Genes and Genes Expression

• Transcription:

  • Creates RNA from DNA
  • Occurs inside the nucleus
  • Catalyzed by RNA polymerase
  • Involves enzymes like helicase, polymerase, and topoisomerase
  • Promoter site and directionality: read in the 3' - 5' direction, created in the 5' - 3' direction
  • Template and coding strands in DNA transcription
• Post-Transcriptional Modification:

  • Prokaryotes: generally do not need modifications, polycistronic mRNA
  • Eukaryotes: transformation of hnRNA into mRNA; 5' cap and 3' poly(A) tail; Introns are excised and exons are coded in mRNA, alternate splicing for protein variations
• Translation:

  • Prokaryotes: occurs simultaneously with transcription, in the nucleoid
  • Eukaryotes: occurs in the cytoplasm, on rough ER or free-floating ribosomes
  • Ribosomes consist of rRNA and have large and small subunits
  • tRNA delivers amino acids, codons determine amino acids brought by tRNA
  • 3 steps: Initiation, elongation, and termination steps
• Post-Translational Modifications:

  • Phosphorylation, glycosylation, ubiquitination
  • Protein folding, proteolytic processing, zymogens, preprohormones
• Gene Expression:

  • Genome in all body cells, exceptions: gametes, mutations
  • Stem cells: totipotent, pluripotent, multipotent
  • Prokaryotic gene expression: operons, positive and negative control, Lac and Trp operons
• Eukaryotic Gene Expression:

  • Promoters, transcription factors, enhancers, silencers
  • microRNA and siRNA for mRNA degradation
  • DNA modification: acetylation, methylation, epigenetics
• Cell Cycle:

  • G0, interphase (G1, S, G2), M-phase with checkpoints
  • Mitosis in somatic cells, 4 phases: prophase, metaphase, anaphase, telophase
  • Mitosis results in 2 identical diploid daughter cells
• Meiosis:

  • Produces 4 haploid genetically different cells in germ cells
  • Two rounds of division: Meiosis I and II, similar stages to mitosis
  • Prophase I with crossing over, metaphase I with tetrads lining up on metaphase plate, anaphase I where separation of homologous pairs occurs, and telophase I and cytokinesis
  • Each daughter cell is now haploid
  • Each chromosome has 2 chromosomes
  • Meiosis II identical to mitosis, results in haploid daughter cells
• Meiosis results in four haploid genetically unique cells
• Meiosis involves crossing over of homologous pairs in prophase I
• RNA polymerase catalyzes transcription
• RNA polymerase locates gene sequences and binds at the promoter (TATA box in eukaryotes)
• RNA synthesizes precursor mRNA molecules from DNA template strands
• RNA reads template strand 3' to 5'; synthesizes 5' to 3' direction
• Template (antisense) is read by RNA polymerase
• Coding strand (sense) is the DNA strand that is identical to the RNA strand being made (except for T and U nucleotides)
• RNA polymerase I: synthesizes rRNA in nucleolus
• RNA polymerase II: synthesizes hnRNA (mRNA precursor)
• RNA polymerase III: synthesize tRNA and some rRNA
• Post-transcriptional modifications not necessary in prokaryotes because it has polycistronic mRNA. translation and trascription occur simultaneously
• Eukaryotic post-transcriptional modification includes the addition of a 3' poly(A) tail, 5' cap, and RNA splicing
• Poly-A tail: the sequence of adenine nucleotides added to 3' end of RNA; increases mRNA stability; facilitates nuclear export; protects mRNA from degradation
• 5' cap: 7-methylguanylate triophosphate cap added to 5' end; protects RNA from degradation; aids in nuclear export
• RNA splicing: non-coding introns are excised from hnRNA; coding exons are spliced together to be expressed later; catalyzed by spliceosome
• Alternative splicing: different proteins generated from the same RNA transcript based on exon combination; increases diversity
• Eukaryotic translation occurs in the cytoplasm, rough ER
• Eukaryotic transcription occurs in the nucleus
• Prokaryote ribosome size: 30S + 50S = 70S
• Eukaryote ribosome: 40S + 60S = 80S
• RNA types in Translation:

  1. mRNA
  2. rRNA
  3. tRNA
• tRNA: carries amino acids to ribosomes, matches them to mRNA
• mRNA: carries genetic information from DNA
• rRNA: composes ribosomes, facilitates protein synthesis
• Wobble hypothesis: one tRNA molecule can recognize several related codons that usually only differ in the third nucleotide; they still encode the same amino acid
• Degeneracy of genetic code: multiple related codons may encode same amino acid
• Initiation:

  • ribosomal small subunit binds mRNA, followed by tRNA at the start codon
  • Start codon: AUG codes for methionine
  • Large subunit binds to small subunit
• Elongation:

  • Ribosome reads mRNA in 5' to 3' direction
  • New tRNA binds at A site (acceptor)
  • Peptide bond forms between amino acids in the A and P (peptidyl) sites
  • Ribosome moves further down mRNA and the tRNA at the E (exit) site leaves the ribosome
• Termination

  • Ribosome encounters a stop codon: UAG, UGA, UAA
  • Peptide sequence is released from ribosome
  • Ribosome releases mRNA molecule; protein synthesis terminated
• Post-translational modifications:

  • Phosphorylation: add phosphate
  • Glycosylation: add carbohydrate for cell signaling and stability
  • Ubiquination: add ubiquitin for degradation
  • Protein folding
  • Proteolytic processing
• Chaperone proteins assist in folding in 3D structures in the cytoplasm and ER