2. DNA and RNA
DNA is a double-stranded molecule that has a long chain of
nucleotides. RNA is a single-stranded molecule which has a
shorter chain of nucleotides. DNA replicates on its own, it is self-
replicating. RNA does not replicate on its own.
Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA), carry genetic information which is read in cells to make
the RNA and proteins by which living things function. The well-
known structure of the DNA double helix allows this information
to be copied and passed on to the next generation.
INTRODUCTION
3. Double helix Structure of DNA
The structure of DNA is referred to as a double helix as it
resembles a twisted staircase. A molecule of the DNA comprises
two strands wound around each other as though a twisted ladder
where each strand comprises a backbone comprising alternating
groups of phosphate and sugar groups. The nitrogenous bases are
centrally placed holding together these two strands.
DNA is made up of four types of nucleotides, each containing a
sugar (deoxyribose), a phosphate group, and a nitrogenous base
(adenine, thymine, cytosine, or guanine). The sequence of these
bases carries the genetic information and determines an organism's
traits. Adenine pairs with thymine, and cytosine pairs with guanine,
forming the double helix structure of DNA.
4. DNA Replication
DNA replication is the process of making an exact copy of DNA.
Enzymes unwind the double helix, create RNA primers, and use
DNA polymerases to synthesize new strands. Adenine pairs with
thymine, and cytosine pairs with guanine. The process includes
proofreading and repair mechanisms. After replication, two
identical DNA molecules are formed and can be distributed to
daughter cells during cell division.
DNA replication's accuracy is vital for preserving genetic
information, preventing diseases, and supporting cell division. It
also contributes to evolution through beneficial mutations and
relies on DNA repair mechanisms to maintain genetic integrity.
5. Role of enzymes in replication process
1) Helicase: Unwinds the double-stranded DNA to create
a replication fork.
2) DNA Primase: Synthesizes short RNA primers at the
replication fork, providing starting points for DNA
synthesis.
3) DNA Polymerases: Add complementary nucleotides to
the template strands to build new DNA strands.
4) DNA Ligase: Seals the gaps between Okazaki
fragments on the lagging strand.
5) Topoisomerases: Help relieve tension during
unwinding by cutting and rejoining DNA strands.
6) DNA Proofreading and Repair Enzymes: Detect and
correct errors to ensure accuracy in the replicated DNA.
6. RNA as Single-stranded Nucleic Acid
RNA (ribonucleic acid) is a single-stranded nucleic acid
composed of ribonucleotides. It plays essential roles in various
cellular processes. Unlike double-stranded DNA, RNA exists as
a single strand, and it contains uracil (U) instead of thymine (T)
as one of its bases. The single-stranded nature of RNA allows it
to fold into intricate secondary and tertiary structures, crucial for
its functions. There are several types of RNA, including
messenger RNA (mRNA), transfer RNA (tRNA), ribosomal
RNA (rRNA), and other functional and regulatory RNAs. Each
type serves specific roles in processes like protein synthesis,
transferring amino acids, and forming the core structure of
ribosomes. RNA's versatility makes it a critical player in gene
expression, cell regulation, and other biological processes within
the cell.
7. Components of RNA:Nucleotides
The components of RNA are nucleotides, just like DNA.
Each RNA nucleotide consists of three main parts:
1)Sugar (Ribose): A five-carbon sugar molecule called
ribose forms the backbone of the RNA strand. It differs from
deoxyribose, which is found in DNA, by having an extra
hydroxyl (-OH) group at the 2' carbon.
2)Phosphate Group: A phosphate group is attached to the
5' carbon of the ribose sugar. It links the RNA nucleotides
together through phosphodiester bonds, forming the sugar-
phosphate backbone of the RNA strand.
3)Nitrogenous Base: One of four nitrogenous bases
(adenine - A, uracil - U, cytosine - C, or guanine - G) is
attached to the 1' carbon of the ribose sugar. The sequence
of these bases carries the genetic information encoded in
RNA and determines its specific functions within the cell.
The unique arrangement of these three components in RNA
8. Three mains types of RNA
Three main types of RNA present in cells include messenger RNA
(mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA).
•Messenger RNA (mRNA) is a single-stranded RNA molecule
responsible for carrying genetic information from the nucleus to
the cytoplasm for protein synthesis by ribosomes. mRNA is
transcribed in the nucleus with DNA acting as its template.
•Ribosomal RNA (rRNA), which is a type of non-coding RNA (i.e.
not translated into protein), is present in the cytoplasm of cells. It
associates with ribosomal proteins to form large and small
ribosome subunits, and together, these subunits are responsible for
translating mRNA into proteins.
•Transfer RNA (tRNA) is a small RNA molecule, typically 75 to
90 nucleotides long, that is present in the cytoplasm of cells. It is
responsible for bringing amino acids to the ribosome that
corresponds to the appropriate codon of mRNA. These amino acids
are then joined together to make proteins.
9. TRANSCRIPTION
Transcription is the process of copying a segment of DNA into
RNA. The segments of DNA transcribed into RNA molecules that
can encode proteins are said to produce messenger RNA (mRNA).
Other segments of DNA are copied into RNA molecules
called non-coding RNAs (ncRNAs). mRNA comprises only 1–3%
of total RNA samples.[1] Less than 2% of the human genome can
be transcribed into mRNA (Human genome#Coding vs. noncoding
DNA), while at least 80% of mammalian genomic DNA can be
actively transcribed (in one or more types of cells), with the
majority of this 80% considered to be ncRNA.[2]
Both DNA and RNA are nucleic acids, which use base
pairs of nucleotides as a complementary language. During
transcription, a DNA sequence is read by an RNA polymerase,
which produces a complementary, antiparallel RNA strand called
a primary transcript.
12. CONCLUSION
In conclusion, the structures of DNA and RNA are essential for
understanding the molecular basis of life. DNA is a double-
stranded helix with complementary base pairs (A-T, C-G) that
ensure accurate replication during cell division. RNA is usually
single-stranded and contains the base pairs (A-U, C-G). DNA
replication and the central dogma of molecular biology (DNA to
RNA to proteins) are critical processes governed by these
structures. The discoveries of DNA and RNA structures have
revolutionized genetics and continue to shape our understanding
of life and its mechanisms.
