In humans and other higher animals, only the sex cells that are passed on to offspring have the haploid number of chromosomes. These sex cells are also called gametes.

 

    RNA

 

    Ribonucleic acid (RNA) helps DNA turn stored genetic messages into proteins. As discussed in the Biochemistry chapter, RNA monomers (nucleotides) are similar to those of DNA, but with three crucial differences:

• DNA’s five-carbon sugar is deoxyribose. RNA nucleotides contain a slightly different sugar, called ribose.
• RNA uses the nitrogenous base uracil in place of DNA’s thymine.
• The RNA molecule takes the form of a single helix—half a spiral ladder—as compared with the double helix structure of DNA.

    Two different types of RNA play important roles in protein synthesis. During transcription, DNA is copied to make messenger RNA (mRNA), which then leaves the nucleus to bring its still encoded to the ribosomes in the cytoplasm. In order to use the contained in the transcribed mRNA to make a protein, a second type of RNA is used. Transfer RNA (tRNA) moves amino acids to the site of protein synthesis at the ribosome according to the code specified by the mRNA strand. There are many different tRNAs, each of which bond to a different amino acid and the mRNA sequence corresponding to that amino acid.