From DNA to Protein: How Cells Read the
Genome
The Central Dogma of Molecular Biology
The flow of genetic information in cells follows a fundamental path: DNA -> RNA Protein. This
process is known as the Central Dogma.
DNA: Stores the genetic blueprint.
Gene: A specific segment of DNA that carries the instructions for producing a functional
product, typically an RNA molecule or a protein.
Gene Expression: The process by which the information encoded in a gene is used to
synthesize a functional product. This involves transcription and, for protein-coding genes,
translation.
Transcription: The synthesis of an RNA molecule from a DNA template.
Translation: The synthesis of a protein from an mRNA template.
While the majority of the human genome is not transcribed, only a small fraction (~1%) codes for
proteins. Many identical copies of RNA can be synthesized from a single gene to produce a high
amount of protein when needed, a process that is carefully regulated.
Differences Between DNA and RNA
RNA and DNA share many similarities but have key distinctions:
Sugar Moiety: RNA uses ribose, which has an extra hydroxyl (-OH) group compared to
deoxyribose, the sugar found in DNA.
Nitrogenous Bases: RNA contains uracil (U) instead of thymine (T). Uracil lacks the methyl
group found on thymine.
Structure: RNA is typically single-stranded, while DNA is double-stranded.
Transcription: Synthesizing RNA from DNA
Transcription is the process of synthesizing an RNA molecule using a DNA strand as a template.
The enzyme responsible for this is RNA polymerase.
Mechanism: RNA polymerase moves along the DNA template strand, unwinding the double
helix ahead of it. It adds complementary ribonucleotides to the growing RNA chain, following
the base-pairing rules (A with U, T with A, C with G, G with C). The newly synthesized RNA
strand peels away from the DNA template, and the DNA helix rewinds behind the polymerase.
A short hybrid DNA/RNA helix exists transiently during this process.
Multiple Transcripts: Many RNA polymerase molecules can transcribe the same gene
simultaneously, creating multiple RNA copies from a single gene. This allows for rapid
, production of RNA when needed.
Bacterial Transcription
Initiation: RNA polymerase, often with the help of a sigma factor, binds to a specific DNA
sequence called the promoter, located upstream of the gene. The promoter region dictates
the direction of transcription and which DNA strand will serve as the template. The
polymerase then unwinds a small section of the DNA, and transcription begins. The sigma
factor is typically released after initiation.
Elongation: RNA polymerase moves along the template strand, synthesizing the RNA
molecule.
Termination: Transcription stops when RNA polymerase reaches a specific DNA sequence
called the terminator. The terminator sequence is transcribed into the RNA, signaling the
polymerase to detach and release the newly synthesized RNA transcript.
Promoter and Terminator Sequences: Bacterial promoters contain conserved sequences,
notably at the -10 and -35 positions relative to the transcription start site. Terminator
sequences signal the end of transcription.
Eukaryotic Transcription
Eukaryotic transcription is more complex and involves several key differences from bacterial
transcription:
Multiple RNA Polymerases: Eukaryotes have three main types of RNA polymerases, each
responsible for transcribing different classes of genes:
RNA polymerase I: Transcribes most ribosomal RNA (rRNA) genes.
RNA polymerase II: Transcribes all protein-coding genes, microRNA (miRNA) genes, and
genes for other noncoding RNAs.
RNA polymerase III: Transcribes transfer RNA (tRNA) genes, the 5S rRNA gene, and other
small RNA genes.
General Transcription Factors: Eukaryotic RNA polymerases, particularly RNA polymerase II,
require a set of general transcription factors (GTFs) to bind to the promoter and initiate
transcription. These GTFs recognize specific DNA sequences within the promoter region.
TATA Box: A common eukaryotic promoter element, typically located about 30 base pairs
upstream of the transcription start site, recognized by the TATA-binding protein (TBP), a
subunit of the GTF TFIID.
Transcription Initiation Complex: The assembly of GTFs and RNA polymerase II at the
promoter forms the transcription initiation complex.
Phosphorylation: TFIIH, a GTF, has kinase activity that phosphorylates RNA polymerase II,
releasing it from the GTFs and allowing transcription elongation to begin. The
polymerase's tail remains phosphorylated during elongation.
Promoter Elements: Eukaryotic promoters can contain various DNA sequences that promote
the binding of GTFs, located both upstream and sometimes downstream of the transcription
start site.
