Having an RNA intermediate allows the information in DNA to travel from the nucleus to the cytoplasm. In addition, in eukaryotes DNA is kept in the nucleus, while translation happens on ribosomes (found in the cytoplasm). This allows cells to be much more responsive to changing conditions. There are also multiple levels of regulation that can control how much mRNA is present, what parts of the mRNA get kept‡, and how frequently the mRNAs get translated. This is because many mRNAs can be transcribed from a gene and then each mRNA can be translated independently (and multiple times). Making mRNA also allows efficient production of proteins from a single gene. (You'd have to redesign life from scratch!) In translation, the sequence of the mRNA is decoded to specify the amino acid. This means that we use RNA because "life" has always used RNA and getting rid of it by evolutionary processes would be essentially impossible. In transcription, the DNA sequence of a gene is copied to make an RNA molecule. One strongly supported hypothesis is that RNA actually came first - this known as the "RNA world hypothesis"§. Color the uracil brown.This is an interesting question to think about, but "why" questions in biology are often very difficult to answer in a meaningful way.īelow I've listed one possible evolutionary reason for the use of mRNA as an intermediary and then some advantages to this system. RNA stands for ribonucleic acid.Ĭolor the ribose a darker blue. In addition to that difference, mRNA has the sugar ribose instead of deoxyribose. Instead of thymine, mRNA contains the base uracil. Messenger RNA is similar to DNA, except that it is a single strand, and it has no thymine. Imagine that the code taken to the ribosomes is telling the ribosome what is needed - like a recipe. Recall that proteins are the body's building blocks and are made of individual amino acids joined into a long chain. It takes the "message" of the DNA to the ribosomes and "tells them" what proteins are to be made. The messenger RNA (mRNA) is small enough to go through the nuclear pores. So a chemical is used read the DNA in the nucleus. The only problem is that the DNA is too big to go through the nuclear pores. We know the nucleus controls the cell's activities through the chemical DNA, but how? It is the sequence of bases that determine what protein is to be made. The two sides of the DNA ladder are held together loosely by hydrogen bonds. Color the nucleotides using the same colors as you colored them in the double helix. Note that the pyrimidines are single ringed and the purines are double ringed. The pyrimidines are thymine and cytosine. Two of the bases are purines - adenine and guanine. DNA is actually a molecule of repeating nucleotides. The combination of a single base, a deoxyribose sugar, and a phosphate make up a nucleotide. Note that the bases attach to the sides of the ladder at the sugars and not the phosphate. A strand of DNA contains millions of bases. For instance A T T A T T would code for a different Trait than A G G A G G. The order of these bases is the code that contains the instructions. The bases can occur in any order along a strand of DNA. These bases always bond in a certain way. The bases are known by their coded letters A,G,T,C. The rungs of the ladder are pairs of 4 types of nitrogen bases. The sugar is deoxyribose.Ĭolor all the phosphates pink (one is labeled with a "P"). The sides of the ladder are made of alternating sugar and phosphate molecules. The structure is a double helix, which is described as a twisted ladder. In 1953, James Watson and Francis Crick established the structure of DNA. We now know that DNA is also found in organelles, the mitochondria and chloroplasts, though it is the DNA in the nucleus that actually controls the cell's workings. DNA is called a nucleic acid because it was first found in the nucleus. Meanwhile, DNA is the chemical that genes and chromosomes are made of. Hence you hear it commonly referred to as the gene for baldness or the gene for blue eyes. A gene is a segment of DNA that codes for a particular protein, which in turn codes for a trait. These proteins in turn, form the structural units of cells and control all chemical processes within the cell.Ĭhromosomes are composed of genes. In simple terms, DNA controls the production of proteins within the cell. How does it do this? Chromosomes, found in the nucleus, are microscopic, threadlike strands composed of the chemical DNA (short for deoxyribonucleic acid). The nucleus is a small spherical, dense body in a cell.It is often called the "control center" because it controls all the activities of the cell including cell reproduction, and heredity.
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