DNA Study Guide: Protein Synthesis

Biological Word Roots and Affixes

• ov-: Egg.
• photo-: Light.
• non-: Not.
• omni-: All.
• phag-: Eat.
• opthal- (or ophthal-): Eye.
• olf-: Smell.
• -phyll: Leaf.
• odont-: Tooth.
• ocul-: Eye.

Structure and Properties of DNA

• Identity and Subcellular Localization:

  • DNA stands for Deoxyribonucleic Acid.
  • It resides within the nucleus of eukaryotic cells.
  • Primary Function: Stores the genetic information and instructions required to direct cellular activities and synthesize proteins.

• Monomeric Structure (Nucleotides):

  • DNA is a macromolecule composed of repeating subunits called nucleotides.
  • Each nucleotide consists of three bonded components:
  • A phosphate group.
  • A deoxyribose sugar (a 5-carbon pentose sugar).
  • A nitrogenous base.

• Structural Backbone and Bonding:

  • The structural exterior of the DNA molecule consists of an alternating sugar-phosphate backbone.
  • The nitrogenous bases project inward from the backbone and join the complementary strands via hydrogen bonds.

• Classification of Nitrogenous Bases:

  • Purines:
  • Characterized by a double-ring chemical structure.
  • Include Adenine (A) and Guanine (G).
  • Pyrimidines:
  • Characterized by a single-ring chemical structure.
  • Include Cytosine (C) and Thymine (T).

• Base-Pairing Rules (Complementary Base Pairing):

  • Adenine always pairs exclusively with Thymine ($\text{A} = \text{T}$) via $2$ hydrogen bonds.
  • Cytosine always pairs exclusively with Guanine ($\text{C} \equiv \text{G}$) via $3$ hydrogen bonds.
  • Mnemonic Devices:
  • Adenine and Thymine: "Apples on a tree" ($\text{A}-\text{T}$).
  • Cytosine and Guanine: "Cars go in a Garage" ($\text{C}-\text{G}$).

• Chargaff's Rule and Quantitative Relationships:

  • Chargaff's Rule dictates that in cellular DNA, the ratio of adenine to thymine is equal ($\text{A} = \text{T}$), and the ratio of cytosine to guanine is equal ($\text{C} = \text{G}$).
  • Mathematical Application:
  • If a DNA molecule is determined to contain $30\%$ adenine:
    • By base-pairing rules, it must also contain $30\%$ thymine:         $\%\text{A} + \%\text{T} = 30\% + 30\% = 60\%$
    • The remaining base percentage constitutes guanine and cytosine:         $100\% - 60\% = 40\%$
    • Because cytosine and guanine must be present in equal proportions:         $\%\text{Cytosine} = \frac{40\%}{2} = 20\%$$\%\text{Guanine} = \frac{40\%}{2} = 20\%$

Structural and Functional Comparison of DNA and RNA

• General Comparison:

  • Both DNA and RNA are classified as nucleic acids assembled from nucleotide monomers.
  • Both nucleic acids contain the nitrogenous bases Adenine, Cytosine, and Guanine.

• Contrasting Properties:

  • Sugar Moiety:
  • DNA contains deoxyribose sugar.
  • RNA contains ribose sugar.
  • Structurally, ribose possesses one more oxygen atom than deoxyribose (deoxyribose has one fewer oxygen atom relative to ribose).
  • Nitrogenous Base Composition:
  • DNA contains Thymine (T), which is complementary to Adenine.
  • RNA lacks thymine; instead, it incorporates Uracil (U), which forms complementary base pairs with Adenine.
  • Number of Strands:
  • DNA is a double-stranded double helix ($2$ strands).
  • RNA is single-stranded ($1$ strand).
  • Cellular Mobility and Localization:
  • DNA is restricted to and remains inside the nucleus under normal physiological conditions.
  • RNA, being single-stranded, can exit the nucleus through nuclear pores and enter the cytoplasm.

DNA Replication Mechanics and Enzymatic Roles

• Semi-Conservative Replication Model:

  • DNA replication is semi-conservative.
  • When a DNA molecule duplicates, each newly formed double-stranded DNA daughter molecule consists of one parental (original/conserved) template strand and one newly synthesized strand.

• Key Replication Enzymes:

  • DNA Helicase: Unzips the double-stranded DNA helix by breaking the hydrogen bonds that hold complementary nitrogenous base pairs together, exposing the single strands for replication.
  • DNA Polymerase: Recruits and incorporates complementary free nucleotides opposite the parental template strands to synthesize the new daughter DNA strands.

• Replication Fork Architecture:

  • Replication Fork: The Y-shaped active region generated when helicase unwinds and separates the parent DNA strands.
  • Leading Strand: The strand that is synthesized continuously in the direction of replication fork unwinding.
  • Lagging Strand: The strand synthesized discontinuously in the direction opposite to replication fork advancement.
  • Okazaki Fragments: The short, discontinuous segments of newly synthesized DNA formed on the lagging strand that are later joined together.

