Nucleic acids are essential biomolecules that play a central role in the storage, transmission, and expression of genetic information. In the International Baccalaureate (IB) Biology Higher Level (HL) curriculum, the study of nucleic acids encompasses several key topics, including DNA structure and replication, transcription and gene expression, and translation. This study note aims to break down these complex ideas into digestible parts, providing detailed explanations and examples to facilitate understanding.
DNA (Deoxyribonucleic Acid) is a double-helix structure composed of two antiparallel strands of nucleotides. Each nucleotide consists of three components:
The strands are held together by hydrogen bonds between complementary bases (A pairs with T, and C pairs with G).
Example:
In 1953, James Watson and Francis Crick proposed the double-helix model of DNA, which was a groundbreaking discovery in molecular biology.
DNA replication is the process by which DNA makes a copy of itself during cell division. This process is semi-conservative, meaning each new DNA molecule consists of one old strand and one new strand. The key steps are:
$$ \text{DNA} \xrightarrow{\text{Replication}} \text{DNA} + \text{DNA} $$
Tip:
DNA polymerase can only add nucleotides to the 3' end of a pre-existing strand, which is why a primer is necessary.
Non-coding DNA does not code for proteins but has regulatory functions and is involved in the structural organization of the genome. Examples include:
Note:
Non-coding DNA makes up a significant portion of the human genome, yet its functions are still being actively researched.
DNA sequencing is the process of determining the precise order of nucleotides within a DNA molecule. Techniques include:
The Hershey and Chase experiment (1952) demonstrated that DNA is the genetic material. They used bacteriophages labeled with radioactive isotopes to show that DNA, not protein, enters bacterial cells and directs viral reproduction.
Nucleosomes are the basic units of DNA packaging in eukaryotes, consisting of DNA wrapped around histone proteins. Molecular visualization software helps in understanding the 3D structure of nucleosomes and other molecular complexes.
Gene expression is regulated by transcription factors that bind to specific DNA sequences. These proteins can either enhance or inhibit the transcription of genes.
Environmental factors, such as temperature, light, and chemicals, can influence gene expression. Epigenetic modifications, like DNA methylation, play a crucial role in this regulation.
Transcription is the process by which RNA is synthesized from a DNA template. The main steps are:
$$ \text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} $$
After transcription, the pre-mRNA undergoes several modifications:
DNA methylation involves the addition of a methyl group to cytosine bases, affecting gene expression. Analyzing methylation patterns helps in understanding epigenetic regulation.
Translation is the process by which mRNA is decoded to synthesize proteins. The key steps are:
$$ \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein} $$
Ribosomes are the molecular machines that facilitate translation. They consist of two subunits (large and small) and are composed of rRNA and proteins.
In prokaryotes, translation can begin even before transcription is complete due to the absence of a nuclear membrane. This process is called coupled transcription-translation.
Bioinformatics involves the use of computational tools to analyze biological data. It plays a crucial role in genomics, proteomics, and understanding complex biological systems.
Proteins have four levels of structure:
Polysomes are clusters of ribosomes translating the same mRNA simultaneously, increasing the efficiency of protein synthesis.
Understanding nucleic acids is fundamental to grasping how genetic information is stored, transmitted, and expressed in living organisms. This study note has covered key aspects of DNA structure and replication, transcription and gene expression, and translation, providing a comprehensive overview of these essential biological processes.