RNA Processing in Eukaryotes
Genome
The Central Dogma of Molecular Biology
The flow of genetic information in cells follows a fundamental path: DNA -> RNA Protein. This
process is known as the Central Dogma.
DNA: Stores the genetic blueprint.
Gene: A specific segment of DNA that carries the instructions for producing a functional
product, typically an RNA molecule or a protein.
Gene Expression: The process by which the information encoded in a gene is used to
synthesize a functional product. This involves transcription and, for protein-coding genes,
translation.
Transcription: The synthesis of an RNA molecule from a DNA template.
Translation: The synthesis of a protein from an mRNA template.
While the majority of the human genome is not transcribed, only a small fraction (~1%) codes for
proteins. Many identical copies of RNA can be synthesized from a single gene to produce a high
amount of protein when needed, a process that is carefully regulated.
Differences Between DNA and RNA
RNA and DNA share many similarities but have key distinctions:
Sugar Moiety: RNA uses ribose, which has an extra hydroxyl (-OH) group compared to
deoxyribose, the sugar found in DNA.
Nitrogenous Bases: RNA contains uracil (U) instead of thymine (T). Uracil lacks the methyl
group found on thymine.
Structure: RNA is typically single-stranded, while DNA is double-stranded.
Transcription: Synthesizing RNA from DNA
Transcription is the process of synthesizing an RNA molecule using a DNA strand as a template.
The enzyme responsible for this is RNA polymerase.
Mechanism: RNA polymerase moves along the DNA template strand, unwinding the double
helix ahead of it. It adds complementary ribonucleotides to the growing RNA chain, following
the base-pairing rules (A with U, T with A, C with G, G with C). The newly synthesized RNA
strand peels away from the DNA template, and the DNA helix rewinds behind the polymerase.
A short hybrid DNA/RNA helix exists transiently during this process.
Multiple Transcripts: Many RNA polymerase molecules can transcribe the same gene
simultaneously, creating multiple RNA copies from a single gene. This allows for rapid
, production of RNA when needed.
Bacterial Transcription
Initiation: RNA polymerase, often with the help of a sigma factor, binds to a specific DNA
sequence called the promoter, located upstream of the gene. The promoter region dictates
the direction of transcription and which DNA strand will serve as the template. The
polymerase then unwinds a small section of the DNA, and transcription begins. The sigma
factor is typically released after initiation.
Elongation: RNA polymerase moves along the template strand, synthesizing the RNA
molecule.
Termination: Transcription stops when RNA polymerase reaches a specific DNA sequence
called the terminator. The terminator sequence is transcribed into the RNA, signaling the
polymerase to detach and release the newly synthesized RNA transcript.
Promoter and Terminator Sequences: Bacterial promoters contain conserved sequences,
notably at the -10 and -35 positions relative to the transcription start site. Terminator
sequences signal the end of transcription.
Eukaryotic Transcription
Eukaryotic transcription is more complex and involves several key differences from bacterial
transcription:
Multiple RNA Polymerases: Eukaryotes have three main types of RNA polymerases, each
responsible for transcribing different classes of genes:
RNA polymerase I: Transcribes most ribosomal RNA (rRNA) genes.
RNA polymerase II: Transcribes all protein-coding genes, microRNA (miRNA) genes, and
genes for other noncoding RNAs.
RNA polymerase III: Transcribes transfer RNA (tRNA) genes, the 5S rRNA gene, and other
small RNA genes.
General Transcription Factors: Eukaryotic RNA polymerases, particularly RNA polymerase II,
require a set of general transcription factors (GTFs) to bind to the promoter and initiate
transcription. These GTFs recognize specific DNA sequences within the promoter region.
TATA Box: A common eukaryotic promoter element, typically located about 30 base pairs
upstream of the transcription start site, recognized by the TATA-binding protein (TBP), a
subunit of the GTF TFIID.
Transcription Initiation Complex: The assembly of GTFs and RNA polymerase II at the
promoter forms the transcription initiation complex.
Phosphorylation: TFIIH, a GTF, has kinase activity that phosphorylates RNA polymerase II,
releasing it from the GTFs and allowing transcription elongation to begin. The
polymerase's tail remains phosphorylated during elongation.
Promoter Elements: Eukaryotic promoters can contain various DNA sequences that promote
the binding of GTFs, located both upstream and sometimes downstream of the transcription
start site.
RNA Processing in Eukaryotes
