1.1 DNA Structure and Replication

• DNA


  Deoxyribonucleic acid
• DNA structure


  • Double helix (twisted ladder)
  • Double stranded (two strands run in opposite directions- anti-parallel 5' to 3')
  • Macromolecule (large molecule – made of smaller units= nucleotides)
• Nucleotides


  • Phosphate
  • Sugar (deoxyribose)
  • Nitrogenous bases (Adenine, Thymine, Cytosine, Guanine)
• Nitrogenous bases


  • Complementary (A and T, C and G)
  • Held together by weak hydrogen bonds (able to break apart easily) (A and T have 2, C and G have 3)
  • Complementary base pairing is consistent pairing of bases (A=T, C=G) allows DNA to accurately replicate itself, and also allows DNA to be transcribed accurately into RNA and then translated from RNA to amino acids (to form proteins)
• Eukaryote DNA


  • Linear (lines)
  • 2 or more chromosomes present (homologous pairs)- DIPLOID
  • Found within the nucleus
  • Bound to histones (proteins)
  • Contains introns (non-coding regions) and exons (coding regions)
• A small circular double stranded DNA is also found in chloroplasts and mitochondria (maternal DNA) within the cells –make up 1% of the DNA in a cell (small amount)
• Homologous chromosomes


  One inherited from mum, one from dad
• Chromosome


  • Tightly coiled DNA (only appear in this way when cell is about to divide)
  • Can be distinguished due to different shape, size, loci (location of genes) different genes found on each chromosome
  • Each species has a different number of chromosomes (46 humans, - 23 pairs of homologous chromosomes)
• Chromatin


  DNA strand and the histone (DNA wraps around histones to keep ordered within the cell). How DNA is found majority of the time in the nucleus (cell cycle)
• Chromatid


  When DNA replicates – the linear DNA appears in a X shape- each strand is called a chromatid (sister chromatids), together make a chromosome. When pulled apart still called a chromosome
• Centromere


  Holds the two sister chromatids together in center of chromosome
• Genes


  Segments of DNA that code for a polypeptide/protein
• Prokaryote DNA


  • Circular
  • Single double stranded DNA molecule- haploid
  • Some have small loops of DNA called plasmids
  • Free loading in cytoplasm (nucleoid- no nucleus)
  • Unbound to histones
  • Exons only (coding DNA)
• Differences between DNA and RNA

  • DNA (deoxyribonucleic acid): Double stranded, Sugar= deoxyribose, Bases A=T, C=G, Present only in nucleus, chloroplasts & mitochondria
  • RNA (ribonucleic acid): Single stranded, Sugar = ribose, Base Uracil instead of Thymine pair (Adenine = Uracil), Present in the nucleus, cytosol, ribosomes, mitochondria
• Structure of DNA allows it to

  • Store and transmit genetic information by using chromosomes
  • Make copies of itself so that genetic information can be inherited or passed on to the next generation
  • Is a code that can be used by cells to manufacture protein molecules
• Importance of DNA structure

  • The double helix structure of DNA provides stability, enables efficient information storage, and reliable replication, which are crucial for the functioning and inheritance of genetic
• Semi-conservative replication of DNA

  1. DNA helicase breaks the hydrogen bonds between the complementary bases joining the two strands and separates the DNA into two strands

  2. Each strand serves as a template for making a new strand which is complementary to the template

  3. Free floating DNA nucleotides base-pair to the exposed bases i.e. A-T, G-C

  4. Enzymes DNA polymerases link the nucleotides to the newly forming strand

  5. Each new double-stranded DNA molecule rewinds into a double helix

  6. Two DNA molecules are made that are semi-conservative