AP Bio unit 6

• Deoxyribonucleic acid (DNA)


  A type of nucleic acid
• Ribonucleic acid (RNA)


  A type of nucleic acid
• DNA and RNA


  • They are polynucleotides made up of many nucleotides linked together in a chain
  • They are both found in all living cells and are both needed to build proteins, which are essential for the proper functioning of cells
  • They are both important information-carrying molecules
• Although their main functions are slightly different, DNA and sometimes RNA are the primary source of heritable information
• Offspring inherit characteristics as a result of the genetic code found in the nucleic acids which are passed from one generation to the next
• Bases in DNA nucleotides

  • Adenine (A)
  • Cytosine (C)
  • Thymine (T)
  • Guanine (G)
• Base pairing in DNA

  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)
• DNA structure


  • The phosphate and sugar section of the nucleotides form the 'backbone' of the DNA strand (like the sides of a ladder) through the formation of strong phosphodiester bonds
  • The base pairs of each strand project out and connect by complementary base pairing to form the rungs of the ladder
• RNA nucleotides


  They contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C) but not thymine (T) – in place of this they contain the nitrogenous base uracil (U)
• RNA molecules


  • They are only made up of one polynucleotide strand (they are single-stranded)
  • They are relatively short compared to DNA
• Phosphodiester bonds


  Strong covalent bonds between different nucleotides in the same strand, with the nitrogenous bases sticking out sideways from the sugar/phosphate backbone
• Eukaryotes


  Cells with membrane-bound organelles
• Prokaryotes


  Do not have membrane-bound organelles
• DNA in prokaryotes


  Circular and free in the cytoplasm
• Nitrogenous base structural forms

  • Purines (adenine, guanine - double-ring structure)
  • Pyrimidines (cytosine, thymine, uracil - single-ring structure)
• A purine always pairs with a pyrimidine to create base pairs of equal length (a single ring base matched to a double ring base)
• This gives stability to the structure of DNA, strengthened by the hydrogen bonds between pairs
• DNA replication


  1. Hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands are broken

  2. This 'unzips' or unwinds the DNA double helix to form two single polynucleotide DNA strands

  3. This process is catalyzed by the enzyme DNA helicase

  4. Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand

  5. The original strand and the new strand then join together to form a new DNA molecule
• Differential gene expression


  The process by which different genes are expressed (activated) in different cells, tissues, or at different times during development
• Gene regulation mechanisms


  • Promotors
  • Suppressors
  • Transcription factors
  • Epigenetics
• Semiconservative replication


  Half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
• RNA


  Regulates gene expression
• Main types of RNA

  • mRNA
  • tRNA
  • rRNA
• Gene regulation mechanisms


  Play a role in differential gene expression
• Small Nuclear RNA (snRNA)

  Noncoding RNAs responsible for splicing introns
• RNA primers


  Mark the position for the DNA polymerase to bind
• Stem cells


  Become specialized through differential gene expression
• Intron removal and mature mRNA formation

  1. Pre-mRNA transcript

  2. Introns removed

  3. Mature mRNA forms
• Nucleoside triphosphates


  Free nucleotides with three phosphate groups, also known as 'activated nucleotides'
• MicroRNA (miRNA)


  Noncoding RNAs mainly involved in gene regulation
• Only certain genes in the DNA of the stem cell are activated and get expressed
• miRNAs are mostly processed from introns
• DNA polymerase


  1. Synthesizes new DNA strands from the two template strands

  2. Catalyzes dehydration synthesis reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar/phosphate backbone of the new DNA strands

  3. Cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)

  4. Can only build the new strand in one direction (5' to 3' direction)
• miRNA regulation of gene expression

  1. Bind to untranslated region on mRNAs to suppress translation

  2. Bind to promoter regions to boost transcription
• Every nucleus within the stem cells of a multicellular organism contains the same genes, that is, all stem cells within an organism have an identical genome
• miRNAs


  • Can function similarly to hormones
  • Released into tissue fluid and taken up by other cells for regulation of cellular activity
• miRNAs are ideal biomarkers for diagnosis of various diseases including cancer through their role in controlling oncogenes and tumor suppressors
• During differentiation certain genes are expressed ('switched' on)
• Small Interfering RNA (siRNA)


  Double-stranded, noncoding RNAs that inhibit gene expression through RNA interference
• Replication fork


  The point where the DNA strands separate