The predominant cells in blood are the red blood cells (RBC) which contains a protein called hemoglobin. Hemoglobin is the principal determinant of the color of blood. Arterial blood and capillary blood are bright red, as oxygen imparts a strong red color to the heme group.
Concept 21 RNA is an intermediary between DNA and protein. DNA is found mostly in the cell nucleus, but another type of nucleic acid, RNA, is common in the cytoplasm. Watson and Crick proposed that RNA must copy the DNA message in the nucleus and carry it out to the cytoplasm, where proteins are synthesized. Crick also predicted the existence of an adaptor molecule that reads the genetic code and selects the appropriate amino acids to add to a growing polypeptide chain.
Although the functions of each type of RNA are different, one type of RNA is called messenger RNA, or simply mRNA. mRNA is created when the DNA recipe is copied in the first step of the central dogma. The information found in mRNA can be interpreted by using two other forms of RNA in the second step of the central dogma. mRNA is translated into protein at a cellular structure known as the ribosome. A second type of RNA helps form the structure of a ribosome. This type of RNA is called ribosomal RNA, or rRNA.