• Transcription


  The relevant gene in the DNA in the molecule nucleus is 'copied' into a molecule of RNA called mRNA
• Transcription process


  1. DNA double helix unzips

  2. Hydrogen bonds between the complimentary bases breaks and the two polynucleotide strands separate

  3. One strand is called the anti sense strand and acts as a template

  4. Free RNA nucleotides complimentary bases pair to the exposed bases on this strand by forming hydrogen bonds between the nucleotides

  5. The mRNA detaches from the sense strand

  6. The two DNA strands join together by complimentary bases pairing and the DNA molecules wind back up into a helix
• The sequences of 3 bases on the mRNA coding for amino acid are called CODONS.

  Not all bases in the DNA code for amino acids so the mRNA removes the non coding regions known as introns by splicing. This is done by enzymes before the mRNA leaves the nucleus.

  This leaves just exons/ coding regions of mRNA.
• Translation is the process in which the mRNA is 'read' and translated into a protein.

  Translation occurs on the ribosomes.

  The process involves another type of RNA molecule known as tRNA.
• Transcription to translation:

  Following the removal of introns, the mRNA moves out through a nuclear pore and attaches to a ribosome .

  A tRNA molecule attached to an amino acid will attach it's complimentary base pair to a complimentary triplet code. Peptide bonds form.

  The mRNA has a start and stop codon and the process will continue happening until the stop codon.

  The anticodons on tRNA are complimentary to CODONS on mRNA.

  More tRNA attaching a hint between the amino acids form and the tRNA leaves. Peptide bonds form and a protein is made.