Classes of RNA

RNA molecules perform a variety of functions in the cell. Ribosomal RNA (rRNA), along with ribosomal protein subunits, makes up the ribosome, the site of protein assembly. We'll take a more detailed look at the ribosome in Chapter 14. Messenger RNA (mRNA) carries the coding instructions for polypeptide chains from DNA to the ri-bosome. After attaching to a ribosome, an mRNA molecule specifies the sequence of the amino acids in a polypeptide chain and provides a template for joining amino acids. Large precursor molecules, which are termed pre-messenger RNAs (pre-mRNAs), are the immediate products of transcription in eukaryotic cells. Pre-mRNAs are modified extensively before they exit the nucleus for translation into protein. Bacterial cells do not possess pre-mRNA; in these cells, transcription takes place concurrently with translation.

Transfer RNA (tRNA) serves as the link between the coding sequence of nucleotides in the mRNA and the amino acid sequence of a polypeptide chain. Each tRNA attaches to one particular type of amino acid and helps to incorporate that amino acid into a polypeptide chain (discussed in Chapter 15).

Additional classes of RNA molecules are found in the nuclei of eukaryotic cells. Small nuclear RNAs (snRNAs) combine with small nuclear protein subunits to form small nuclear ribonucleoproteins (snRNPs, affectionately known as "snurps"). The snRNPs are analogous to ribosomes in structure, only smaller, and they typically contain a single RNA molecule combined with approximately 10 small nuclear protein subunits. Some snRNAs participate in the processing of RNA, converting pre-mRNA into mRNA. Small nucleolar RNAs (snoRNAs) take part in the processing of rRNA. Small

[Table 13.2 Locations and functions of different classes of RNA molecules 1

Class of RNA

Cell Type

Location of Function* in Eukaryotic Cells

Function

Ribosomal RNA (rRNA)

Bacterial and eukaryotic

Cytoplasm

Structural and functional components of the ribosome

Messenger RNA (mRNA)

Bacterial and eukaryotic

Nucleus and cytoplasm

Carries genetic code for proteins

Transfer RNA (tRNA)

Bacterial and eukaryotic

Cytoplasm

Helps incorporate amino acids into polypeptide chain

Small nuclear RNA (snRNA)

Eukaryotic

Nucleus

Processing of pre-mRNA

Small nucleolar RNA (snoRNA)

Eukaryotic

Nucleus

Processing and assembly of rRNA

Small cytoplasmic RNA (scRNA)

Eukaryotic

Cytoplasm

Variable

*All eukaryotic RNAs are transcribed in the nucleus.

*All eukaryotic RNAs are transcribed in the nucleus.

RNA molecules also are found in the cytoplasm of eukaryotic cells; these molecules are called small cytoplasmic RNAs (scRNAs). The different classes of RNA molecules are summarized in Table 13.2. __

Concepts 9

RNA differs from DNA in that it possesses a hydroxyl group on the 2'-carbon atom of its sugar, contains uracil instead of thymine, and is normally single stranded. Several classes of RNA exist within bacterial and eukaryotic cells.

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