Lesson 2.3: Molecular Biology and Gene Expression

Introduction

In this lesson, we will explore the fundamental processes of molecular biology that enable the expression of genes, which in turn dictate the functionality of living organisms. Understanding these processes is crucial as they form the basis for many medical conditions that arise from defects in molecular and cellular functioning. By the end of this lesson, students will be able to:

• Explain DNA replication, repair, transcription, translation, and post-transcriptional regulation.
• Classify mutation types and connect DNA repair defects to disease syndromes.
• Understand epigenetics and its role in gene expression during development and disease.
• Describe the flow of genetic information and identify potential disruptions.

DNA Replication

DNA replication is the process by which a cell makes a copy of its DNA. This ensures that when a cell divides, each daughter cell has an identical set of DNA. Key enzymes involved in this process include DNA helicase, DNA polymerase, and ligase.

Mechanism of DNA Replication

1. Initiation: Hydrogen bonds between complementary bases (adenine-thymine, guanine-cytosine) break, and the double helix unwinds at specific sites known as origins of replication. This unwound section is stabilized by single-strand binding proteins.
2. Elongation: DNA polymerase synthesizes a new strand by adding nucleotides complementary to the template strand in a 5' to 3' direction. This process occurs continuously on the leading strand while the lagging strand is synthesized in short segments called Okazaki fragments.
3. Termination: Once the entire DNA molecule has been replicated, RNA primers are removed and replaced with DNA, and the fragments are joined by DNA ligase.

Example of DNA Replication

Consider the following simple example:

Let’s say we have a small section of DNA on the template strand:

$$\text{DNA template: 5' - ATGCCG - 3'}$$

To replicate this section, the complementary nucleotides are added:

• Adenine pairs with thymine,
• Thymine pairs with adenine,
• Guanine pairs with cytosine,
• Cytosine pairs with guanine.

So the new complementary strand will be:

$$\text{New strand: 5' - TACGGC - 3'}$$

Common Misconceptions

One common misunderstanding is that DNA replication can occur randomly. In reality, this process is highly regulated and requires a multitude of proteins and enzymes to ensure fidelity and efficiency.

DNA Repair Mechanisms

Despite the high fidelity of DNA replication, errors do occur. DNA repair mechanisms are crucial for correcting these mutations. Major types of DNA repair include:

1. Base Excision Repair (BER): Targets and repairs non-helix-distorting base lesions. An enzyme called DNA glycosylase recognizes and removes the damaged base, followed by the action of other enzymes to fill in the gap.
2. Nucleotide Excision Repair (NER): This mechanism removes bulky DNA adducts. In this process, proteins recognize and excise the damaged segment of DNA, and DNA polymerase synthesizes a new segment.
3. Mismatch Repair (MMR): This mechanism corrects errors that escape proofreading during DNA replication. Specialized proteins recognize and repair mismatched base pairs.

Example of DNA Repair

Suppose a cytosine is mistakenly converted to uracil in DNA. The BER process would identify this as a flaw, and the glycosylase enzyme removes the uracil, allowing DNA polymerase to insert the correct cytosine.

Connection to Disease

Defects in DNA repair mechanisms can lead to a variety of diseases, including cancer. For example, mutations in the BRCA1 and BRCA2 genes (which are involved in the repair of double-strand breaks) greatly increase the risk of breast and ovarian cancer.

Transcription

Once DNA is replicated, the next step in expressing a gene is transcription, where a specific segment of DNA is copied into mRNA.

Steps of Transcription

1. Initiation: RNA polymerase binds to the promoter region of the gene, unwinding the DNA helix.
2. Elongation: As RNA polymerase moves along the DNA template strand, it synthesizes the mRNA strand by adding RNA nucleotides complementary to the DNA template strand.
3. Termination: Transcription ends when RNA polymerase reaches a termination sequence, releasing the newly synthesized mRNA.

Example of Transcription

For a segment of DNA:

$$\text{DNA: 5' - ATGCCG - 3'}$$

The mRNA transcribed would be:

$$\text{mRNA: 5' - AUGCCG - 3'}$$

Post-Transcriptional Regulation

After transcription, eukaryotic mRNA undergoes several modifications before it is translated. This includes:

• 5' Capping: A modified guanine nucleotide is added to the 5' end, which is crucial for mRNA stability and initiation of translation.
• Polyadenylation: A tail of adenine residues (poly-A tail) is added to the 3' end, enhancing mRNA stability.
• Splicing: Non-coding regions (introns) are removed from the mRNA transcript, while coding regions (exons) are spliced together.

Example of Splicing

Consider a pre-mRNA sequence:

$$\text{Pre-mRNA: 5' - EXON1 - INTRON - EXON2 - 3'}$$

After splicing, the resulting mRNA will be:

$$\text{mRNA: 5' - EXON1 - EXON2 - 3'}$$

Translation

The final step in gene expression is translation, where the mRNA is decoded to produce a protein. This process takes place in ribosomes.

Steps of Translation

1. Initiation: The ribosome assembles around the start codon of the mRNA.
2. Elongation: Transfer RNAs (tRNA) bring the respective amino acids to the ribosome based on the codons in the mRNA, elongating the protein chain.
3. Termination: Translation stops when a stop codon is reached, releasing the newly synthesized polypeptide.

Example of Translation

If the mRNA sequence is:

$$\text{mRNA: 5' - AUG UCC GAA UAA - 3'}$$

The corresponding amino acids could be:

• Methionine (AUG)
• Serine (UCC)
• Glutamic acid (GAA)

Epigenetics and Gene Regulation

Epigenetics refers to heritable changes in gene expression that do not involve changes to the underlying DNA sequence. These changes can be influenced by environmental factors and play a critical role in development and disease. Common mechanisms include:

• DNA Methylation: Often represses gene expression.
• Histone Modification: Alters chromatin structure, impacting accessibility of the DNA for transcription.

Real-world Implication

Environmental factors such as diet, stress, and exposure to toxins can influence epigenetic markers, potentially leading to diseases like cancer.

Conclusion

Through the processes of replication, repair, transcription, translation, and regulation, the flow of genetic information is maintained and manipulated. Understanding these processes not only provides insight into the normal functioning of cells but also highlights the potential points of failure that can lead to disease.

Study Notes

• DNA replication is essential for cell division.
• DNA repair mechanisms include BER, NER, and MMR.
• Transcription and translation are critical processes in gene expression.
• Post-transcriptional modifications include capping, polyadenylation, and splicing.
• Epigenetic changes can affect gene expression without altering the DNA sequence.
• Understanding these molecular processes is crucial for recognizing genetic and metabolic disorders.