DNA, Genes and Chromosomes

• Genes

  A sequence of DNA nucleotide bases that codes for a polypeptide or a functional RNA.
• DNA is a double stranded molecule, each strand is a polynucleotide made up of individual nucleotides.
• Each nucleotide in DNA compromises a phosphate, deoxyribose sugar and a nitrogenous base.
• The bases from one strand of DNA pair in complementary fashion with the bases on the other strand.

  Adenine pairs with Thymine.

  Guanine pairs with Cytosine.
• DNA molecules are many millions of base pairs long.
• Where is DNA found in prokaryotes?

  Prokaryotic (bacterial) cells have a single molecule of DNA formed into a circle. Some cells also have smaller loops called plasmids. The DNA is naked and not associated with histone proteins.
• DNA in the chloroplasts and mitochondria of eukaryotic cells is...

  ... a short, circular molecule not associated with histone proteins.
• DNA in the nuclei of eukaryotic cells is...

  ... a very long linear molecule which is associated with histone protein. Together a DNA molecule and the histone proteins form a chromosome.
• The Components of Chromosomes:

  A) Nucleus

  B) Chromosome

  C) Solenoid

  D) Histones

  E) Gene

  F) Centromere

  
• Homologous Chromosomes

  • The same genes in the same loci/ potentially different alleles
  • Same size and shape with the centromere in the same place
  • One is maternal and one is paternal
• The DNA molecule is wrapped around histone proteins to fix it into position. The DNA histone complex is then coiled into solenoids and then further coiled. This means that a very long molecule of DNA can be condensed into a single chromosome. A lot of genetic information can be stored in the nucleus of each cell.
• When the cell is in interphase, the chromosomes are dispersed throughout the nucleus and are not visible as separate structures.
• A chromosome can only be seen as a distinct structure when the nucleus is about to divide and the DNA molecule is already replicated by semi-conservative replication.
• Each sister chromatid contains a genetically identical DNA molecule. The two sister chromatids are held together by a centromere.
• Diploid (2n) organisms inherit a full set of chromosomes from each parent. As a consequence, diploid cells have homologous pairs of chromosomes.
• Humans have a diploid number 2n = 46.

  However, other species have different diploid numbers (eg dogs have 78 chromosomes and potato plants have 48).
• A gene occupies a fixed position called a locus on a particular DNA molecule. One DNA molecule carries many genes.
• In eukaryotes, most of the DNA molecule does not code for amino acid sequences in polypeptides. There are non-coding multiple repeat sequences between genes.
• The parts of the genes which code for amino acid sequences are called exons.
• Exons are separated by one or more non-coding sequences called introns.
• Genome

  The complete set of all of the genes in a cell, including the genes in the nucleus, mitochondria and chloroplasts.
• Proteome

  The full range of all the proteins that a cell is able to produce using its genome.
• Allele

  Some genes have two or more alternative forms. Each allele has a different sequence of bases and consequently codes for a different polypeptide.
• Gene pool

  All the different alleles of all the genes found within a population.