Overview and Terminology of Protein Synthesis

• The Central Dogma Flow of Genetic Information:

  • The general pathway of protein synthesis follows a directed flow:     $\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

• Genetic Code Units Across Nucleic Acids:

  • Triplet: A sequence of $3$ consecutive nitrogenous bases within a DNA molecule that encodes specific instructions.
  • Codon: A sequence of $3$ consecutive nucleotides in mRNA (or DNA) that specifies a single amino acid or signals a termination event.
  • Anticodon: A sequence of $3$ consecutive nitrogenous bases located on a tRNA molecule that is complementary to a corresponding mRNA codon.

• Composition of Proteins:

  • Proteins are biopolymers formed from smaller subunit monomers called amino acids.
  • Individual amino acids are covalently linked together during synthesis by peptide bonds to construct a polypeptide chain.

Molecular Mechanisms of Transcription

• Definition: The enzymatic process of reading a DNA template sequence to produce a complementary single strand of messenger RNA (mRNA).
• Subcellular Site: Takes place entirely inside the nucleus.
• Enzymatic Machinery:
  • RNA Polymerase: The primary enzyme responsible for synthesizing RNA from a DNA template. It separates the DNA strands and adds complementary RNA nucleotides.
• Complementary Base Pairing During Transcription:
  • DNA Adenine ($\text{A}$) templates RNA Uracil ($\text{U}$).
  • DNA Thymine ($\text{T}$) templates RNA Adenine ($\text{A}$).
  • DNA Cytosine ($\text{C}$) templates RNA Guanine ($\text{G}$).
  • DNA Guanine ($\text{G}$) templates RNA Cytosine ($\text{C}$).

Molecular Mechanisms of Translation

• Definition: The cellular process of decoding genetic information delivered by mRNA to assemble a specific sequence of amino acids into a functional protein.

• Subcellular Site: Takes place in the cytoplasm at the surface of a ribosome.

• Primary Classes of RNA in Translation:

  • Messenger RNA (mRNA): Carries the transcribed genetic blueprint and coding instructions from DNA in the nucleus out to the ribosomes.
  • Transfer RNA (tRNA): Delivers specific amino acids from the cytoplasm to the ribosome, utilizing its anticodon to recognize complementary codons along the mRNA.
  • Ribosomal RNA (rRNA): Structural and catalytic RNA that constitutes the chemical and structural building blocks of ribosomes.

• Step-by-Step Translation Pathway:

  • Ribosomal Binding: The newly synthesized mRNA transcript exits the nucleus into the cytoplasm and binds to a ribosome.
  • Initiation: Translation commences at the designated start codon, AUG.
  • Codon-Anticodon Recognition: The ribosome coordinates positioning such that the mRNA AUG codon attracts a tRNA bearing the complementary anticodon UAC, delivering the initial amino acid Methionine.
  • Elongation:
  • As the ribosome translocates along the mRNA transcript, successive codons are presented.
  • Complementary tRNAs carrying specified amino acids (such as Phenylalanine, Lysine, etc.) bind to their corresponding codons.
  • The ribosome catalyzes peptide bond formation between adjacent amino acids, progressively lengthening the growing polypeptide chain.
  • Key Components of the Translation Complex:
  • mRNA (messenger strand containing sequential codons).
  • Ribosome (the macromolecular catalytic complex coordinating translation).
  • tRNA (adapter molecule bearing the anticodon at one pole and a specific amino acid at the other).
  • Amino Acid (individual building blocks added sequentially).
  • Polypeptide (the emerging chain of covalently linked amino acids).

Worked Example: Protein Synthesis Execution

• DNA Template Sequence:   $\text{TAC - GGC - AAT - CCC - ATT}$

• Step 1: mRNA Transcription (Complementary Codons):

  • Complementary pairings ($\text{T}\rightarrow\text{A}$, $\text{A}\rightarrow\text{U}$, $\text{C}\rightarrow\text{G}$, $\text{G}\rightarrow\text{C}$):
  • TAC $\rightarrow$ AUG
  • GGC $\rightarrow$ CCG
  • AAT $\rightarrow$ UUA
  • CCC $\rightarrow$ GGG
  • ATT $\rightarrow$ UAA
  • Resulting mRNA Codons: $\text{AUG - CCG - UUA - GGG - UAA}$

• Step 2: tRNA Anticodon Pairing:

  • Complementary anticodons ($\text{A}\rightarrow\text{U}$, $\text{U}\rightarrow\text{A}$, $\text{C}\rightarrow\text{G}$, $\text{G}\rightarrow\text{C}$):
  • AUG $\rightarrow$ UAC
  • CCG $\rightarrow$ GGC
  • UUA $\rightarrow$ AAU
  • GGG $\rightarrow$ CCC
  • UAA $\rightarrow$ AUU
  • Resulting tRNA Anticodons: $\text{UAC - GGC - AAU - CCC - AUU}$

• Step 3: Amino Acid Sequence Translation:

  • AUG $\rightarrow$ Met (Methionine)
  • CCG $\rightarrow$ Pro (Proline)
  • UUA $\rightarrow$ Leu (Leucine)
  • GGG $\rightarrow$ Gly (Glycine)
  • UAA $\rightarrow$ Stop (Termination codon; does not incorporate an amino acid into the chain)
  • Polypeptide Sequence: $\text{Met - Pro - Leu - Gly - Stop}$

• Total Amino Acids in Synthesized Protein:

  • The completed polypeptide contains exactly $4$ amino acids.