DNA & RNA Structure
Structure of DNA:
Structure of DNA:
1.
Double Helix: DNA (Deoxyribonucleic acid) typically exists
as a double-stranded helix. The structure was elucidated by James
Watson and Francis Crick in 1953, based on X-ray diffraction data collected by
Rosalind Franklin and Maurice Wilkins.
2. Backbone: The backbone of DNA is formed by alternating sugar and phosphate groups. The sugar component in DNA is deoxyribose, a five-carbon sugar. The phosphate groups are negatively charged, providing stability to the structure.
3. Bases: The bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). These nitrogenous bases extend from the sugar-phosphate backbone and form hydrogen bonds with their complementary bases on the opposing strand: adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G).
4. Base Pairing: The complementary base pairing between adenine and thymine, and cytosine and guanine, allows for the precise replication of DNA during cell division and serves as a mechanism for maintaining genetic integrity.
5. Antiparallel Strands: In the double helix structure, the two DNA strands run in opposite directions, known as antiparallel orientation. This means that one strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.
1. Single Stranded: RNA (Ribonucleic acid) is typically single-stranded, although it can form secondary structures through intermolecular base pairing.
2. Backbone: Like DNA, the backbone of RNA is formed by alternating sugar and phosphate groups. However, the sugar component in RNA is ribose, which contains a hydroxyl (-OH) group at the 2' position.
3. Bases: The bases in RNA are adenine (A), cytosine (C), guanine (G), and uracil (U). Uracil replaces thymine in RNA and pairs with adenine through hydrogen bonds (A-U pairing).
4. Function-specific RNA Molecules: RNA serves various functions within the cell, mediated by different types of RNA molecules:
- Messenger RNA (mRNA):
Carries the genetic information from DNA to the ribosomes, where it serves as a
template for protein synthesis.
- Transfer RNA (tRNA): Carries
specific amino acids to the ribosome during protein synthesis, based on the
codons on the mRNA.
-
Ribosomal RNA (rRNA): Forms the core of the ribosome structure and
catalyzes protein synthesis.
5. Secondary Structures: RNA molecules can form secondary structures through intermolecular base pairing, creating stem-loop structures or more complex tertiary structures. These secondary structures are essential for RNA's diverse functions, including mRNA stability, tRNA folding, and regulatory roles.
Structure of Polynucleotide:
Differences between DNA & RNA:
|
|
DNA |
RNA |
|
Structure |
DNA
(Deoxyribonucleic acid) typically exists as a double-stranded helix. |
RNA
(Ribonucleic acid) is typically single-stranded. |
|
Sugar Component |
The
sugar component in DNA is deoxyribose, a five-carbon sugar. |
The sugar component in RNA is ribose, which contains a hydroxyl (-OH) group at the 2' position. |
|
Base |
The
bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). |
The
bases in DNA are adenine (A), cytosine (C), guanine (G), and Uracil (U). |
|
Base Pairing |
The
complementary base pairing between adenine and thymine, and cytosine and
guanine. A = T or T=A C =G or G =C |
The
complementary base pairing between adenine and uracil, and cytosine and
guanine. A =U or U=A C =G or G =C |
|
(Note: = denotes double hydrogen bond = and
denotes triple hydrogen bond) |
||
DNA Replication:
DNA
replication is the process by which a cell makes an identical copy of its DNA. It
occurs in nucleus before a cell divides so that each new cell produced has a
complete set of chromosome. This process is crucial for cell division, growth,
and repair. It occurs during the S phase of the cell cycle.
DNA Replication Steps:
1.
Initiation:
The
process begins at specific sites on the DNA called origins of replication.
Enzymes called helicases unwind and separate the double-stranded DNA into two
single strands.
2.
Elongation:
Enzymes
called DNA polymerases move along each of the separated DNA strands, adding
nucleotides complementary to the template strand. DNA polymerases can only add
nucleotides in the 5' to 3' direction, so replication occurs in a
semi-discontinuous manner. The leading strand is synthesized continuously in
the direction of the replication fork, while the lagging strand is synthesized
discontinuously in short fragments called Okazaki fragments.
3.
Primase:
Primase
synthesizes a short RNA primer on the lagging strand, providing a starting
point for DNA polymerase to begin synthesis.
4.
DNA Polymerase:
DNA
polymerase then extends the RNA primer by adding DNA nucleotides, synthesizing
the lagging strand in short fragments.
5.
Ligase:
Enzymes
called ligases join the Okazaki fragments together on the lagging strand by
forming phosphodiester bonds, producing a continuous DNA strand.
6.
Termination:
Replication
continues bidirectional until the entire DNA molecule is copied. At the
termination point, specific proteins halt the replication process and ensure
that the newly synthesized DNA strands are properly separated.
Genetic codes
Genetic codes are sequences of
nucleotide, the building blocks of DNA, which contain instructions for
building proteins. These codes are made up of combinations of four nucleotide:
adenine (A), thymine (T), cytosine (C), and guanine (G). Each set of three
nucleotide, called a codon, corresponds to a specific amino acid or a signal
for the start or end of protein synthesis. This process is fundamental to the
functioning and development of all living organisms.
There are 64 possible codons, and each
one either codes for an amino acid or serves as a punctuation mark in the
protein-building process.
There are some exceptions and variations
to the standard genetic code. For example, certain organisms have slightly
different codon assignments or incorporate modified amino acids into their
proteins. These variations are often found in mitochondria, chloroplasts, and
some bacteria. Additionally, some viruses have unique genetic codes that differ
from those of their host organisms.