CONCEPTS SUMMARY

• Early life used RNA both as the carrier of genetic information and as a biological catalyst.

• RNA is a polymer consisting of nucleotides joined together by phosphodiester bonds. Each RNA nucleotide consists of a ribose sugar, a phosphate, and a base. RNA contains the base uracil and is usually single stranded.

• Cells possess a number of different classes of RNA. Ribosomal RNA is a component of the ribosome, messenger RNA carries coding instructions for proteins, and transfer RNA helps incorporate the amino acids into a polypeptide chain.

• The template for RNA synthesis is single-stranded DNA. In transcription, RNA synthesis is complementary and antiparallel to the DNA template strand. A transcription unit consists of a promoter, an RNA-coding region, and a terminator.

• The substrates for RNA synthesis are ribonucleoside triphosphates.

• RNA polymerase in bacterial cells consists of a core enzyme, which catalyzes the addition of nucleotides to an RNA molecule, and other subunits. The sigma factor controls the binding of the core enzyme to the promoter. Eukaryotic cells contain multiple types of RNA polymerase.

• Transcription begins at the start site, which is determined by consensus sequences. RNA is synthesized from a single template strand of DNA. RNA synthesis ceases after a terminator sequence has been transcribed.

• Introns—noncoding sequences that interrupt the coding sequences (exons) of genes—are common in eukaryotic genes but rare in bacterial genes.

• An mRNA molecule has three primary parts: a 5′ untranslated region, a protein-coding sequence, and a 3′ untranslated region.

• The pre-mRNA of a eukaryotic protein-encoding gene is extensively processed: a modified nucleotide and methyl groups, collectively termed the 5′ cap, are added to the 5′ end of pre-mRNA; the 3′ end is cleaved and a poly(A) tail is added; and introns are removed.

• Transfer RNAs, which attach to amino acids, are short molecules that assume a common secondary structure and contain modified bases.

• Ribosomes, the sites of protein synthesis, are composed of several ribosomal RNA molecules and numerous proteins.

• Small interfering RNAs, microRNAs, Piwi-interacting RNAs, and CRISPR RNAs play important roles in gene silencing and in a number of other biological processes.

• Long noncoding RNAs are RNA molecules that do not encode proteins. Evidence increasingly suggests that many of these molecules function in the control of gene expression.

• Caenorhabditis elegans is a nematode that is widely used as a model genetic organism.