Transcription and Gene Expression

Fundamental Concepts: Properties of Life and Genetic Information

Living organisms store and follow instructions encoded in their DNA. The Central Dogma of molecular biology describes the flow of genetic information: DNA is transcribed into mRNA, which is then translated into protein. This process is essential for cellular structure and function.

• DNA stores instructions for cellular work.

• mRNA directs the production of proteins.

• Proteins provide structure and essential activities in a cell.

• Gene expression is the process by which instructions in DNA are converted into functional proteins or other products.

The Central Dogma: DNA → RNA → Protein

The Central Dogma explains how genetic information is transferred from DNA to RNA and then to protein. Transcription is the process of copying DNA into RNA, while translation converts RNA into protein.

• Transcription: Rewriting genetic information from DNA to RNA in the same language (nucleic acids).

• Translation: Changing the language from nucleic acids (RNA) to amino acids (protein).

• Changes in DNA sequence may alter protein function.

Genes and Chromosomes

Genes are segments of DNA that encode instructions for making proteins. Chromosomes contain many genes, each with a specific function.

• The average human chromosome has about 1000 genes.

• Each gene consists of a promoter region and a transcription region.

Cellular Differentiation and Gene Expression

All cells in an organism have the same DNA, but different cell types express different genes and make different proteins. This process, called cellular differentiation, allows cells to develop specialized functions.

• Muscle cells, pancreas cells, and blood cells have the same DNA but express different genes.

• Gene expression is regulated so that only certain genes are active in each cell type.

Regulation of Gene Expression

Gene expression is regulated at multiple levels, but the initiation of transcription is the most common point of control. Transcription factors (proteins) bind to promoter sequences on DNA to regulate transcription.

• Transcription factors are proteins that bind to specific DNA sequences (promoters) to control the initiation of transcription.

• The promoter region determines when, where, and how much a gene is transcribed.

Steps of Transcription

Transcription occurs in three main steps: initiation, elongation, and termination.

1. Initiation: RNA polymerase binds to the promoter region with the help of transcription factors.

2. Elongation: RNA polymerase synthesizes the RNA strand by adding nucleotides complementary to the DNA template strand.

3. Termination: Transcription ends at terminator sequences, and the RNA polymerase detaches from the DNA.

Transcription: Template and Coding Strands

During transcription, one strand of DNA (the template strand) is used to synthesize mRNA. The coding strand has the same sequence as the mRNA (except RNA uses uracil instead of thymine).

• mRNA is synthesized in the 5’ to 3’ direction.

• The template strand is read in the 3’ to 5’ direction.

• The coding strand is complementary and antiparallel to the template strand.

Promoter Sequences and Transcription Factors

Promoter sequences are regions of DNA where transcription factors and RNA polymerase bind to initiate transcription. The sequence of the promoter determines which transcription factors bind and regulates gene expression.

• Transcription factors recruit RNA polymerase to the promoter.

• RNA polymerase uses the DNA template to synthesize mRNA.

• Which strand is transcribed can differ from one gene to the next.

mRNA Processing: Capping, Polyadenylation, and Splicing

After transcription, mRNA undergoes several processing steps before it is exported to the cytoplasm for translation.

• 5’ Cap: A 7-methylguanosine cap is added to the 5’ end, stabilizing mRNA and aiding in export and translation.

• Poly(A) Tail: A string of adenine nucleotides is added to the 3’ end, promoting translation and stabilizing RNA.

• Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together. Alternative splicing allows for multiple proteins from a single gene.

Summary Table: Key Steps in Transcription and mRNA Processing

Fundamental Concept: Structure and Polarity of DNA and RNA

The structure and polarity of DNA and RNA govern information transfer. DNA and RNA strands have directionality, with a 5’ and 3’ end, which is crucial for transcription and translation.

• New nucleotides are added to the 3’ OH group of the growing strand.

• RNA has uracil (U) instead of thymine (T).

• Polarity and sequence determine the flow of genetic information.

Equations and Formulas

Phosphodiester bond formation during RNA synthesis:

Directionality of transcription:

Additional info:

• Alternative splicing increases protein diversity.

• Gene expression can be regulated at multiple levels, including transcription, mRNA processing, and translation.