lecture 4-2 (protein synthesis)

• Protein synthesis follows the central dogma that transcription of DNA leads to the synthesis of RNA, and the translation of RNA leads to protein synthesis
• RNA stands for ribonucleic acid. It is a single-stranded nucleic acid with ribose sugar and nucleotide bases consisting of adenine, guanine, cytosine, and uracil.
• Label this diagram

  A) transcription

  B) DNA

  C) RNA processing

  D) mRNA

  E) translation

  F) ribosome

  G) polypeptide

  
• Genes are units found on the DNA sequence which determine the amino acids that compose the protein to be produced by using the genetic code
• A gene is composed of a promoter region, introns, and exons.
• Exons are spliced together to create RNA and used in translation, while introns are spliced out of pre-processed RNA.

  
• Transcription initiates at the promoter region (which contains the TATA sequence) at the 5' end of the gene, which then binds to the transcription initiation complex (RNA complex II + transcription factors)
• The transcription initiation complex is composed of RNA polymerase II and transcription factors
• Once binded to DNA, RNA polymerase begins a new mRNA strand in the 5' to 3' direction.
• The polyadenylation signal (AAUAAA) terminates transcription once the polymerase reaches it.
• The 5' cap, which is a modified form of guanine, helps protect mRNA from hydrolytic enzymes and is a site for ribosome attachment.
• The poly-A tail is added in RNA processing. It helps transport the RNA out of the nucleus and provides greater protection to the mRNA.
• Spliceosomes remove introns and join exons together

  
• The 5’ G-P-P-P region protects the mRNA from being degraded by enzymes in the cytoplasm. It also provides a site to signal ribosomes where to attach
• The poly-A tail (AAA- AAA) inhibits mRNA degradation and facilitates export out of the nucleus
• A DNA gene sequence may produce a much shorter mRNA molecule
• The three steps of RNA processing are the addition of a 5' cap region and a poly-AAA tail, and then splicing out of introns and merging of exons. after this, the mRNA can now leave the nucleus.
• The genetic code specifies 20 amino acids.

  • This code is redundant, meaning that the same amino acid can be coded by different codons.
  • It is unambiguous, meaning that one codon only codes for one amino acid.
  • It is universal, meaning that all organisms have the same code.
• Label this ribosomal unit.

  A) exit

  B) binding

  C) Aminoacyl

  D) peptidyl trna

  E) trna

  F) binding

  G) large

  H) small

  
• What is the most abundant type of nucleic acid?

  ribosomal RNA
• The steps of translation are

  1. Initiation
  2. Elongation
  3. Termination
• The small sub unit of ribosome binds to the 5' cap region of mRNA. Then a t-RNA molecule carrying a methionine arrives to the P-site which requires energy. The large sub unit of ribosome binds to the small subunit to form the translation initiation complex.
• Anti-codon sites on t-rna determine which amino acids the t-rna corresponds with. There are 20 t-rna molecules for 20 amino acids .
• The binding of a specific amino acid to the correct t-RNA is catalyzed by aminoacyl-tRNA synthetase
• Elongation is done by t-RNAs carrying amino acids arriving to the A-site. The amino acid from the t-RNA at the P-site is transferred the the t-RNA at the A site by the formation of a polypeptide bond. Then, the t-rna moves down the sites.
• To initiate termination, the ribosome must reach the stop codon (IAA, UAG, UGA). This stop codon codes for a water molecule, which acts as a release factor, releasing the polypeptide chain, the departure of the last r-RNA molecule from the ribosome complex, and the separation of the small and large subunits.
• It takes about 1 minute for a cell to produce a new polypeptide
• A point mutation is caused by a single base pair chain in the DNA sequence.
• A silent mutation does not change the amino acid sequence.
• Sickle cell hemoglobin is a mutation of the hemoglobin protein which leads to a higher resistance against malaria
• A mis-sense mutation can lead to a change in the amino acid sequence of the protein.
• A nonsense mutation leads to an early stop codon and the protein cannot be made
• Mutations can lead to diseases such as Parkinson’s, Alzheimer’s, and cancer. However, it can also increase genetic diversity and lead to good mutations. For example, a mutation made humans less hairy than chimpanzees.