Molecular biology is a branch of biology that focuses on the molecular mechanisms that govern the flow of genetic information. The core processes involved are DNA replication, transcription, and translation, which together allow cells to store, process, and express genetic information. These mechanisms are fundamental to understanding how cells function, replicate, and transmit genetic material to future generations.
DNA Replication
Definition:
DNA replication is the process by which a cell duplicates its DNA before cell division. This ensures that each new cell receives an identical copy of the DNA. Replication is semiconservative, meaning that each daughter DNA molecule consists of one old strand and one newly synthesized strand.
Steps of DNA Replication:
- Initiation: The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between base pairs, forming a replication fork.
- Elongation: DNA polymerase adds new nucleotides complementary to the parent strand in the $5′ \rightarrow 3’$ direction.
- Leading and Lagging Strands: The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments known as Okazaki fragments.
- Termination: The enzyme ligase seals any nicks in the sugar-phosphate backbone, ensuring the DNA strands are complete.
Example 1: Semiconservative DNA Replication
Question: If a DNA molecule has a sequence $5′ – ATCGGA – 3’$ on one strand, what is the sequence of the new complementary strand during replication?
Answer:
Step 1: Given Data:
- Original DNA sequence: $5′ – ATCGGA – 3’$.
Step 2: Solution:
- The complementary base pairs are:
- $A$ pairs with $T$,
- $T$ pairs with $A$,
- $C$ pairs with $G$,
- $G$ pairs with $C$.
Step 3: Final Answer: The new complementary strand will be $3′ – TAGCCT – 5’$.
Transcription
Definition:
Transcription is the process by which an RNA molecule is synthesized from a DNA template. This is the first step in gene expression, where genetic information in DNA is transcribed into messenger RNA (mRNA).
Steps of Transcription:
- Initiation: The enzyme RNA polymerase binds to the promoter region on the DNA, signaling the start of transcription.
- Elongation: RNA polymerase moves along the DNA, adding RNA nucleotides complementary to the DNA template strand.
- Termination: When RNA polymerase reaches a terminator sequence, transcription stops, and the RNA strand is released.
Example 2: Transcription of DNA to mRNA
Question: What is the mRNA sequence transcribed from the DNA template strand $3′ – TACGGA – 5’$?
Answer:
Step 1: Given Data:
- DNA template strand: $3′ – TACGGA – 5’$.
Step 2: Solution:
- Transcription produces an RNA strand complementary to the DNA template.
- $T$ pairs with $A$,
- $A$ pairs with $U$ (since RNA uses Uracil instead of Thymine),
- $C$ pairs with $G$,
- $G$ pairs with $C$.
Step 3: Final Answer: The mRNA sequence is $5′ – AUGCCU – 3’$.
Translation
Definition:
Translation is the process in which the mRNA sequence is translated into a polypeptide chain (protein) by the ribosome. This occurs in the cytoplasm of the cell and requires the participation of transfer RNA (tRNA) and ribosomes.
Steps of Translation:
- Initiation: The ribosome assembles around the mRNA strand. The start codon ($AUG$) is recognized, and translation begins.
- Elongation: tRNA molecules bring amino acids to the ribosome based on the codons in the mRNA sequence. The ribosome reads the mRNA sequence in groups of three bases (codons), each corresponding to a specific amino acid.
- Termination: When the ribosome encounters a stop codon ($UAA$, $UAG$, $UGA$), translation stops, and the newly synthesized protein is released.
Example 3: Translation of mRNA to Protein
Question: If the mRNA sequence is $5′ – AUGCCUAAU – 3’$, what is the corresponding amino acid sequence?
Answer:
Step 1: Given Data:
- mRNA sequence: $5′ – AUGCCUAAU – 3’$.
Step 2: Solution:
- Using the genetic code:
- $AUG$ codes for Methionine (Met),
- $CCU$ codes for Proline (Pro),
- $AAU$ codes for Asparagine (Asn).
Step 3: Final Answer: The amino acid sequence is Met-Pro-Asn.
The Central Dogma of Molecular Biology
The Central Dogma of Molecular Biology describes the flow of genetic information in cells, from DNA to RNA to protein. It explains the processes of transcription and translation and emphasizes that information flows in one direction: DNA $\rightarrow$ RNA $\rightarrow$ Protein.
Example 4: Central Dogma Pathway
Question: Explain the flow of genetic information in a cell according to the Central Dogma.
Answer:
Step 1: Given Data:
- Starting point: DNA.
Step 2: Solution:
- DNA Replication: DNA is replicated to produce identical copies before cell division.
- Transcription: DNA is transcribed into mRNA in the nucleus.
- Translation: mRNA is translated into proteins in the cytoplasm.
Step 3: Final Answer: The Central Dogma pathway is DNA $\rightarrow$ RNA $\rightarrow$ Protein.
Mutations and Their Effects
Definition:
A mutation is a change in the DNA sequence. Mutations can occur spontaneously or as a result of environmental factors (mutagens). They can affect protein function, leading to genetic disorders or diseases.
Types of Mutations:
- Point Mutation: A change in a single nucleotide.
- Frameshift Mutation: An insertion or deletion of nucleotides that shifts the reading frame.
- Silent Mutation: A mutation that does not affect the amino acid sequence.
- Missense Mutation: A mutation that changes one amino acid in the protein.
- Nonsense Mutation: A mutation that introduces a stop codon, prematurely ending translation.
Example 5: Point Mutation and Its Effect on Translation
Question: What happens if the codon $AUG$ (coding for Methionine) mutates to $AUC$ (coding for Isoleucine)?
Answer:
Step 1: Given Data:
- Original codon: $AUG$ (Methionine).
- Mutated codon: $AUC$ (Isoleucine).
Step 2: Solution:
- $AUG$ codes for Methionine, which is the start codon.
- If $AUG$ mutates to $AUC$, the codon now codes for Isoleucine, which is not a start codon. As a result, translation may not initiate properly.
Step 3: Final Answer: A mutation from $AUG$ to $AUC$ would likely prevent the initiation of translation, affecting protein synthesis.
Genetic Code
Definition:
The genetic code is the set of rules by which the information encoded in mRNA is translated into proteins. The genetic code is universal, meaning it is shared by almost all organisms, and degenerate, meaning that multiple codons can code for the same amino acid.
Example 6: Codon Table
Question: What amino acids are encoded by the codons $GGA$, $UGG$, and $UAA$?
Answer:
Step 1: Given Data:
- Codons: $GGA$, $UGG$, and $UAA$.
Step 2: Solution:
- $GGA$ codes for Glycine (Gly).
- $UGG$ codes for Tryptophan (Trp).
- $UAA$ is a stop codon.
Step 3: Final Answer: The codons correspond to Glycine, Tryptophan, and a stop signal.
Conclusion
Molecular biology provides critical insights into how genetic information is stored, processed, and expressed. The processes of replication, transcription, and translation are central to life, ensuring that genetic material is faithfully replicated and expressed as proteins, the workhorses of the cell. Understanding these molecular mechanisms allows scientists to explore the basis of diseases, develop therapies, and even manipulate genetic material for biotechnology applications.
