Chapter 17

RNA processing.

AUG

UAG UGA UAA

CAC GCA UGC AUC ACU AAA

met- ala-arg-lys An amino acid sequence is determined by strings of three-letter codons on the mRNA, each of which codes for a specific amino acid or a stop signal. The mRNA is translated in a 5' → 3' direction.

Example mRNA sequence 5−AUGUCGACUAAGGGA−3′ Codon sequence AUG-UCG-ACU-AAG-GGA Amino acid sequence(three-letter abbreviation) Met Ser Thr Lys Gly

Before mRNA can be translated into an amino acid sequence, the mRNA must first be synthesized from DNA through transcription. Base pairing in mRNA synthesis follows slightly different rules than in DNA synthesis: uracil (U) replaces thymine (T) in pairing with adenine (A). The codons specified by the mRNA are then translated into a string of amino acids.

promoter gene terminator The gene itself, the promoter, and the terminator are all DNA deoxyribonucleic acid) sequences. The RNA (ribonucleic acid) that is produced is another. The RNA polymerase enzyme that performs transcription is a protein and therefore not a nucleic acid.

1. RNA polymerase binds promoter. 2. RNA polymerase transcribes gene. 3.RNA polymerase reaches terminator. 4. RNA polymerase exits gene, RNA is released.

A specific sequence of three adjacent bases on a strand of DNA or RNA that provides genetic code information for a particular amino acid

group of three bases on a tRNA molecule that are complementary to an mRNA codon

(genetics) the organic process whereby the DNA sequence in a gene is copied into mRNA

Process by which mRNA is decoded and a protein is produced

a long chain of chemically bonded amino acids

transcription.

charging of tRNA.

translation,

In eukaryotes, pre-mRNA is produced by the direct transcription of the DNA sequence of a gene into a sequence of RNA nucleotides. Before this RNA transcript can be used as a template for protein synthesis, it is processed by modification of both the 5' and 3' ends. In addition, introns are removed from the pre-mRNA by a splicing process that is catalyzed by snRNAs (small nuclear RNAs) complexed with proteins. The product of RNA processing, mRNA (messenger RNA), exits the nucleus. Outside the nucleus, the mRNA serves as a template for protein synthesis on the ribosomes, which consist of catalytic rRNA (ribosomal RNA) molecules bound to ribosomal proteins. During translation, tRNA (transfer RNA) molecules match a sequence of three nucleotides in the mRNA to a specific amino acid, which is added to the growing polypeptide chain. RNA primers are not used in protein synthesis. RNA primers are only needed to initiate a new strand of DNA during DNA replication.

mRNA, pre-MRNA, snRNA

tRNA, rRNA

RNA primers

There are three principles to keep in mind when predicting the sequence of the mRNA produced by transcription of a particular DNA sequence. The RNA polymerase reads the sequence of DNA bases from only one of the two strands of DNA: the template strand. The RNA polymerase reads the code from the template strand in the 3' to 5' direction and thus produces the mRNA strand in the 5' to 3' direction. In RNA, the base uracil (U) replaces the DNA base thymine (T). Thus the base-pairing rules in transcription are A→U, T→A, C→G, and G→C, where the first base is the coding base in the template strand of the DNA and the second base is the base that is added to the growing mRNA strand.

Some transcription factors bind to both strands of the DNA, whereas others bind specifically to either the template or coding strand. Binding of a transcription factor to DNA requires a specific sequence of bases in the DNA binding region. Some transcription factors must bind to the promoter before RNA polymerase II can bind. Transcription factors are a group of DNA-binding proteins involved in the initiation and regulation of transcription. Many transcription factors bind to the promoter sequence of a gene. These transcription factors enable RNA polymerase II to bind to the promoter in the correct orientation (facing in the right direction on the DNA) to copy the template strand.

the base sequence of the gene's promoter In eukaryotes, binding of RNA polymerase II to DNA involves several other proteins known as transcription factors. Many of these transcription factors bind to the DNA in the promoter region (shown below in green), located at the 3' end of the sequence on the template strand. Although some transcription factors bind to both strands of the DNA, others bind specifically to only one of the strands. Transcription factors do not bind randomly to the DNA. Information about where each transcription factor binds originates in the base sequence to which each transcription factor binds. The positioning of the transcription factors in the promoter region determines how the RNA polymerase II binds to the DNA and in which direction transcription will occur.

Noncoding sequences called introns are spliced out by molecular complexes called spliceosomes. A poly-A tail (50-250 adenine nucleotides) is added to the 3' end of the pre-mRNA. A cap consisting of a modified guanine nucleotide is added to the 5' end of the pre-mRNA. Once RNA polymerase II is bound to the promoter region of a gene, transcription of the template strand begins. As transcription proceeds, three key steps occur on the RNA transcript: Early in transcription, when the growing transcript is about 20 to 40 nucleotides long, a modified guanine nucleotide is added to the 5' end of the transcript, creating a 5' cap. Introns are spliced out of the RNA transcript by spliceosomes, and the exons are joined together, producing a continuous coding region. A poly-A tail (between 50 and 250 adenine nucleotides) is added to the 3' end of the RNA transcript. Only after all these steps have taken place is the mRNA complete and capable of exiting the nucleus. Once in the cytoplasm, the mRNA can participate in translation.

introns.

splicing

spliceosomes

exons

It could cause a silent, missense, or nonsense mutation because those are the types that can be caused by a nucleotide-pair substitution like this one. When the sequence of bases in a gene is changed by a single nucleotide-pair substitution, it can have different effects on the product depending on where it is and whether or not the mutated codon codes for the same amino acid as the original codon.

A substitution missense affects only one codon, but a frameshift missense affects all codons downstream of the frameshift. Many genetic diseases are caused by missense mutations.