DNA and RNA Structure and Function in Cells
DNA and RNA: Structure and Function
Alternative DNA Structures
While the Watson-Crick double helix (B-form DNA) is well-known, other forms exist, notably A-DNA and Z-DNA. B-DNA is the predominant form in biological systems and is crucial for DNA-protein interactions within the nucleus.
B-Form DNA
This is the standard double helix structure observed in most biological contexts.
A-Form DNA
A-DNA arises from B-DNA under dehydrating conditions. It is a right-handed double helix, but wider and shorter than B-DNA, with base pairs tilted at a 20° angle relative to the helix axis.
Z-Form DNA
Z-DNA is a left-handed double helix, narrower and longer than B-DNA, with a zig-zag backbone.
DNA’s Biological Role
DNA stores genetic information and provides instructions for protein synthesis. It replicates itself based on the complementarity of its nitrogenous bases. Organism complexity generally correlates with DNA content.
DNA in Eukaryotic and Prokaryotic Cells
DNA organization differs between cell types:
- Prokaryotes: Circular DNA molecule (bacterial chromosome) and smaller circular plasmids.
- Eukaryotes: Linear DNA molecules within the nucleus, forming chromatin fibers. During cell division, chromatin condenses into chromosomes. Some DNA also exists in mitochondria and chloroplasts.
- Viruses: Single or double-stranded DNA, either linear or circular.
RNA: Structure and Function
RNA, similar to DNA, is composed of ribonucleotides (adenine, guanine, cytosine, and uracil) linked by phosphodiester bonds. It is typically single-stranded, except in some viruses. Single-stranded RNA can form double-stranded regions (hairpins) through complementary base pairing.
RNA’s primary function is to direct protein synthesis using DNA’s genetic information. All RNAs are transcribed from DNA.
Types of RNA
Messenger RNA (mRNA)
mRNA constitutes 2-5% of total RNA. It has a linear structure with occasional hairpin loops. Its function is to transcribe genetic information from DNA and carry it to ribosomes for protein synthesis. Eukaryotic mRNA is monocistronic (codes for one protein), while prokaryotic mRNA is often polycistronic (codes for multiple proteins). mRNA has a short lifespan due to rapid degradation by ribonucleases.
Ribosomal RNA (rRNA)
rRNA makes up 80% of total cellular RNA. These large, single-stranded molecules have some double-stranded regions due to base pairing. rRNA is a structural component of ribosomes.
Transfer RNA (tRNA)
tRNA transports amino acids to ribosomes for protein synthesis. It contains 70-90 nucleotides, with double-stranded regions and loops. tRNA features include:
- 5′ end: Guanine triplet and free phosphoric acid.
- 3′ end: Three unpaired bases for amino acid attachment.
- Anticodon: Triplet of bases complementary to the mRNA codon.
- T-arm: Ribosome binding site.
- D-arm.
Other RNAs
Various other RNAs exist in the nucleus and cytoplasm, some with catalytic functions (ribozymes) and others forming ribonucleoproteins that modify mRNA.
The Cell: Basic Unit of Life
The cell is the fundamental anatomical and physiological unit of living organisms.
Cell Theory Postulates
- All organisms are composed of one or more cells.
- The cell is the basic unit of life.
- All cells arise from pre-existing cells.
- Hereditary material is passed from parent to daughter cells.
Cellular Origin and Evolution
: The beginning of life: The emergence of life occurred about 3800 Ma ago experiences have been made to give an explanation of The different steps occurring until the rise of life, these experiences form the designation prebiotic chemistry.