Cytoplasm genetic information

Those cytoplasmic RNA molecules that serve as templates for protein synthesis (ie, that transfer genetic information from DNA to the protein-synthesizing machinery) are designated messenger RNAs, or mRNAs. Many other cytoplasmic RNA molecules (ribosomal RNAs rRNAs) have strucmral roles wherein they con-... 

Whereas DNA has a single role as the storehouse of genetic information, RNA plays many roles in the operation of a cell. There are several different types of RNA, each having its own function. The principal job of RNA is to provide the information needed to synthesize proteins. Protein synthesis requires several steps, each assisted by RNA. One type of RNA copies the genetic information from DNA and carries this blueprint out of the nucleus and into the cytoplasm, where construction of the protein takes place. The protein is assembled on the surface of a ribosome, a cell component that contains a second type of RNA. The protein is consfructed by sequential addition of amino acids in the order specified by the DNA. The individual amino acids are carried to the growing protein chain by yet a third type of RNA. The details of protein synthesis are well understood, but the process is much too complex to be described in an introductoiy course in chemistry. 

The eytoplasm is a viscous fluid and contains within it systems of paramount importance. These are the nucleus, responsible for the genehc make-up of the cell, and the ribosomes, whieh are the site of protein synthesis, hi addihon are found granules of reserve material suehas polylydioxybutyric add, an energy reserve, and polyphosphate or volutin granules, the exact funchon of which has not yet been elucidated. The prokaiyohc nueleus or bacterial chromosome exists in the cytoplasm in the form of a loop and is not surrounded by a nuclear membrane. Bacteria cany other chromosomal elements episomes, which are portions of the main chromosome that have become isolated firm it, and plasmids, whieh may be called miniature chromosomes. These are small annular pieees of DNA whieh carry a limited amount of genetic information. 

Nucleic acids Complex biopolymers Storage and transfer of genetic information and makeup of proteins Nuclei and cytoplasm of living cells about 2... 

Messenger RNAs (mRNAs) transfer genetic information from the cell nucleus to the cytoplasm. The primary transcripts are substantially modified while still in the nucleus (mRNA maturation see p.246). Since mRNAs have to be read codon by codon in the ribosome, they must not form a stable tertiary structure. This is ensured in part by the attachment of RNA-binding proteins, which prevent base pairing. Due to the varying amounts of information that they carry, the lengths of mRNAs also vary widely. Their lifespan is usually short, as they are quickly broken down after translation. 

The presence of RNA in the cytoplasm had been linked to protein synthesis by experiments done in the early 1940s. After the discovery of the double helix, the concept followed quickly that DNA was the master "blueprint" from which secondary blueprints or transcripts of RNA could be copied. The RNA copies, later identified as messenger RNA (mRNA), provided the genetic information for specifying protein sequence. The flow of information from DNA to RNA to proteins could be symbolized as in Eq. 26-1. 

The DNA does not transfer its genetic information directly to protein. Rather, this information passes through an intermediary, the messenger RNA (mRNA). The mRNA is made on a DNA template in the nucleus of a eukaryotic cell and then passes into the cytoplasm, where it serves in turn as a template for the synthesis of the polypeptide chain. The overall process of information transfer from DNA to mRNA (transcription) and from mRNA to protein (translation) is depicted in figure 1.22. 

In prokaryotes DNA, RNA, and protein synthesis all take place in the same cellular compartment. In eukaryotes the DNA is compartmentalized in the cell nucleus, and it became clear long before the biochemistry of these three processes was understood that DNA synthesis takes place in the nucleus, whereas the bulk of protein synthesis takes place in the cytoplasm. From these observations on eukaryotes it was self-evident that DNA cannot be directly involved in the synthesis of protein but must somehow transmit its genetic information for protein synthesis to the cytoplasm. Careful experiments with radioactive labels were used to demonstrate that RNA synthesis takes place in the nucleus much of this RNA is degraded rather quickly, but the portion that survives is mostly transferred to the cytoplasm (fig. 28.1). From observations of this kind it became clear that RNA was the prime candidate for the carrier of genetic information for the synthesis of proteins. 

