chapter 7

RNA differs from DNA chemically in two respects: (1) the nucleotides in RNA are _—that is, they contain the sugar _ (hence the name ribonucleic acid) rather than the _ found in DNA; and (2) although, like DNA, RNA contains the bases adenine (A), guanine (G), and cytosine (C), it contains _ (U) instead of the _ (T) found in DNA

The first step a cell takes in expressing one of its many thousands of genes is to copy the nucleotide sequence of that gene into RNA. The pro- cess is called _

Whereas DNA always occurs in cells as a double-stranded helix, RNA is largely _.

RNA polymer- ases make about one mistake for every _ nucleotides copied into RNA

The majority of genes carried in a cell’s DNA specify the amino acid sequences of proteins. The RNA molecules encoded by these genes— which ultimately direct the synthesis of proteins—are called _ (_).

When an RNA polymerase collides randomly with a DNA molecule, the enzyme sticks weakly to the double helix and then slides rapidly along its length. RNA polymerase latches on tightly only after it has encountered a gene region called a _. As it binds tightly to this sequence, the RNA polymerase opens up the double helix immediately in front of the _ to expose the nucleotides on each strand of a short stretch of DNA.

Elongation then continues until the enzyme encounters a second signal in the DNA, the _, where the polymerase halts and releases both the DNA template and the newly made RNA transcript

The terminator sequence itself is _, and it is the interaction of this _ with the polymerase that causes the enzyme to let go of the template DNA.

a segment of DNA will be transcribed only if _.

RNA polymerases I and III transcribe the genes encoding _, _ RNA, and various other RNAs that _ in the cell.

RNA polymerases _ transcribe the genes encoding transfer RNA, ribosomal RNA, and various other RNAs that play structural and catalytic roles in the cell

RNA polymerase _ transcribes the rest, including all those that encode _—which constitutes the majority of genes in eukaryotes

WhereasthebacterialRNApolymerase(alongwithitssigmasubunit) is able to initiate transcription on its own, eukaryotic RNA polymerases require the assistance of a _ Principal among these are the _, which must assemble at each promoter, along with the polymerase, before transcription can begin.

And before it can be exported to the cytosol, a eukaryotic RNA must go through several RNA processing steps, which include _, _, and _

In short, capping modifies the _ of RNA, and polyadenylation adds _ to _ in eukaryotic cells. These changes help the RNA to be _, _, and _.

RNA splicing is carried out largely by RNA molecules rather than proteins. These RNA molecules, called _, are packaged with additional _ to form _

Together, these snRNPs form the core of the _, the large assembly of RNA and protein molecules that carries out RNA splicing in the nucleus.

the transcripts of many eukaryotic genes can be spliced in different ways, each of which can produce a distinct protein called:_

the translation of mRNA into protein depends on adaptor molecules that bind to a codon with one part of the adaptor and to an amino acid with another. These adaptors consist of a set of small RNA molecules known as _, each about 80 nucleotides in length.

ecognition and attachment of the correct amino acid depend on enzymes called _

This _ tRNA always carries the amino acid methionine

In eukaryotes, an initiator tRNA, charged with methionine, is first loaded into the _ site of the small ribosomal subunit, along with additional proteins called _

These enzymes, which degrade proteins, first to short peptides and finally to individual amino acids, are known collectively as _. proteins are broken down by large protein machines called _

Proteins must then fold into the correct, three-dimensional shape (as we discuss in Chapter 4). Some proteins do so spontaneously, as they emerge from the ribosome. Most, however, require the assistance of _, which steer them along productive folding path- ways and prevent them from aggregating inside the cell