MELC 15 DNA Replication and Protein Synthesis

• DNA

  Number of strands - double helix

  Type of sugar - deoxyribose

  Nitrogenous bases - ATCG (adenine, thymine, cytosine, guanine)

  Location in cell - nucleus
• RNA

  Number of strands - single helix

  Type of sugar - ribose

  Nitrogenous bases - AUCG (adenine, uracil, cytosine, guanine)

  Location in the cell - cytoplasm
• deoxyribonucleic acid (DNA) - is a nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.
• nucleotides - the building blocks of DNA. Each nucleotide is composed of one phosphate group, the sugar deoxyribose and the bases Adenine, Thymine, Cytosine and Guanine (A, T, C, G).
• two types of bases of DNA

  purines - two carbon rings (guanine, adanine)

  pyrimidines - one carbon ring (thymine, uracil)
• James Watson and Francis Crick - in 1953, took some of Rosalind’s x-ray diffraction images of DNA and correctly determined that it was a DNA is a double helix.
• ribonucleic acid (RNA) - are much shorter and are typically single stranded. perform a variety of roles in the cell but are mainly involved in the process of protein synthesis (translation) and its regulation.
• deoxyribonucleic acid (DNA) - blueprint of all living things

   contains hereditary material that controls the activities of each cell.
• gene expression - use of genetic information to make proteins. also known as protein synthesis. during this, the messages encoded in DNA direct all cellular activities.
• DNA replication - process during which a double-stranded DNA molecule is copied to produce two identical DNA molecules.
• base pairing - principle in which the nitrogenous bases of the DNA molecules bond with one another
• gene expression occurs in two parts:

  transcription and translation
• gene expression

  Watson and Crick describes this as the “central dogma”
• replication - the process of making copies of DNA.

  semi-conservative process - each new strand of DNA produced is made of one parental and one new strand (described by Watson and Crick)
• helicase - enzyme that unwinds and unzips the helix at the replication forks.
• single-strand binding protein (SSBP’s) - binds to unpaired DNA strand to keep them from re-pairing.
• topoisomerase - enzyme that relieves tension ahead of the replication fork (from untwisting of strand).
• primase - enzyme that synthesizes the RNA primer for replication.
• DNA polymerase - several enzymes that catalyze the synthesis of new DNA (in eukaryotes there are 11 total); also checks for errors.
• ligase - links new fragmented DNA segments together.
• EVENTS WHILE DNA COPIES ITSELF

  STEP 1: An enzyme called helicase breaks the bond between nitrogenous bases. The two strands of DNA split.

  STEP 2: The bases attached to each strand then pair up with the free nucleotides found in the cytoplasm.

  STEP 3: The complementary nucleotides are added to each strand by DNA polymerase to form new strands. Two new DNA molecules, each with a parent strand and each with a new strand are formed. The DNA replication is known as semi-conservative replication because one of the old strands is conservative replication of DNA.
• messenger RNA (mRNA) - brings information from the DNA in the nucleus to the protein manufacturing area, the cytoplasm. In the cytoplasm, this becomes the template of information to make proteins.
• ribosomal RNA (rRNA) holds tightly into the mRNA using its information to assemble the amino acids in correct order.
• transfer RNA (tRNA) - supplies amino acids to the ribosome to be assembled as protein.
• transcription - is the sequence of nucleotides in DNA directs the order of nucleotides in messenger RNA.

  • synthesis of RNA (mRNA) using DNA as a template (protein instructions)
  • occurs in nucleus (eukaryotes) or cytoplasm (prokaryotes)
  • Prokaryotes can begin translation before transcription is finished.
  • Eukaryotes have an extra step during transcription before translation can begin.
• TRANSCRIPTION PROCESS

  SET 1: Ribonucleic Acid polymerase enzyme binds and opens the DNA molecules that will be transcribed.

  SET 2: As the DNA molecule opens, the RNA polymerase slides along the DNA strands and links free RNA nucleotides that pair with nitrogenous bases of the complementary DNA strand. Hence, if the sequence of bases on the DNA strand were CCG TTA CAT, the sequence of bases on the RNA strand would be GGC AAU GUA.

  STEP 3: When the process of base-pairing is completed, the RNA molecule breaks away as the DNA strands rejoin. The RNA leaves the nucleus and goes to the cytoplasm.
• SYNTHESIS OF RNA

  • A gene consists of several sections.
  • The promotor sequence is the site where transcription factor proteins and RNA polymerase bind initially.
  • The core of the gene consists of the protein code, the information required for synthesizing a protein.
  • The termination sequence indicates the end of the gene, the end for transcription.
• TRANSCRIPTION OCCURS IN THREE PARTS:

  initiation - the key enzyme, RNA polymerase, finds the correct region of DNA to begin the process.

  elongation - the DNA double helix unwinds a bit and RNA polymerase makes a RNA copy of the DNA template.

  termination - the RNA polymerase reaches the end of the gene and releases the RNA molecule.
• TRANSCRIPTION - INITIATION

  • In eukaryotes, proteins called transcription factors bind to the promotor region.
  • They assist the binding of RNA polymerase to the correct spot.
• TRANSCRIPTION - ELONGATION

  • Once RNA polymerase binds, the DNA double helix begins to unwind.
  • One strand has the information for the gene, the coding strand.
  • RNA polymerase copies the coding strand as an RNA strand.
• TRANSCRIPTION - TERMINATION

  • Elongation continues until RNA polymerase reaches a specific DNA sequence called a terminator.
  • At this point the new RNA strand is either released or cut free.
• translation - is the process of converting the information in messenger RNA into a sequence of amino acids that make a protein.

  
  

  instructions for producing a protein uses three letters on mRNA (triplet code) called a "codon"
• both are situated differently, the codon which is a set of three nucleotide is found on the messenger RNA whereas tRNA carrying amino acid contains anti codon in one of its loop structures.

  codons are in multiple sequences where start codon initiates and stop codon ends, anti codons appear individually in each tRNA molecule.
• the reading frame of the codon is 5′ to 3′ and the anti codons follows directions 3′ to 5′.

  the nucleotide in codon set compliments with that of DNA from the transcription process but anti codons are complementary to its codon.

  codon carries the genetic information to the mRNA from the transcription process whereas anti codon brings amino acid in the tRNA structure during translation.