2.1 DNA, RNA, Protein Synthesis & Gene Expression

2.1 DNA, RNA, Protein Synthesis & Gene Expression

GED Exam Focus: Questions on molecular biology test your understanding of how genetic information flows from nucleic acids to functional proteins. You will be expected to compare DNA and RNA, trace the steps of transcription and translation, translate codon sequences using a reference chart, and analyze how mutations impact protein structure and gene expression.

Molecular Architecture of Nucleic Acids

Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are the two primary nucleic acids responsible for storing, transmitting, and expressing genetic information in living organisms. Both are polymers constructed from monomer subunits called nucleotides.

Each nucleotide consists of three fundamental components:

1. A five-carbon sugar (deoxyribose in DNA; ribose in RNA)
2. A phosphate group ($PO_4^{3-}$)
3. A nitrogenous base

Structural Comparison: DNA vs. RNA

DNA Double Helix (Antiparallel Strands):
5'- Phosphate --- Sugar --- Base (Adenine)  ====  Base (Thymine) --- Sugar --- Phosphate - 3'
                  |                                                |
5'- Phosphate --- Sugar --- Base (Cytosine) ==== Base (Guanine)  --- Sugar --- Phosphate - 3'

Chargaff's Rules & Base Pairing

According to Chargaff's Rules, in any double-stranded DNA molecule, the amount of adenine equals thymine ($%A = %T$), and the amount of cytosine equals guanine ($%C = %G$). The two strands run in opposite directions (antiparallel, $5' ightarrow 3'$ versus $3' ightarrow 5'$) and are bound together by weak hydrogen bonds between nitrogenous bases.

The Central Dogma: Transcription & Translation

The flow of genetic information within a biological system is described by the Central Dogma of Molecular Biology: $\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}$

   +------------------+         +------------------+         +------------------+
   |       DNA        | ------> |       mRNA       | ------> |     Protein      |
   |  (Gene Record)   |         |    (Messenger)   |         |  (Amino Acid Chain)
   +------------------+         +------------------+         +------------------+
        [Nucleus]                   [Cytoplasm]                 [Ribosome]
     (Transcription)               (Transport)                 (Translation)

Step 1: Transcription (DNA $\rightarrow$ mRNA)

Transcription takes place inside the nucleus of eukaryotic cells. It is the enzymatic process where a specific DNA gene sequence is copied into a complementary single-stranded messenger RNA (mRNA) molecule.

1. Initiation: The enzyme RNA Polymerase binds to a specialized region on DNA called a promoter. It unwinds and unzips the DNA double helix.
2. Elongation: RNA Polymerase moves along the DNA template strand in the $3' \rightarrow 5'$ direction, synthesizing a complementary mRNA transcript in the $5' \rightarrow 3'$ direction.
   • If DNA template reads A, RNA adds U.
   • If DNA template reads T, RNA adds A.
   • If DNA template reads C, RNA adds G.
   • If DNA template reads G, RNA adds C.
3. Termination: RNA Polymerase reaches a termination signal, releases the pre-mRNA strand, and the DNA double helix rewinds.
4. Processing (Eukaryotes): Pre-mRNA undergoes splicing where non-coding regions (introns) are cut out and coding regions (exons) are spliced together before leaving the nucleus via nuclear pores.

Step 2: Translation (mRNA $\rightarrow$ Protein)

Translation occurs in the cytoplasm at the ribosome (composed of ribosomal RNA or rRNA and proteins). It decodes the mRNA message into a chain of amino acids linked by peptide bonds to form a functional protein.

• Codons: mRNA is read by the ribosome in groups of three consecutive nucleotides called codons. Each codon specifies one specific amino acid.
• Transfer RNA (tRNA): tRNA molecules act as molecular adaptors. At one end, tRNA carries a specific amino acid; at the other end, it features a three-base sequence called an anticodon that base-pairs with the complementary mRNA codon.
• Process Flow:
  1. Initiation: The ribosome attaches to mRNA at the start codon ($5'\text{-AUG-}3'$), which codes for the amino acid Methionine (Met).
  2. Elongation: The ribosome moves along the mRNA. Matching tRNAs enter the ribosome, deliver their amino acids, and form peptide bonds to grow the polypeptide chain.
  3. Termination: When the ribosome encounters a stop codon ($5'\text{-UAA-}3'$, $5'\text{-UAG-}3'$, or $5'\text{-UGA-}3'$), translation ceases. The completed polypeptide chain is released to fold into a 3D functional protein.

