Therefore, purines form hydrogen bonds to pyrimidines, with A bonding only to T, and C bonding only to G. Furthermore, DNA is never translated directly to protein.
The Central Dogma of Molecular Biology. See also: The central dogma external link. Cell division is essential for cells to multiply and organisms to grow. As the final step in the Central Dogma, DNA replication must occur in order to faithfully transmit genetic material to the progeny of any cell or organism.
When a cell divides, it must correctly replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent. This enzyme makes the complementary strand by finding the correct base through complementary base pairing.
In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with a perfect copy of its DNA. This process typically takes place during S phase of the cell cycle. The process by which DNA achieves its control of cell life and function through protein synthesis is called gene expression.
A gene is a DNA sequence that contains genetic information for one functional protein. Proteins are essential for the modulation and maintenance of cellular activities. The amino acid sequence of each protein determines its conformation and properties e.
Directed protein synthesis follows two major steps: gene transcription and transcript translation. Transcription is the process by which the genetic information stored in DNA is used to produce a complementary RNA strand. Genes consist of sequences encoding mRNA exons that are interrupted by non-coding sequences of variable length, called introns. Introns are removed and exons joined together before translation begins in a process called mRNA splicing.
Messenger RNA splicing has proved to be an important mechanism for greatly increasing the versatility and diversity of expression of a single gene. It takes place in the nucleus in eukaryotes and in the cytoplasm in bacteria and archaea and leads to the formation of mature mRNA. Several different mRNA and protein products can arise from a single gene by selective inclusion or exclusion of individual exons from the mature mRNA products. This phenomenon is called alternative mRNA splicing.
It permits a single gene to code for multiple mRNA and protein products with related but distinct structures and functions 1. Once introns are excised from the final mature mRNA molecule, this is then exported to the cytoplasm through the nuclear pores where it binds to protein-RNA complexes called ribosomes 2.
DNA transcription. Although every somatic cell in the human body contains the same genome, activation and silencing of specific genes in a cell-type-specific manner is necessary.
Moreover, a cell must silence expression of genes specific to other cell types to ensure genomic stability. This type of repression must be maintained throughout the life of each cell in normal development. Epigenetic modifications that are defined as heritable, yet reversible changes that influence the expression of certain genes but with no alteration in the primary DNA sequence are ideal for regulating these events. The best studied epigenetic modification in human is DNA methylation, however it becomes increasingly acknowledged that DNA methylation does not work alone, but rather occurs in the context of other epigenetic modifications such as the histone modifications.
Epigenetic Modifications. RNA, is another macromolecule essential for all known forms of life. The chemical structure of RNA is very similar to that of DNA: each nucleotide consists of a nucleobase a ribose sugar, and a phosphate group. Among the ncRNAs, microRNAs miRNAs represent the best-studied class to date and have been shown to regulate the expression of their protein-coding gene targets in a sequence-dependent manner 10 — An RNA molecule is said to be monocistronic when it captures the genetic information for a single molecular transcriptional product, e.
Most eukaryotic mRNAs are indeed monocistronic. In the case of polycistronic mRNAs, the primary transcript comprises several back-to-back mRNAs, each of which will be eventually translated into an amino acid sequence polypeptide.
Such polypeptides usually have a related function they often are the subunits composing a final complex protein and their coding sequences are grouped into a single primary transcript, which in turn permits them to share a common promoter and to be regulated together. MRNAs carry the genetic information that directs the synthesis of proteins by the ribosomes. All cellular organisms use mRNAs. The structure of an mRNA. RNA interference is a process that moderates gene expression in a sequence dependent manner.
The RNAi pathway is found in all higher eukaryotes and was recently found in the budding yeast as well. SiRNAs are double-stranded ncRNAs that are mainly delivered to the cell experimentally by various transfection methods although they have been described to be produced form the cell itself SiRNAs are typically designed to be perfectly complementary to their targets.
RNA interference in mammalian cells. Designer siRNAs are now widely used in the laboratory to down-regulate specific proteins whose function is under study. At the same time, the ability to engage the RNAi pathway in an on demand manner suggests the possibility that RNAi can be used in the clinic to reduce the production of those proteins that are over-expressed in a given disease context.
The delivery method remains an important consideration for the development of RNAi-based therapies as the active molecule needs to be delivered efficiently and in a tissue-specific manner in order to maximize impact and diminish off-target effects. See also: RNAi external link. The expression of proteins is determined by genomic information, and their presence supports the function of cell life.
Things began to change with the discovery of microRNAs more than 20 years ago in plants 16 and animals 17 , These RNA transcripts have been referred to as ncRNAs and there is increased appreciation that many of them are indeed functional and affect key cellular processes. There are many recognizable classes of ncRNAs, each having a distinct functionality. The full extent of distinct classes of ncRNAs that are encoded within the human genome is currently unknown but are believed to be numerous.
