Gene Expression - Transcription and Translation

Learning Objectives

Define and explain the “central dogma of life”

Outline the process of genetic transcription (eukaryotes)

Summarize the process of genetic translation

Decode the sequence of an mRNA into the amino acid sequence of a protein

Understand the impact that various DNA mutations can have on the functioning of organisms

The Central Dogma of Life

Information flow in organisms follows the sequence: DNA → RNA → Protein. This principle, first articulated by Francis Crick in 1957, describes the process by which genetic information is transferred from DNA to RNA through transcription, and from RNA to proteins through translation.

DNA is transcribed to RNA using complementary base pairing rules:

• Adenine (A) pairs with Uracil (U)

• Thymine (T) pairs with Adenine (A)

• Guanine (G) pairs with Cytosine (C)

• Cytosine (C) pairs with Guanine (G)

Processing:

• Pre-mRNA transforms into mature mRNA after processing, which is crucial for its functionality in protein synthesis.

• This processing includes splicing out non-coding regions called introns while retaining coding regions called exons.

• Addition of a 5' cap and 3' poly-A tail to protect mRNA from degradation and assist in ribosome binding, ensuring the mRNA is stable and recognized by the cellular machinery for translation.

mRNA is translated into an amino acid sequence by ribosomes in the cytoplasm, where the actual protein synthesis occurs based on the genetic code.

Final Steps:

• The polypeptide is folded and modified into functional proteins at the Golgi apparatus, where it undergoes necessary post-translational modifications.

• Modifications may include glycosylation (addition of sugar molecules) or phosphorylation (addition of phosphate groups) which are crucial for protein function and stability.

Regulating Gene Expression

Cells control the synthesis of proteins from DNA information through gene expression, allowing for the adaptability and specialization of cells.

• Transcription factors play a key role in regulating which genes are expressed, leading to various cell types with different functionalities, such as muscle cells versus nerve cells. This regulation is vital during development and in response to environmental changes.

• This regulation conserves energy and space, ensuring cells only produce necessary proteins when needed, thus optimizing metabolic resources.

Transcription and Translation

Transcription

Occurs in the nucleus, using DNA as a template for mRNA synthesis via RNA polymerase. Steps in Transcription:

The first step of protein synthesis is transcription, which occurs in the nucleus of eukaryotic cells. During this process:

1. Initiation: RNA polymerase binds to the promoter region of the gene, unwinding the DNA and preparing to transcribe the DNA into mRNA.

2. Elongation: RNA polymerase synthesizes the new mRNA strand by adding nucleotides complementary to the DNA template strand, following base pairing rules (A pairs with U, T with A, G with C, and C with G).

3. Termination: Transcription ends when RNA polymerase reaches a termination sequence, releasing the fully formed pre-mRNA strand, which then undergoes processing to become mature mRNA. This mature mRNA is crucial for the next step, translation, where it is used for protein synthesis in the cytoplasm.

RNA Processing

Eukaryotic pre-mRNA undergoes several steps before translation:

• Introns are spliced out to form mature mRNA.

• A 5' cap and 3' poly-A tail are added for stability and ribosome recognition, enhancing mRNA longevity in the cytoplasm.

• Splicing can occur via spliceosomes, which are complex molecular machines that facilitate the removal of introns and ligation of exons. This process ensures only protein-coding sequences are included in the mature mRNA.

Translation: Making Proteins

Occurs in the cytoplasm, where ribosomes utilize mRNA to synthesize polypeptides:

Key Features of Translation:

• Codons on mRNA correspond to amino acids, with each codon comprising three nucleotides.

• tRNA molecules bring the correct amino acids to the ribosome, binding to codons via their anticodons. Each tRNA is specific to an amino acid.

Steps in Translation:

1. Initiation: Starts with an initiator tRNA recognizing the start codon on the mRNA.

2. Elongation: The polypeptide chain is formed as the mRNA moves through the ribosome, adding amino acids one by one according to the codon sequence.

3. Termination: A release factor enters the A site when a stop codon is encountered, terminating translation and releasing the newly synthesized polypeptide.

Mutations in DNA

Definition: A mutation is a DNA sequence change, either from copying errors during cell division or environmental factors such as radiation or chemicals.

Types of mutations:

• Nonsense: Alters a codon to a stop codon, truncating protein synthesis, which can lead to nonfunctional proteins.

• Missense: Alters one amino acid in a protein, which can significantly affect its structure and function, potentially leading to diseases.

• Silent: Does not affect the amino acid sequence, thereby not impacting protein function, often due to the redundancy of the genetic code.

• Frameshift: The insertion or deletion of nucleotides shifts the reading frame of the genetic message, possibly rendering the protein nonfunctional.

Hereditary vs. Acquired Mutations

• Hereditary mutations: Present in every cell of the body, inherited from parents, and can be passed to offspring.

• Acquired mutations: Occur during a person's life due to environmental factors or errors in DNA replication and are not inherited, often responsible for conditions like cancer.

Genetic Code

Amino acids are encoded by triplet sequences called codons, with the genetic code being degenerate. This means multiple codons can specify the same amino acid, which provides a level of protection against mutations that could otherwise harm protein function.

Protein Modifications Post-Translation

Proteins undergo folding and modifications:

• Changes in amino acids or their sequence influence protein function, often determining the protein's activity and interactions with other molecules.

• Some proteins may need molecular chaperones to assist in proper folding, ensuring they achieve their functional conformation to perform specific biological tasks.

Mosaicism and Polymorphisms

• Mosaicism: Refers to somatic mutations that can lead to variations within a single individual's cells, which are not inherited; this can create a patchwork of genetically distinct cells within the same organism.

• Polymorphism: Refers to common genetic variations found in more than 1% of the population; these variations can influence traits such as hair color, blood type, and susceptibility to certain diseases, contributing to genetic diversity.

Summary of Conceptual Links

Outline eukaryotic transcription processes, summarize translation, decode mRNA to amino acid sequences, and understand the central dogma of life. This understanding is crucial for fields such as genetics, molecular biology, and biotechnology, where manipulation of gene expression holds the key to advancements in medicine, agriculture, and biotechnology applications.

Protein synthesis is a fundamental biological process that occurs in two main stages: transcription and translation. This process enables cells to produce proteins by converting genetic information encoded in DNA into functional polypeptides.

1. Transcription

• Location: Nucleus (in eukaryotes)

• Process:

  1. Initiation: RNA polymerase binds to the promoter region of a gene, unwinding the DNA.

  2. Elongation: RNA polymerase synthesizes mRNA by adding complementary RNA nucleotides, following base pairing rules (A pairs with U, T with A, G with C, C with G).

  3. Termination: Transcription ends when RNA polymerase reaches a termination sequence, releasing the pre-mRNA strand.

• Processing: Before mRNA can be translated, it undergoes several processing steps:

  • Introns (non-coding regions) are spliced out.

  • A 5' cap and a 3' poly-A tail are added, enhancing stability and facilitating ribosome binding.

2. Translation

• Location: Cytoplasm (at ribosomes)

• Process:

  1. Initiation: The small ribosomal subunit binds to the mRNA, and the initiator tRNA recognizes the start codon.

  2. Elongation: The ribosome travels along the mRNA, and tRNA molecules bring the corresponding amino acids to the growing polypeptide chain.

  3. Termination: The process concludes when a stop codon is reached, releasing the completed polypeptide from the ribosome.

Summary

• Protein synthesis is vital for cell function, allowing the expression of genes and the production of proteins essential for various biological functions. Understanding this process is crucial in fields such as molecular biology and genetics, particularly in applications related to gene manipulation, biotechnology, and medicine.