Chp 3

• nucleic acids are large biomolecules that store and transmit hereditary information
• DNA:

  • carries instructions that code for RNA
  • can self replicate
  • stores and transmits hereditary information
• RNA:

  • has different forms (rRNA, tRNA, mRNA)
  • read and translate DNA information (tRNA)
• nucleotides have three basic units:

  • phosphate group
  • five-carbon sugar (deoxyribose in DNA, ribose in RNA)
  • a nitrogenous base (A,G,C,T,U)
• Purines are nitrogenous bases with two rings in their structure

  • adenine
  • guanine
• pyrimidines are nitrogenous bases with one ring in their structure

  • thymine
  • uracil
  • cytosine
• complementary base pairs

  • adenine and thymine/uracil (joined by two weak hydrogen bonds)
  • guanine and cytosine (joined by three hydrogen bonds)
• DNA is a long, coiled, double-stranded nucleic acid that forms a double helix
• RNA is single stranded and are shorter than DNA molecules
• histones are proteins in eukaryotic cells that tightly package DNA into structures called nucleosomes
• nucleic acids are made up of nucleotides
• in DNA the 5 prime end is up and the 3 prime end is down and the double strands are antiparallel
• anticodons pair with mRNA codons
• genes are region sof DNA that may be translated into a polypeptide or an RNA molecule that can be functional
• the genetic code is a set of rules that defines how the information in nucleic acids is translated into proteins and functional RNA molecules
• transcription is the production of single-stranded mRNA from DNA
• translation is the process where the sequence of an mRNA molecule is used to produce the amino acid sequence of a polypeptide
• the genetic code is degenerate because more than one codon can code for the same amino acid - the genetic code uses 4 nucleotides while 3 nucleotides code for an amino acid
• structure of eukaryotic genes

  • promoter region
  • exons
  • introns
• promoter regions are upstream binding regions where RNA polymerase attaches to the gene
• exons are coding regions of DNA
• introns are non-coding regions of DNA
• RNA polymerase is needed to separate the double helix in a small section of DNA
• gene expression is the process where the information stored in a gene is used to synthesise a functional gene product (RNA or a protein)
• triplets are a sequence of 3 nucleotides in genes that corresponds to a specific amino acid in a polypeptide chain
• universal - the same triplet will code for the same amino acid in almost all organisms
• ribosomal RNA is a structural component of ribosomes
• messenger RNA (mRNA) is a single stranded molecule that carries the information from the DNA to the ribosome
• transfer RNA bring amino acids in the order specified by the mRNA and rRNA to the ribosomes to help build a polypeptide chain
• Protein synthesis has three phases

  • transcription
  • RNA processing
  • translation
• Transcription has three phases

  • initiation - RNA polymerase attaches to the promoter and begins unwinding the double helix, this strand is complementary to te non-template strand (coding strand)
  • elongation - moving in a 3' to 5' direction, RNA polymerase catalyses condensation polymerisation of free m RNA nucleotides, DNA triplets are being transcribed into RNA codons
  • termination when RNA polymerase hots the stop codon the mRNA transcript detaches from the RNA polymerase
• RNA processing has two steps:

  • addition of a 5' cap and a poly-A tail to the 5' and 3' end of the pre-mRNA
  • splicing - introns are removed and exons are spliced together, the mature mRNA moves into the cytoplasm
• Transcription and RNA processing occurs in the nucleus and translation occurs in the cytoplasm in a ribosome
• Translation has three stages:
• Initiation:

  • Ribosomal subunit attaches to the 5' end of the mRNA molecule and moves along until it finds the start codon
  • A loaded tRNA molecule with the anti-codon attaches to the codon
  • A large ribosomal subunit attaches to the tRNA and the small ribosomal subunit
• Elongation:

  • Another loaded tRNA molecule with an anticodon for the next triplet on the mRNA molecule attaches
  • Amino acids form a hydrogen bond to create a growing polypeptide chain
  • Unloaded tRNA molecule with the anticodon detaches and leaves
• Termination:

  • Polypeptide chain continues to form until a stop codon is reached
  • The polypeptide chain is released from the ribosome
• alternative splicing is when different combinations of exons form different proteins when some exons are spliced along with the introns
• there are three sites for tRNA to bind, aminoacyl site, peptidyl site, exit site
• gene expression in eukaryotes can be regulated at any stage of the three stages