The Genetic Code & Codon Chart Mechanics

The genetic code is the set of rules by which information encoded in mRNA is translated into amino acids. It has three critical properties:

• Triplet-based: 3 bases = 1 codon = 1 amino acid.
• Degenerate / Redundant: There are $4^3 = 64$ possible codons, but only 20 standard amino acids. Therefore, multiple codons can code for the same amino acid (e.g., $5'\text{-GCU-}3'$, $5'\text{-GCC-}3'$, $5'\text{-GCA-}3'$, and $5'\text{-GCG-}3'$ all code for Alanine).
• Universal: Nearly all living organisms use the exact same genetic code, providing strong evidence for shared evolutionary ancestry.

Worked Example: Translating a DNA Sequence

GED Problem: A segment of a DNA template strand has the following nucleotide sequence: $3'\text{-TAC GGC TTA CTI ACT-}5'$ Determine the corresponding mRNA codon sequence and the resulting amino acid sequence.

Step 1: Perform Transcription (DNA Template $\rightarrow$ mRNA) Apply complementary RNA base pairing ($3' \rightarrow 5'$ DNA template maps to $5' \rightarrow 3'$ mRNA):

• DNA: $3'\text{- TAC - GGC - TTA - CTG - ACT -}5'$
• mRNA: $5'\text{- AUG - CCG - AAU - GAC - UGA -}3'$

Step 2: Perform Translation (mRNA Codons $\rightarrow$ Amino Acids) Consult a standard genetic codon chart:

1. Codon 1 ($5'\text{-AUG-}3'$): Methionine (Start Codon)
2. Codon 2 ($5'\text{-CCG-}3'$): Proline
3. Codon 3 ($5'\text{-AAU-}3'$): Asparagine
4. Codon 4 ($5'\text{-GAC-}3'$): Aspartic Acid
5. Codon 5 ($5'\text{-UGA-}3'$): STOP (Terminates translation)

Final Polypeptide Chain: Methionine - Proline - Asparagine - Aspartic Acid

Gene Mutations & Gene Expression

A mutation is any change in the nucleotide sequence of an organism's DNA. Mutations can occur spontaneously during DNA replication or be induced by physical or chemical mutagens (such as UV radiation, X-rays, or toxic chemicals).

1. Point Mutations (Substitutions)

A point mutation occurs when a single nucleotide base is substituted for another:

• Silent Mutation: The base change alters a codon, but due to genetic code degeneracy, it still codes for the same amino acid. Protein function remains unaffected.
• Missense Mutation: The base substitution changes one codon, causing a different amino acid to be inserted. Example: Sickle-cell anemia results from a missense mutation changing glutamic acid to valine in hemoglobin.
• Nonsense Mutation: The base change converts an amino acid codon into a premature STOP codon, terminating translation early and producing a truncated, nonfunctional protein.

2. Frameshift Mutations (Insertions & Deletions)

A frameshift mutation occurs when one or two nucleotides are inserted or deleted from a DNA sequence (in amounts not divisible by three).

• Because mRNA is read in strict triplet codons, adding or removing a base shifts the entire downstream reading frame.
• Every amino acid added after the mutation point is altered, almost always producing a completely nonfunctional protein.

Original Sequence:   AUG - CAG - GUA - UCG (Met - Gln - Val - Ser)
Insertion of 'U':    AUG - UCA - GGU - AUC - G... (Met - Ser - Gly - Ile...)
                      ^ Shifted reading frame changes all downstream amino acids!

Epigenetics & Gene Expression

Although every somatic cell in a multicellular organism contains identical DNA, cells differentiate into specialized types (e.g., neuron vs. muscle cell) through gene expression regulation. Environmental factors like diet, stress, temperature, and toxins can alter gene expression through epigenetic modifications (such as DNA methylation or histone modification) without altering the underlying DNA sequence.