The of Molecular Biology explains how genetic info flows from DNA to RNA to proteins. It's the backbone of modern genetics, showing how our genes make the molecules that run our bodies. This concept revolutionized our understanding of life.
Understanding the Central Dogma is key to grasping DNA's role in life. It's led to breakthroughs in medicine, helping us fight diseases and develop new treatments. This idea changed how we see genes and their impact on our health.
Central Dogma of Molecular Biology
Overview of the Central Dogma
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The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to proteins, the main functional molecules in cells
The three key processes involved in the central dogma are replication, , and
The central dogma is a unidirectional process, with information flowing from DNA to RNA to proteins, but not in the reverse direction
Exceptions to the central dogma include reverse transcription in retroviruses (HIV) and direct translation from DNA to protein in some viruses (Hepatitis B)
Importance of the Central Dogma
The central dogma provides a framework for understanding how genetic information is stored, transmitted, and expressed in living organisms
It explains the relationship between genes, RNA, and proteins, which are essential for cellular function and the development of complex traits
Understanding the central dogma is crucial for advancements in fields such as molecular biology, genetics, biotechnology, and medicine (personalized medicine, gene therapy)
Disruptions in the processes of the central dogma can lead to various genetic disorders (sickle cell anemia) and diseases (cancer)
DNA Replication and Genetic Integrity
Mechanism of DNA Replication
is the process by which a cell duplicates its entire genome before cell division, ensuring that each daughter cell receives an identical copy of the genetic material
Replication begins at specific sites called origins of replication and proceeds bidirectionally, creating two replication forks
The enzyme DNA helicase unwinds the double-stranded DNA, separating the two strands to form a replication bubble
Single-stranded DNA binding proteins stabilize the separated single strands of DNA
DNA primase synthesizes short RNA primers complementary to the single-stranded DNA, providing a starting point for DNA synthesis
DNA polymerase III extends the primers, synthesizing the new DNA strands in the 5' to 3' direction
The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as Okazaki fragments
DNA polymerase I replaces the RNA primers with DNA nucleotides, and DNA ligase joins the Okazaki fragments to create a continuous strand
Maintaining Genetic Integrity
Accurate DNA replication is crucial for maintaining genetic integrity and preventing mutations that can lead to genetic disorders (Down syndrome) or cancer
DNA polymerases have proofreading activity, which allows them to correct errors during replication
Telomerase, a specialized RNA-dependent DNA polymerase, maintains the telomeres at the ends of linear chromosomes to prevent loss of genetic information during replication
DNA repair mechanisms, such as mismatch repair, base excision repair, and nucleotide excision repair, fix damage or errors in the DNA sequence
Checkpoints in the cell cycle ensure that DNA replication is complete and accurate before the cell proceeds to division (mitosis or meiosis)
Transcription and RNA's Role in Gene Expression
Mechanism of Transcription
Transcription is the process by which the genetic information in DNA is copied into a complementary RNA sequence
Transcription is catalyzed by the enzyme RNA polymerase, which synthesizes RNA from a DNA template
Transcription occurs in three stages: initiation, elongation, and termination
During initiation, RNA polymerase binds to a promoter sequence on the DNA and separates the two strands to form a transcription bubble
In elongation, RNA polymerase moves along the DNA template, synthesizing the RNA strand in the 5' to 3' direction by adding ribonucleotides complementary to the DNA sequence
Termination occurs when RNA polymerase encounters a termination signal, releasing the newly synthesized RNA and dissociating from the DNA template
The primary transcript undergoes post-transcriptional modifications, such as 5' capping, 3' polyadenylation, and splicing (in eukaryotes) to produce a mature
RNA's Roles in Gene Expression
RNA plays a crucial role in by serving as a template for (mRNA)
RNA also has regulatory functions, such as microRNA (miRNA) and small interfering RNA (siRNA), which can silence gene expression post-transcriptionally
Some RNA molecules, called ribozymes, have catalytic functions and can perform enzymatic reactions (self-splicing introns)
Ribosomal RNA (rRNA) and transfer RNA () are essential components of the translation machinery
Long non-coding RNAs (lncRNAs) are involved in various cellular processes, including transcriptional regulation, chromatin remodeling, and nuclear organization
Translation and Protein Synthesis from mRNA
Mechanism of Translation
Translation is the process by which the genetic information encoded in an mRNA molecule is used to synthesize a polypeptide chain
Translation occurs in the cytoplasm of a cell and involves , tRNAs, and various protein factors
The mRNA sequence is read in triplets called codons, each coding for a specific amino acid or serving as a stop signal
tRNAs act as adaptor molecules, carrying specific amino acids and having anticodon sequences complementary to the codons on the mRNA
Translation occurs in three stages: initiation, elongation, and termination
During initiation, the small ribosomal subunit binds to the start (AUG) on the mRNA, and the initiator tRNA carrying methionine is recruited
In elongation, the large ribosomal subunit joins, and the ribosome moves along the mRNA, with tRNAs bringing in the corresponding amino acids. Peptide bonds are formed between the amino acids, creating a growing polypeptide chain
Termination occurs when the ribosome encounters a stop codon (UAA, UAG, or UGA). Release factors bind to the stop codon, triggering the release of the completed polypeptide chain and the dissociation of the ribosome
Post-translational Modifications and the Genetic Code
The newly synthesized polypeptide chain undergoes post-translational modifications, such as folding, cleavage, and the addition of functional groups, to form a mature, functional protein
Examples of post-translational modifications include phosphorylation, glycosylation, and disulfide bond formation
The is nearly universal across all life forms, with a few exceptions in some organelles (mitochondria) and microorganisms (mycoplasma)
The genetic code is degenerate, meaning that multiple codons can code for the same amino acid (synonymous codons)
The universality of the genetic code allows for the expression of genes from one organism in another (recombinant DNA technology)