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. All proteins thus begin with a methionine residue, but this is often removed later in the translational process. The completed polypeptide chain then folds into a functional three-dimensional protein molecule and is transferred to other organelles for further processing or released into cytosol for association of the newly completed chain with other subunits to form complex multimeric proteins.
Protein translation. Post-translational modification is the chemical modification of a peptide that takes place after its translation. They represent one of the later steps in protein biosynthesis for many proteins. During protein synthesis, 20 different amino acids can be incorporated in order to form a polypeptide.
In addition, enzymes may remove amino acids from the amino end of the protein, or even cut the peptide chain in the middle. This amino acid is usually taken off during post-translational modification. Other modifications, like phosphorylation, are part of common mechanisms for controlling the behavior of a protein, for instance activating or inactivating an enzyme.
See also: Inside a cell external link. Home Learn! DNA 1. DNA transcription 1. Regions of DNA in the human genome, ranging from 0. Approximately half of all gene promoters have CpG islands that when methylated lead to transcriptional silencing. Aberrant DNA methylation patterns have been described in various human malignancies.
In particular, global hupomethylation has been implicated in the earlier stages of carcinogenesis, whereas hypermethylation of tumour suppressor genes has been implicated in cancer progression 3. DNA hypomethylating agents are used for the treatment of certain haematological malignancies. Histone modifications: Histones are proteins around which DNA winds to form nucleosomes.
Nucleosome is the basic unit of DNA packaging within the nucleus and consists of base pairs of genomic DNA wrapped twice around a highly conserved histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3 and H4. The histone tails may undergo many posttranslational chemical modifications, such as acetylation, methylation, phosphorylation, ubiquitylation, and sumoylation.
Histone modifications act except for chromatin condention and transcriptional repression in various other biological processes including gene activation and DNA repair 4. Epigenetic Modifications 2. Untranslated regions: Untranslated regions UTRs are nucleotide stretches that flank the coding region and are not translated into amino acids. These regions are part of the primary transcript and remain after the splicing of exons into the mRNA. As such UTRs are exonic regions.
Several functional roles have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency. Coding regions begin with the start codon and end with a stop codon. This tail promotes export from the nucleus, translation, and stability of mRNA 13 , The structure of an mRNA 3.
RNA interference in mammalian cells Designer siRNAs are now widely used in the laboratory to down-regulate specific proteins whose function is under study.
Non protein coding RNAs a. More than one thousand miRNAs are currently known for the human genome, and each of them has the ability to down regulate the expression of possibly thousands of protein coding genes Alternative pathways non-canonical Drosha independent pathways: As mentioned above, most miRNAs either originate form their own transcription units or derive from the exons or introns of other genes 33 and require both Drosha and Dicer for cleavage in their maturation.
It was recently shown however first in Droshophila 33 and later in mammals 34 that short hairpin introns, called mirtrons can be alternative sources of miRNAs. Although there are several differences between mammalian and invertebrate mirtrons, both are Drosha independent.
Beyond the ladder-like structure described above, another key characteristic of double-stranded DNA is its unique three-dimensional shape.
The first photographic evidence of this shape was obtained in , when scientist Rosalind Franklin used a process called X-ray diffraction to capture images of DNA molecules Figure 5. Although the black lines in these photos look relatively sparse, Dr. Franklin interpreted them as representing distances between the nucleotides that were arranged in a spiral shape called a helix.
Around the same time, researchers James Watson and Francis Crick were pursuing a definitive model for the stable structure of DNA inside cell nuclei. Watson and Crick ultimately used Franklin's images, along with their own evidence for the double-stranded nature of DNA, to argue that DNA actually takes the form of a double helix , a ladder-like structure that is twisted along its entire length Figure 6. Franklin, Watson, and Crick all published articles describing their related findings in the same issue of Nature in Most cells are incredibly small.
For instance, one human alone consists of approximately trillion cells. Yet, if all of the DNA within just one of these cells were arranged into a single straight piece, that DNA would be nearly two meters long! So, how can this much DNA be made to fit within a cell? The answer to this question lies in the process known as DNA packaging , which is the phenomenon of fitting DNA into dense compact forms Figure 7.
During DNA packaging, long pieces of double-stranded DNA are tightly looped, coiled, and folded so that they fit easily within the cell. Eukaryotes accomplish this feat by wrapping their DNA around special proteins called histones , thereby compacting it enough to fit inside the nucleus Figure 8. Together, eukaryotic DNA and the histone proteins that hold it together in a coiled form is called chromatin. It is impossible for researchers to see double-stranded DNA with the naked eye — unless, that is, they have a large amount of it.
