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DNA Replication

DNA Replication

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  • Nuclear division is the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis
  • Cytokinesis follows the nuclear division and is the process where the rest of the cell divides
  • Before the nucleus can divide the DNA must be replicated to ensure that the resulting daughter cells have the same genetic code for to produce the correct enzymes and other proteins

Conservative Replication

  • The conservative replication model suggests that the original DNA molecule remains intact and that a separate daughter DNA copy was built from new molecules of deoxyribose, phosphate and organic bases. Of the two molecules produced, one would be mode of entirely new materials whilst the other would be entirely original material.

Semi Conservative Replication

  • The semi conservative model suggests that the original DNA molecule splits into two separate strands that new nucleotides fill in the opposite chain. The result is one strand of new polynucleotide chains and one strand of the original polynucleotide chain.
  • The semi conservative replication to occur it needs 4 requirements
  1. The 4 types of nucleotides (adenine, guanine, thymine and cytosine)
  2. Both strands of the DNA molecule act as a template for the attachment of the nucleotide
  3. The enzyme polymerase
  4. A source of chemical energy which is required to fuel the process

Process of Semi Conservative Replication

  1. The enzyme DNA Helicase breaks the hydrogen bonds linking the base pairs of DNA. The DNA molecule therefore unwinds into two separate strands
  2. Each separate strand has the exposed polynucleotide that acts like a template to which complementary free nucleotides that are abundant around in the nucleus bind by specific base pairings
  3. The nucleotides are joined through a condensation reaction by the enzyme DNA polymerase to form the new polynucleotide strand on each of the two original polynucleotide strands of DNA
  4. The result is that there are now 2 DNA molecules, with each have 1 strand of the old DNA and 1 strand of the new DNA

Proving the Semi Conservative Model

  • Meselsohn and Stahl devised an experiment to discover which model was correct
  • They based their experiment on 3 key areas
  • All the bases in DNA contain nitrogen
  • Nitrogen has 2 forms, the lighter nitrogen (14N) and the heavier isotope (15N)
  • Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make
  • Bacteria was grown in 2 mediums, one in the 14N and the other in 15N
  • The grew the 15N bacteria for several generations before transferring them to 14N for one generation to replicate.
  • The mass of this molecule of DNA was measured using a centrifuge, with the heavier molecules being at the bottom and the lighter at the top.
  • The 14N was at the top
  • The 15N was at the bottom
  • The 14N and 15N was in the middle proving that half of the DNA comes from the old DNA and the other half made of new DNA; thus the semi conservative model was correct.
Nucleic Acid

Nucleic Acid

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  • Nucleic acids are a group of molecules that include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

    Nucleotide Structure

  • A nucleotide is made up of 3 components:
  • Pentose Sugar
  • Phosphate group
  • A nitrogen containing organic base (Adenine, cytosine, thymine, guanine and uracil)
  • All 3 components are joined through a condensation reactions to form 1 nucleotide.
  • Two mononucleotides can join together as a result of a condensation reaction between one of the phosphate groups on one mononucleotides to the deoxyribose sugar on the other mononucleotides. The bond formed is a phosphodiester bond. The resulting product is a dinucleotide where if more nucleotides are added it then results in a polynucleotide
  • Ribonucleic Acid (RNA) Structure

  • Ribonucleic acid is a polymer made up of nucleotides
  • It is relatively short chain (especially when compared to DNA)
  • RNA is a polynucleotide chain where the pentose sugar is always ribose and the organic bases are Adenine, Guanine, Cytosine and Uracil.
  • One example of RNA transfers genetic information from the DNA to the ribosomes which has its own RNA strand to create proteins and other types of RNA. Other RNA can be found in the production of protein synthesis
  • Deoxyribonucleic Acid (DNA) Structure

  • Deoxyribonucleic acid is a polymer made up of nucleotides
  • It is long chain (especially when compared to RNA)
  • DNA is a polynucleotide chain where the pentose sugar is always ribose and the organic bases are Adenine, Guanine, Cytosine and Thymine.
  • DNA is made of 2 strands, with hydrogen bonds joining them together
  • The phosphate groups and the deoxyribose molecule alternate u[p the polynucleotide chain
  • Base Pairing

  • The bases on the two strand are joined together through hydrogen bond which holds the entire DNA molecule together
  • The base pairs are specific to each other:
  • Adenine and Thymine (uracil in bacteria)
  • Guanine and Cytosine
  • Guanine will always and only bond to cytosine, therefore guanine’s complimentary base pair is cytosine. and adenine complimentary base pair is thymine (or uracil)

  • The amount of adenine and thymine, and guanine and cytosine are always the same however it is the ratio of the adenine and thymine to guanine and cytosine which varies from between different species
  • The stability of DNA

  • The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
  • The hydrogen bonds between the organic base pairs form bridges (rungs) between the phosphodiester uprights.
  • Between the cytosine and guanine are 3 hydrogen bonds, therefore increasing the stability of the molecule as a whole compared to the adenine and thymine which has 2 hydrogen bonds.
  • As the molecule is so stable, it is rare for mutations to occur which makes it ideal for carrying hereditary material that is passed on generation after generation
  • Function of DNA

  • DNA is a hereditary material responsible for passing genetic information from cell to cell and generation to generation
  • There are in total 3.2 billion base pairs in DNA for a typical mammalian cell
  • During DNA replication, the weak hydrogen bonds between the base pairs are able to separate to reveal their code
  • By having the actual genetic code embedded into the phosphate backbone, it protects the material from being corrupted by outside chemicals or physical forces
  • Base pairing allows for DNA to replicate and transfer information as mRNA
  • Prime Location

  • The carbon atom in the pentose sugar are numbered of particular importance as 3’ (3 prime) and 5’ (5 prime) carbon atoms.
  • 5’ has an attached phosphate group
  • 3’ has a hydroxyl group
  • When nucleotides are organised in to the double helix stand, one of the strands run in the 5’ to 3’ direction whilst the opposite strand runs in a 3’ to 5’ direction
  • The two strands are said to be antiparallel

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DNA and Chromosomes

DNA and Chromosomes

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Chromosome Structure

  • Chromosomes are only visible as distinct structures when a cell is dividing
  • Through interphase, the chromosomes are free to move in the nucleus
  • The first time they are visible is during cell division in which they appear as 2 threads (chromatids) joined at a single point (centromere)
  • Each chromatid has already replicated to give two identical DNA molecule
  • The DNA is wrapped around histones (protein) molecules which are then further wrapped to form chromosomes

Homologous Chromosomes

  • All chromosomes occur in pairs, one from the mother (maternal chromosome) and one from the father (paternal chromosome)
  • These are known as homologous chromosome pairs and that total number is reffered to as the diploid number
  • A homologous pair is always two chromosomes that correlate into the same genetic characteristic however determining the same genetic characteristic is not the same as being identical e.g. both homologous pairs have the allele for hair colour, but the maternal chromosome might code for blond hair whilst the paternal chromosome codes for brown hair.
  • Through meiosis, the division of the number of chromosomes is carried out to ensure that each daughter cell receives one chromosome from each homologous pair. Each cell therefore has one chromosome for each characteristic of the organism. When these haploid cells combine, the diploid state with paired homologous chromosome is restored


  • A small section of DNA is a gene. Each gene exists in 2 (and sometimes more) forms. Each form is called an allele
  • Each cell has one allele from its maternal and paternal cells. These two alleles may be the same or different. If they are different each allele will code for a different polypeptide