DNA, genes and chromosomes

• Describe DNA in eukaryotic and prokaryotic cells

  Although the structure of DNA is the same in all organisms, eukaryotic and prokaryotic cells store DNA in different ways.

  
  

  Eukaryotic Cells:

  -Contain linear DNA molecules that exist as chromosomes, thread like structures, each made up of one long molecule of DNA.

  -Chromosomes are found in the nucleus.

  -The DNA molecules is wound around proteins called histones.

  -Histone proteins help to support the DNA.

  -The DNA (and protein) is then coiled up tightly to make a compact chromosome.

  -The mitochondria and chloroplasts in the eukaryotic cells also have their own DNA. This is pretty similar to prokaryotic DNA because it is circular and shorter than DNA molecules in the nucleus. This is also not associated with histones.

  
  

  Prokaryotic Cells:

  -Prokaryotes also carry DNA as chromosomes, but the DNA molecules are shorter and circular.

  -DNA isn't wound around histones, but instead, condenses to fit into the cell by supercoiling.

  
• Describe the importance of genes

  -A sequence of DNA bases that codes for either a polypeptide or functional RNA.

  -The sequence of amino acids in a polypeptide forms the primary structure of a protein.

  -Different polypeptides have a different number and order of amino acids. It is order of bases in a gene that determines the order of amino acids in a particular polypeptide.

  -Each amino acid is coded for by a sequence of three bases in a gene called a triplet.

  -To make a polypeptide, DNA is first copied in messenger RNA (mRNA). This is the first stage of protein synthesis.

  -Genes that don't code for a polypeptide code for functional RNA instead. Functional RNA is the RNA molecules, other than mRNA which perform special tasks during protein synthesis, e.g. tRNA and ribosomal RNA (rRNA), which forms part of ribosomes.

  
• Genome definition


  The complete set of genes in the cell.
• Proteome definition


  The full range of proteins that the cell is able to produce.
• Importance of Introns and Exons

  -Some genes do not code for polypeptides at all, they code for functional RNA.

  -In eukaryotic cells, genes that do code for polypeptides (exons) contain sections that do not code for amino acids (introns).

  -These sections of DNA are called introns. There can be several introns within a gene.

  -All the bits of a gene that do code for amino acids are called exons.

  -Introns are removed during protein synthesis, so they don't affect the amino acid order. Their purpose isn't known for sure as prokaryotic DNA does not contain introns.

  -Eukaryotic DNA also contains regions of multiple repeats outside of genes.

  -These are DNA sequences that repeat over and over e.g. CCTTCCTTCCTT.

  -These areas don't code for amino acids either, so they are non-coding repeats.

  
• Alleles definition


  -Different forms of a gene.

  -A gene can exist in more than one form, these forms are called alleles.

  -The order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide. For example, the gene that determines blood type exists as one of three alleles, one determines O, another type A and the other type B.
• Homologous Chromosomes definition


  -In a eukaryotic cell nucleus, DNA is stored as chromosomes.

  -Humans have 23 pairs of chromosomes, 46 in total - two number 1s, two number 2s ect.

  -Pairs of matching chromosomes are called homologous pairs. One chromosome is from the mother and one is from the father.

  -In a homologous pair, both chromosomes are the same size and have the same genes, although they could have different alleles.

  -Alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosomes in a homologous pair.

  
• RNA definition


  -A single polynucleotide strand.

  -Contains uracil (U) as a base, instead of thymine.

  -Uracil always pairs with adenine during protein synthesis.