DNA, along with RNA and proteins, is one of the three major macromolecules that are essential for life. Within the nucleus of eukaryotic cells, DNA is organized into structures called chromosomes. The complete set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes. DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds.
These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called nucleobases bases. It is the sequence of these four bases along the backbone that encodes information. The sequence of these bases comprises the genetic code, which subsequently specifies the sequence of the amino acids within proteins. The structure of DNA. Bases are classified into two types: the purines, A and G, and the pyrimidines, the six-membered rings C, T and U.
Uracil U , takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine. In the DNA double helix, each type of base on one strand normally interacts with just one type of base on the other strand.
This is complementary base pairing. 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. Related Posts. Long-term storage of genetic information; transmission of genetic information to make other cells and new organisms. 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. B-form double helix. Essentially, it copies just some of the information found in DNA, and carries it whenever it's needed by the cell. RNA can be thought of like a copy of a segment of DNA that you can throw away once it's not needed anymore.
Sometimes it is even a copy of one specific gene. Neat, huh? There are four main types in organisms: carbohydrates , proteins, lipids , and nucleic acids. DNA vs. Biology Molecular Genetic Code. Explanations 5 Gabi Slizewska.
Image source: By Gabi Slizewska. Functions: DNA deoxyribonucleic acid and RNA ribonucleic acid are very similar molecules that serve very different functions. The differences start to show up when we take a closer look at their structures. Related Lessons. View All Related Lessons. Sarah Morgan. Image source: By Sarah Morgan. This Venn diagram can help remind you. Deena Hauze. Samantha Broders. This picture shows how the two strands of DNA bond with each other. It also shows the bonds within the strand itself.
Function The primary function of DNA is to provide genetic information for living organisms. Vocabulary Review: Monomer : Any molecule that can react with other molecules of the same or different makeup to form a chain, or polymer Polymer : A chain of monomers made of a specific type of compound Macromolecule : A polymer that has a large macro molecular mass. There are four main types in organisms: carbohydrates , proteins , lipids , and nucleic acids Nucleotides : The monomers of nucleic acids.
Made up of a nitrogen group, a phosphorus group, and a 5-carbon ring Nucleic Acid : Complex polymer that stores information in a cell in the form of a code.
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