The biological role of long ncRNAs as a class remains largely elusive. Several specific cases have been shown to be involved in transcriptional gene silencing, and the activation of critical regulators of development and differentiation: these exerted their regulatory roles by interfering with transcription factors or their co-activators, though direct action on DNA duplex, by regulating adjacent protein-coding gene expression, by mediating DNA epigenetic modifications, etc.
This is known to occur in the case of retroviruses, such as HIV, as well as in eukaryotes, in the case of retrotransposons and telomere synthesis.
Post-transcriptional modification is a process in cell biology by which, primary transcript RNA is converted into mature RNA. This process is vital for the correct translation of the genomes of eukaryotes as the human primary RNA transcript that is produced as a result of transcription contains both exons, which are coding sections of the primary RNA transcript and introns, which are the non coding sections of the primary RNA transcript.
The cap and tail protect the mRNA from enzyme degradation and aid its attachment to the ribosome. In addition, iii introns non-coding sequences are spliced out of the mRNA and exons coding sequences are spliced together. The mature mRNA transcript will then undergo translation A protein is a molecule that performs reactions necessary to sustain the life of an organism.
One cell can contain thousands of proteins. Following transcription, translation is the next step of protein biosynthesis. In translation, mRNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or a polypeptide, that will later fold into a protein.
Ribosomes read mRNA sequence in a ticker tape fashion three bases at a time, inserting the appropriate amino acid encoded by each three-base code word or codon into the appropriate position of the growing protein chain. This process is called mRNA translation. Each amino acid is encoded by a sequence of three successive bases. Some specialized codons serve as punctuation points during translation.
The methionine codon AUG , serves as the initiator codon signaling the first amino acid to be incorporated. Copy Number Variation. Copy Number Variation and Genetic Disease. Copy Number Variation and Human Disease. Tandem Repeats and Morphological Variation. Chemical Structure of RNA. Eukaryotic Genome Complexity. RNA Functions. Citation: Clancy, S. Nature Education 7 1 The more researchers examine RNA, the more surprises they continue to uncover.
What have we learned about RNA structure and function so far? Aa Aa Aa. Figure 1. Figure Detail. Figure 2. Figure 3. Figure 4. Ribosomal RNA rRNA , which forms the structural and functional components of the ribosome, is present in bacterial and eukaryotic cells. In eukaryotic cells, it is present in the cytoplasm. In eukaryotes, it's found in the nucleus and cytoplasm. Transfer RNA tRNA , which helps incorporate amino acids into growing polypeptide chains, is present in bacterial and eukaryotic cells.
In eukaryotes, it's found in the cytoplasm. Figure 8. Figure 9. More and More RNAs. For instance, short RNAs are not only part of organelles like ribosomes and spliceosomes, but also of some enzymes. For example, the enzyme telomerase , which adds nucleotides to the ends of chromosomes, is composed of a nucleotide RNA and several proteins. Juli Feigon at the University of California, Los Angeles, together with postdoctoral scholar Carla Theimer and graduate student Craig Blois, first solved the structure of an essential piece of this RNA by nuclear magnetic resonance spectroscopy Theimer et al.
References and Recommended Reading Berget, S. Cell 20 , — Evans, R. Cell 12, — Holley, R. New York, Freeman, Rich, A. Cell , — Theimer, C. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend.
Submit Cancel. This content is currently under construction. Explore This Subject. Applications in Biotechnology. DNA Replication. Jumping Genes. Discovery of Genetic Material. Gene Copies. Updated February 02, Used to transfer the genetic code from the nucleus to the ribosomes to make proteins.
RNA is used to transmit genetic information in some organisms and may have been the molecule used to store genetic blueprints in primitive organisms. Structural Features B-form double helix. DNA is a double-stranded molecule consisting of a long chain of nucleotides.
A-form helix. RNA usually is a single-strand helix consisting of shorter chains of nucleotides. Composition of Bases and Sugars deoxyribose sugar phosphate backbone adenine, guanine, cytosine, thymine bases ribose sugar phosphate backbone adenine, guanine, cytosine, uracil bases Propagation DNA is self-replicating.
The small grooves in the helix also serve as protection, providing minimal space for enzymes to attach. RNA is not stable under alkaline conditions, plus the large grooves in the molecule make it susceptible to enzyme attack. RNA is constantly produced, used, degraded, and recycled. Featured Video. View Article Sources. Cite this Article Format. Helmenstine, Anne Marie, Ph. What Are the 3 Parts of a Nucleotide? How Are They Connected?
Nitrogenous Bases - Definition and Structures. The Difference Between Purines and Pyrimidines. Nucleic Acids - Structure and Function.
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