transcription

Gene Expression Overview

  • Genes express through a process outlined in the Central Dogma of molecular biology.

  • Key processes involved are:

    • Replication

    • Transcription (Txn)

    • Translation (Tsn)

  • The trajectory of information flows from DNA to mRNA and then to protein.

  • In prokaryotes, transcription occurs in the cytosol, whereas in eukaryotes, it takes place in the nucleus, and translation occurs in the cytosol.

Understanding Gene Expression

  • Gene expression involves the decoding and utilization of the information stored in DNA.

  • Differential Gene Expression

    • Cells selectively express specific genes necessary for their specialized functions.

Modern Definition of a Gene

  • Definition: A gene is a sequence of DNA along with its associated regulatory regions that holds the instructions to produce a functional polypeptide or a functional RNA (excluding mRNA).

Differences Between RNA and DNA

Structural Differences

  • Sugar:

    • RNA: Ribose

    • DNA: Deoxyribose

  • Bases:

    • RNA: A, C, U, G

    • DNA: A, C, G, T

    • Some RNA bases may be modified, while DNA bases are not.

  • Number of Polynucleotide Strands:

    • RNA: Single-stranded

    • DNA: Double-stranded

  • Three-Dimensional Structure:

    • RNA typically forms more complex structures than DNA, which usually forms a double helix.

Functional Differences

  • RNA serves as both instruction and machine for protein synthesis.

  • Stability: RNA tends to be less stable than DNA, indicating shorter lifespans in cellular processes.

RNA World Hypothesis

  • Proposes that RNA was the first molecule to serve as genetic material in evolutionary history, preceding DNA.

  • Suggests that early life forms used RNA for both information storage and as a catalyst for biochemical reactions.

Types of RNA

  • Messenger RNA (mRNA):

    • Encodes the information to synthesize a polypeptide, acting as a "photocopy" of genes.

  • Ribosomal RNA (rRNA):

    • Integral to the ribosome's construction, helps form ribosomes with polypeptides.

    • Ribosomes are the sites where proteins are synthesized.

  • Transfer RNA (tRNA):

    • Functions as the decoding unit, linking amino acids to the corresponding three RNA nucleotides (part of the genetic code).

  • Other types of RNA:

    • Includes various small RNAs involved in different cellular processes.

Transcription Process

  • Transcription: Making an RNA copy of information in a DNA gene.

  • Cellular Location: In eukaryotes, transcription occurs in the nucleus.

  • RNA Polymerase (RNAP):

    • Enzyme that reads DNA and synthesizes RNA.

    • Binds to DNA and unwinds the helix to separate the strands.

    • Only transcribes the template strand of DNA, which is used to generate the RNA transcript.

    • The non-template (coding) strand has the same sequence as the RNA transcript, except for the base uracil (U) instead of thymine (T).

Stages of Transcription

  1. Initiation:

    • Transcription factors are proteins that guide RNA polymerase on where to begin transcription.

    • The Promoter is a DNA sequence signaling the start of transcription.

  2. Elongation:

    • This is the major part of transcription where nucleotides are added to form the complete RNA molecule.

  3. Termination:

    • A termination signal in the DNA sequence marks the end of transcription.

    • Unique to prokaryotes: mRNA can be translated into proteins before transcription completion.

    • For eukaryotes, RNA processing occurs, where the primary mRNA transcript undergoes modifications before leaving the nucleus for cytoplasm translation.

RNA Processing

  • Transformation from pre-mRNA primary transcript to mature mRNA involves several key steps:

    1. 5' Capping: A backward G nucleotide is added at the 5' end, facilitating stability.

    2. Poly-A Tail Addition: A poly-A tail is attached to the 3' end to increase mRNA stability in the cytoplasm.

    3. Splicing: Removal of introns to produce a mature mRNA coding sequence. Exons (coding sequences) exit the nucleus, while introns (non-coding sequences) are discarded and degraded within the nucleus.

    • Spliceosome: A complex of small RNAs and proteins that cuts out introns and joins exons, ensuring a precise nucleotide sequence for the mature mRNA.

    • Ribozymes: Certain RNA molecules act as biological catalysts, cutting and joining RNA sections during splicing processes.

  • Alternative Splicing: Some genes can produce multiple mRNA variants from one primary transcript by including or excluding certain exons, allowing for increased protein diversity. For instance, if a primary transcript contains four exons, various combinations can lead to different functional mRNAs, such as mRNA #1 containing exons 1, 2, and 4, or mRNA #2 containing exons 1, 2, and 3.

Summary

  • Understanding gene expression, transcription, and RNA processing provides insight into how genetic information is utilized to create proteins and perform cellular functions.