Peran Asam Nukleat

The Roles of Nucleic Acids

The two types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), enable living organisms to reproduce their complex components from one generation to the next. Unique among molecules, DNA provides directions for its own replication. DNA also directs RNA syntheSIS and, through RNA, controls protein synthesis (Figure 5.26). DNA is the genetic material that organisms inherit from their parents. Each chromosome contains one long DNA molecule, usually carrying several hundred or more genes. Then a cell reproduces it self by dividing, its DNA molecules are copied and passed along from one generation of cells to the next. Encoded in the structure ofDNA is the information that programs all the cell's activities. The DNA, however, is not directly involved in running the operations ofthe cell, any more than computer software by itselfcan print a bank statement or read the bar code on a box ofcereal. Just asa printer is needed to print outa statement and a scanner is needed to read a bar code, proteins are required to implement genetic programs. The molecular hardware of the cell-the tools for biological functions-consists mostly of proteins. For example, the oxygen carrier in red blood cells is the protein hemoglobin, not the DNA that specifies its structure. How does RNA, the other type of nucleic acid, fit into the flow ofgenetic information from DNA to proteins? Each gene along a DNA molecule directs synthesis of a type of RNA called messenger RNA (mRNA). The mRNA molecule interacts with the cell's protein-synthesizing machinery to direct production of a polypeptide, which folds into all or part of a protein. We can summarize the flow of genetic information as DNA ---> RNA ---> protein (see Figure 5.26). The sites of protein synthesis are tiny structures called ribosomes. In a eukaryotic cell, ribosomes are in the cytoplasm, but DNA resides in the nucleus. Messenger RNA conveys genetic instructions for building proteins from the nucleus to the cytoplasm. Prokaryotic cells lack nuclei but still use RNA to convey a message from the DNA to ribosomes and other cellular equipment that translate the coded information into amino acid sequences. RNA also plays many other roles in the cell.

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