Modern laboratory techniques allow scientists to extract DNA from tissue samples, thereby pooling together miniscule amounts of DNA from thousands of individual cells. When this DNA is collected and purified, the result is a whitish, sticky substance that is somewhat translucent.
To actually visualize the double-helical structure of DNA, researchers require special imaging technology, such as the X-ray diffraction used by Rosalind Franklin. However, it is possible to see chromosomes with a standard light microscope, as long as the chromosomes are in their most condensed form. To see chromosomes in this way, scientists must first use a chemical process that attaches the chromosomes to a glass slide and stains or "paints" them.
Staining makes the chromosomes easier to see under the microscope. In addition, the banding patterns that appear on individual chromosomes as a result of the staining process are unique to each pair of chromosomes, so they allow researchers to distinguish different chromosomes from one another. Then, after a scientist has visualized all of the chromosomes within a cell and captured images of them, he or she can arrange these images to make a composite picture called a karyotype Figure This page appears in the following eBook.
Aa Aa Aa. What components make up DNA? Figure 1: A single nucleotide contains a nitrogenous base red , a deoxyribose sugar molecule gray , and a phosphate group attached to the 5' side of the sugar indicated by light gray.
Opposite to the 5' side of the sugar molecule is the 3' side dark gray , which has a free hydroxyl group attached not shown. Figure 2: The four nitrogenous bases that compose DNA nucleotides are shown in bright colors: adenine A, green , thymine T, red , cytosine C, orange , and guanine G, blue. Although nucleotides derive their names from the nitrogenous bases they contain, they owe much of their structure and bonding capabilities to their deoxyribose molecule.
The central portion of this molecule contains five carbon atoms arranged in the shape of a ring, and each carbon in the ring is referred to by a number followed by the prime symbol '.
Of these carbons, the 5' carbon atom is particularly notable, because it is the site at which the phosphate group is attached to the nucleotide. Appropriately, the area surrounding this carbon atom is known as the 5' end of the nucleotide. The coordination of the protein complexes required for the steps of replication and the speed at which replication must occur in order for cells to divide are impressive, especially considering that enzymes are also proofreading , which leaves very few errors behind.
The study of DNA replication started almost as soon as the structure of DNA was elucidated, and it continues to this day. Currently, the stages of initiation, unwinding, primer synthesis, and elongation are understood in the most basic sense, but many questions remain unanswered, particularly when it comes to replication of the eukaryotic genome.
Scientists have devoted decades to the study of replication, and researchers such as Kornberg and Okazaki have made a number of important breakthroughs. Nonetheless, much remains to be learned about replication, including how errors in this process contribute to human disease.
Annunziato, A. Split decision: What happens to nucleosomes during DNA replication? Journal of Biological Chemistry , — Bessman, M. Enzymatic synthesis of deoxyribonucleic acid. General properties of the reaction. Kornberg, A. The biological synthesis of deoxyribonucleic acid. Nobel Lecture, December 11, Biological synthesis of deoxyribonucleic acid. Science , — Lehman, I. Preparation of substrates and partial purification of an enzyme from Escherichia coli.
Losick, R. DNA replication: Bringing the mountain to Mohammed. Mackiewicz, P. Where does bacterial replication start? Rules for predicting the oriC region. Nucleic Acids Research 32 , — Ogawa, T. Molecular and General Genetics , — Okazaki, R. Mechanism of DNA chain growth. Possible discontinuity and unusual secondary structure of newly synthesized chains. Proceedings of the National Academy of Sciences 59 , — Restriction Enzymes.
Genetic Mutation. Functions and Utility of Alu Jumping Genes. Transposons: The Jumping Genes. DNA Transcription. What is a Gene? Colinearity and Transcription Units. 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. Pray, Ph. Citation: Pray, L. Nature Education 1 1 Arthur Kornberg compared DNA to a tape recording of instructions that can be copied over and over.
How do cells make these near-perfect copies, and does the process ever vary? Aa Aa Aa. Initiation and Unwinding. Primer Synthesis.
The Challenges of Eukaryotic Replication. References and Recommended Reading Annunziato, A. Journal of Biological Chemistry , — Bessman, M. Journal of Biological Chemistry , — Kornberg, A. Science , — Lehman, I. Journal of Biological Chemistry , — Losick, R.
Science , — Mackiewicz, P. Nucleic Acids Research 32 , — Ogawa, T. Molecular and General Genetics , — Okazaki, R.
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