STRUCTURE AND FUNCTION OF DNA

• DNA and RNA Structure

  Both DNA and RNA are nucleic acids, which consists of long chains (polymers) of chemical units (monomers) called nucleotides.
• DNA and RNA Structure

  Nucleotides are joined together by covalent bonds between the sugar of one nucleotide and the phosphate of the next.

  • This results in a repeating pattern of sugar-phosphate, which is known as sugar-phosphate backbone.
• DNA and RNA Structure

  Each nucleotide consists of three components:

  • a nitrogenous base
  • a sugar (blue)
  • a phosphate group
• DNA and RNA Structure

  The four nucleotides found in DNA differ only in their nitrogenous bases. The bases can be divided into two types:

  SINGLE RING STRUCTURES:

  • Thymine T
  • Cytosine C

  DOUBLE RING STRUCTURES:

  • Adenine A
  • Guanine G
• DNA and RNA Structure

  Instead of thymine, RNA has a similar base called uracil U.
• DNA and RNA Structure

  For DNA, the sugar is called deoxyribose because, compared with sugar ribose. it is missing an oxygen atom.

  DNA - deoxyribonucleic acid where nucleic referring to DNA's location in the nuclei of eukaryotes
• DNA and RNA Structure

  For RNA, it contains a slightly different sugar than DNA (ribose instead of deoxyribose, accounting for the names RNA versus DNA).
• DNA and RNA Structure

  RNA and DNA polynucleotides have the same chemical structure
• Watson and Crick's Discovery of the Double Helix

  Watson and Crick worked out the three-dimensional structure of DNA: two polynucleotide strands wrapped around each other in a double helix.

  • hydrogen bonds between bases hold the strands together
  • Each base pairs with a complementary partner: A&T, G&C
• DNA Replication

  The structure of DNA, with its complementary base pairing, allows it to function as the molecule of heredity through DNA replication.
• DNA Replication

  The two strands of parental DNA separate, and each becomes a template for the assembly of a complementary strand from a supply of free nucleotides.
• DNA Replication

  The nucleotides are lined up one at a time along the template strand in accordance with the base-pairing rules.

  • Enzymes link the nucleotides to form new DNA strands.
  • The completed new molecules, identical to the parental molecule, are known as daughter DNA molecules
• DNA Replication

  DNA polymerases are the enzymes that make the covalent bonds between the nucleotides of a new DNA strand.

  • As an incoming nucleotide base-pairs with its complement on the template strand, a DNA polymerase adds it to the end of the growing daughter strand.
• The Central Dogma: From DNA to RNA to Protein

  How an Organism's Genotype determines its Phenotype?

  • The information constituting an organism's genotype is carried in the sequence of its DNA bases.
  • The genotype controls phenotype through the expression of proteins.
• How an Organism's Genotype determines its Phenotype?

  DNA specifies the synthesis of proteins. However, a gene does not build a protein directly.

  • Instead, DNA dispatches instructions in the form of RNA, which in turn programs protein synthesis.
• How an Organism's Genotype determines its Phenotype?

  The molecules "chain of command" is from DNA in the nucleus to RNA to protein synthesis in the cytoplasm.
• How an Organism's Genotype determines its Phenotype?

  Replication - the process by which DNA makes a copy of itself during cell division.
• How an Organism's Genotype determines its Phenotype?

  The two stages are:

  1. Transcription - the transfer of genetic information from DNA to an RNA molecule.
  2. Translation - the transfer of the information from RNA into a polypeptide (protein strand)
  3. The start codon marks the site at which translation into protein sequence begins,
  4. AUG - the start codon
  5. UGA, UAA, UAG - stop codons
• How an Organism's Genotype determines its Phenotype?

  The function of a DNA gene is to dictate the production of a polypeptide.
• Nucleotides to Amino Acids

  The DNA of a gene is transcribed into RNA using the usual base-pairing rules, except that an A in DNA pairs with U in RNA.
• Nucleotides to Amino Acids

  In the translation of a genetic message, each triplet of nucleotide bases in the RNA, called a codon, specifies one amino acid in the polypeptide.
• The Genetic Code

  In addition to codons that specify amino acids, the genetic code has one codon that is a start signal and three that are stop signals for translation.
• Transcription: From DNA to RNA

  RNA polymerase binds to the promoter of a gene, opens the DNA double helix there, and catalyzes the synthesis of an RNA molecule using one DNA strand as a template.

  As the single-stranded RNA transcript peels away from the gene, the DNA strands rejoin.
• The Processing of Eukaryotic RNA

  The RNA transcribed from a eukaryotic gene is processed before leaving the nucleus to serve as messenger RNA (mRNA). Introns are spliced out, and a cap and tail are added.
• Translation: The Process

  There are three steps of translation:

  Initiation - a ribosome assembles with the mRNA and the initiator tRNA bearing the first amino acid

  Elongation - at the start codon, the codons of the mRNA are recognized one by one by tRNA's bearing succeeding amino acids.

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  The ribosome bonds the amino acids together. With each addition, the mRNA moves by one codon through the ribosome

  Termination - when a stop codon is reached, the completed polypeptide is released
• Summary of Central Dogma

  DNA --> RNA --> Protein