RNA that carries genetic information for a specific polypeptide from the nuclear gene to the cytoplasm where it combines with transfer RNA (tRNA) to initiate protein synthesis. 

The essence of life is contained in deoxyribonucleic acid (DNA), which stays in the cell nucleus, and ribonucleic acid (RNA), which functions in the cell cytoplasm. These substances, which are known collectively as nucleic acids, store and pass on essential genetic information that controls reproduction and protein synthesis. 

Genetic information is accessed by a process known as transcription, in which the double-stranded DNA splits and the genetic code is transcribed onto a single-strand messenger RNA(mRNA). The mRNA is comprised of the same bases as the DNA, arranged in the same sequence, but in a complementary fashion. The mRNA migrates out of the nucleus and into the cytoplasm, where it attaches to ribosomes. The ribosomes assemble amino acids to form protein molecules through a process known as translation. 

Messenger RNA (mRNA) The form of RNA that carries a copy of a specific sequence of genetic information (a gene) from the DNA in the cell nucleus to the ribosomes in the cytoplasm, where it is translated into a protein. 

The genetic information in a gene is copied (transcribed) into a messenger RNA molecule (mRNA), preserving the sequence by complementary base-pairing. The introns are cut and the mRNA molecule is transported into the cytoplasm where it directs the synthesis of protein at the ribosomes. The sequence of bases is translated into a sequence of amino acid residues by a triplet code wherein three bases specify one amino acid. 

The principle distinguishing features of the procaryotic cell are 1) absence of internal membranes which separate the resting nucleus from the cytoplasm, and isolate the enzymatic machinery of photosynthesis and of respiration in specific organelles 2) nuclear division by fission, not by mitosis, a character possibly related to the presence of a single structure which carries all the genetic information of the cell and 3) the presence of a cell wall which contains a specific mucopeptide as its strengthening element, (p. 21 in Stanier and van Neil 1962) . 

DNA stores the genetic information, while RNA molecules are responsible for transmitting this information to the ribosomes, where protein synthesis actually occurs. This complex process involves, first, the construction of a special RNA molecule called messenger RNA (mRNA). The mRNA is built in the cell nucleus on the appropriate section of DNA (the gene) the double helix is unzipped, and the complementarity of the bases is used in a process similar to that used in DNA replication. The mRNA then migrates into the cytoplasm of the cell where, with the assistance of the ribosomes, the protein is synthesized. 

Prior to cell division, the nucleus replicates itself so that the two new cells will each contain genetic information. Several nuclear enzymes coordinate the replication of DNA. During cell division, the nuclear envelope breaks down, and equal copies of DNA and cytoplasm are partitioned into two daughter cells. After division, the nuclear envelopes reform in each daughter cell around its own copy of DNA. This fundamental sequence of events allows for the continuation of eukaryotic life during embryonic development and cellular regeneration throughout life. 

Hepatic protein synthesis proceeds via the subcellular stages of gene transcription (in the nucleus) and gene translation (in the cytoplasm). The DNA is transcribed into various types of RNA by the action of the different DNA-dependent RNA polymerases I (A), II (B) and III (C). RNA polymerase I is responsible for the transcription of ribosomal RNA, RNA polymerase II mediates the transcription of messenger RNA, and RNA polymerase III forms transcriptal RNA. These three different RNA types move out of the nucleus into the cytoplasm. Here the ribosomes acquire the genetic information needed for protein synthesis via mRNA, and tRNA transports the activated amino acids to the ribosomes, which are themselves activated (and if necessary replicated) by rRNA. (s. figs. 2.9, 2.17 3.5)... 

The primary cellular function of RNA is to direct biosynthesis of the thousands of diverse peptides and proteins required by an organism—at least 100,000 in a human. The mechanics of protein biosynthesis appear to be catalyzed by mRNA rather than by protein-based enzymes and lake place on ribosomes, small granular particles in the cytoplasm of a cell that consist of about 60% ribosomal RNA and 40% protein. On the ribosome, mRNA serves as a template to pass on the genetic information it has transcribed from DNA. 

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