The two strands of a DNA molecule are antiparallel, meaning they run in opposite directions. One strand is oriented in the $5'to3'$ direction, while the other runs $3'to5'$. The connection between the sugar of one nucleotide and the phosphate of the next is a strong covalent bond called a phosphodiester bond, which creates a sturdy "sugar-phosphate backbone".
The two strands are held together in the center by hydrogen bonds between the nitrogenous bases. These bonds follow the rules of complementary base pairing:
- Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.
- Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
Semi-Conservative DNA Replication
Before a cell divides during mitosis, it must duplicate its DNA so that each daughter cell receives a complete set of instructions. This occurs during the S phase of the cell cycle through a process called semi-conservative replication.
The process begins when the hydrogen bonds between the base pairs break, causing the double helix to "unzip". Each original strand then serves as a template for the creation of a new strand. The term "semi-conservative" refers to the fact that each new DNA molecule consists of one original (conserved) strand and one newly synthesized strand.
The Role of DNA Polymerase
The enzyme DNA polymerase is responsible for synthesizing the new strands. It uses free "activated nucleotides" (nucleoside triphosphates) to build the new chain. As it adds nucleotides, it catalyzes condensation reactions to form phosphodiester bonds and uses the energy released from breaking the extra phosphates to power the process.
However, DNA polymerase can only synthesize new DNA in a $5'to3'$ direction. This creates two different methods of synthesis at the replication fork:
- The Leading Strand: Synthesized continuously toward the replication fork.
- The Lagging Strand: Synthesized discontinuously in short segments known as Okazaki fragments.
Because these fragments are disconnected, an enzyme called DNA ligase must eventually come in to join them together, creating a continuous sugar-phosphate backbone.
RNA and the Path to Protein Synthesis
While DNA stores the master blueprint, RNA (ribonucleic acid) is typically a single-stranded molecule that helps translate that blueprint into functional proteins. RNA differs from DNA in two key ways: it uses the sugar ribose instead of deoxyribose, and it uses the base uracil (U) instead of thymine (T).
From Gene to Polypeptide
A gene is a specific sequence of nucleotides within a DNA molecule that codes for a specific protein (polypeptide). The process of turning a gene into a protein happens in two main stages:
1. Transcription (In the Nucleus)
During transcription, the DNA unwinds to expose a gene. An enzyme (RNA polymerase) builds a complementary strand of messenger RNA (mRNA) using the DNA as a template. In eukaryotic cells, this initial transcript contains both coding sequences (exons) and non-coding sequences (introns). Before the mRNA can leave the nucleus, the introns are removed through a process called splicing, leaving only the mature mRNA.
2. Translation (In the Cytoplasm)
The mature mRNA travels to a ribosome in the cytoplasm. Here, transfer RNA (tRNA) molecules act as adapters. Each tRNA carries a specific amino acid and possesses an anticodon—a triplet of bases that matches a specific codon (triplet of bases) on the mRNA. As the ribosome moves along the mRNA, amino acids are joined together by peptide bonds until a "stop" codon is reached, completing the polypeptide chain.
Exam Tip: Remember that the genetic code is degenerate. This means that multiple different codons can code for the same amino acid. This is a biological safeguard; if a small mutation occurs, it might not actually change the resulting protein!
Gene Mutations
A mutation is a change in the DNA base sequence. While many mutations are harmless due to the degeneracy of the code, others can be significant:
- Substitution: A single base is swapped for another. This can be silent (no change), missense (changes one amino acid), or nonsense (creates a premature stop signal).
- Insertion/Deletion: A nucleotide is added or removed. These often cause a frameshift, altering every subsequent triplet and potentially changing the entire protein structure.
