DNA, Genes, Chromosomes

• HOMOLOGOUS CHROMOSOMES:

  • ALLELES: One number of alternative forms of a gene. Each gene (that contains coded info) exists in 2 or more different forms (the alleles). Each individual inherits one allele per parent. Different alleles have different base sequences so produce different polypeptides.
• HOMOLOGOUS CHROMOSOMES:

  • HOMOLOGOUS CHROMOSOMES: Sexually produced organisms: from fusion of sperm & egg (which contribute 1 complete set of chromosomes) -> one of each pair is derived from chromosomes of the mother (maternal chromosomes) and the father (paternal chromosomes). They are homologous pairs, the total number= diploid number (in humans= 46). A homologous pair is always 2 chromosomes that carry the same genes, but not always the same alleles of the genes
• CHROMOSOME STRUCTURE:

  • CHROMOSOME STRUCTURE: Only visible when a cell is dividing, in all other times are widely dispersed in the nucleus. When first visible: as 2 threads joined at a single point. Each thread= chromatid. Centre= centromere.
  • OVERALL CHROMOSOME STRUCTURE: DNA is a double helix -> The double helix is wound around histones (to fix into position) -> The DNA-histone complex is then coiled -> The coil is looped & further coiled -> The looped coil is packed into the chromosome (therefore a chromosome contains 1 molecule of DNA, while DNA contains several genes on loci)
• THE GENETIC CODE:

  1)DEGENERATE: some amino acids are coded by more than one codon (there are 64 possible triplet codes and only 20 amino acids)

  2) NON OVERLAPPING: each base is only read once/ each triplet code is separate from other triplets in the chromosome

  3) UNIVERSAL: the same 4 bases A,T,C,G are used in the DNA of every organism & codons in DNA are transcribed into mRNA and then translated into amino acids in every organism (few exceptions, but nearly all triplet codes code for same amino acids in all organisms) -> indirect evidence for evolution.
• GENE:

  • TRIPLET CODE: The genetic code is a degenerate code as one amino acid can be coded by several triplets (the code has 3 bases so 1 code= triplet). A triplet is read in a particular direction along the DNA strand. Start of DNA sequence (that codes for a polypeptide): always same triplet (methionine). 3 triplets do not code for amino acids= stop codons & mark the end of the polypeptide chain. Coding sequences= exons, non-coding sequences= introns.
• GENE:

  • GENES: A section of DNA containing coded info for making polypeptides & functional RNA (eg tRNA or rRNA). Each gene has a specific base sequence. The coded info= a specific sequence of bases along DNA. It codes for an amino acid sequence or functional RNA (eg rRNA or tRNA) Genes are located at a locus (at a particular position) Polypeptides determine proteins/ enzymes (which are responsible for an organism's development by controlling chem reactions) and genes determine proteins so genes determine the nature and development of all organisms.
• MITOCHONDRIAL & CHLOROPLAST DNA:

  • ENDOSYMBIOTIC THEORY: Explains presence of DNA in mitochondria & chloroplasts. Bacterial cells were engulfed by a larger cell during evolution (they formed a beneficial symbiotic relationship & the bacterial cell became incorporated into the larger cell)
  • ROLE OF THE DNA: in mitochondria encodes enzymes used in respiration. In chloroplasts encodes enzymes used in photosynthesis
  • NATURE OF THE DNA: Like prokaryotic DNA: is short, circular, & not associated with proteins
• EUKARYOTIC DNA:

  • HISTONES: The DNA in eukaryotes are tightly wrapped around proteins called histones. DNA coils around the histones and is condensed in the chromosomes
  • LOCATED IN THE NUCLEUS: Eukaryotes have a nucleus, where DNA is located
  • LONG & LINEAR: Chromosomes in eukaryotic cells are long and linear (form a line)
  • HOMOLOGOUS PAIRS OF CHROMOSOMES: Diploid eukaryotes have homologous pairs of chromosomes (number of pairs varies per species e.g. humans have 23 pairs of chromosomes). Homologous means the mother and father have the same genes at the same loci but the alleles may differ
• PROKARYOTIC DNA:

  • SHORT & CIRCULAR: Chromosomes have a short and circular shape
  • LOCATED IN CYTOPLASM: Prokaryotes have no nucleus. DNA is located in the cytoplasm
  • SUPERCOILED: Prokaryotic chromosomes coil around themselves in a process called supercoiling. Supercoiling condenses the circular chromosomes so they can fit inside the cell
  • NO PROTEIN: Prokaryotic DNA are not associated with protein molecules so do not have chromosomes