what step could you add after you transcribe the dna to make

9.ii: Transcription

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What you'll learn to do: Outline the process of transcription

Have you ever had to transcribe something? Maybe someone left a message on your voicemail, and you had to write it down on newspaper. Or perhaps you took notes in grade, and then rewrote them neatly to assist y'all review.

As these examples show, transcription is a procedure in which information is rewritten. Transcription is something we do in our everyday lives, and it'due south also something our cells must do, in a more specialized and narrowly defined mode. In biology, transcription is the process of copying out the DNA sequence of a cistron in the similar alphabet of RNA.

Learning Objectives

• Understand the basic steps in the transcription of Deoxyribonucleic acid into RNA
• Understand the difference between pre-RNA and mRNA

Steps of Transcription

The process of Transcription takes place in the cytoplasm in prokaryotes and in nucleus in eukaryotes. It uses DNA every bit a template to make an RNA (mRNA) molecule. During transcription, a strand of mRNA is made that is complementary to a strand of Deoxyribonucleic acid. Figure 1 shows how this occurs. Eventually portions of the transcribed mRNA volition exist fabricated into functional proteins.

Figure one. Overview of Transcription. Transcription uses the sequence of bases in a strand of Deoxyribonucleic acid to make a complementary strand of mRNA. Triplets are groups of three successive nucleotide bases in Dna. Codons are complementary groups of bases in mRNA.

You lot tin also lookout this more than detailed video about transcription.

Figure 2. Transcription occurs in the three steps—initiation, elongation, and termination—all shown here.

Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in Figure ii.

Step ane: Initiation

Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can ''read'' the bases in one of the Dna strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.

Step 2: Elongation

Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule, using complementary base of operations pairs. During this process, an adenine (A) in the Deoxyribonucleic acid binds to an uracil (U) in the RNA.

Step iii: Termination

Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is consummate, and it detaches from Deoxyribonucleic acid.

This video provides a review of these steps. You lot tin can stop watching the video at v:35. (Later this point, information technology discusses translation, which nosotros'll discuss in the next upshot.)

Visit this BioStudio animation to see the process of prokaryotic transcription.

pre-RNA and mRNA

After transcription, eukaryotic pre-mRNAsouthward must undergo several processing steps before they can be translated. Eukaryotic (and prokaryotic) tRNAs and rRNAs also undergo processing before they tin part every bit components in the poly peptide synthesis machinery.

mRNA Processing

The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. The boosted steps involved in eukaryotic mRNA maturation create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical E. coli mRNA lasts no more than five seconds.

The 3 about important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5′ and 3′ ends of the molecule, and the removal of intervening sequences that do not specify the appropriate amino acids.

five′ Capping

A cap is added to the 5′ terminate of the growing transcript by a phosphate linkage. This addition protects the mRNA from degradation. In improver, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.

3′ Poly-A Tail

Once elongation is consummate, an enzyme called poly-A polymerase adds a string of approximately 200 A residues, called the poly-A tail to the pre-mRNA. This modification further protects the pre-mRNA from degradation and signals the consign of the cellular factors that the transcript needs to the cytoplasm.

Pre-mRNA Splicing

Eukaryotic genes are composed of exons, which correspond to poly peptide-coding sequences (ex-on signifies that they are expressed), and intervening sequences called introns (intron denotes their intervening role), which are removed from the pre-mRNA during processing. Intron sequences in mRNA practise non encode functional proteins.

All of a pre-mRNA's introns must exist completely and precisely removed before protein synthesis. If the procedure errs by fifty-fifty a single nucleotide, the reading frame of the rejoined exons would shift, and the resulting poly peptide would be dysfunctional. The process of removing introns and reconnecting exons is called splicing (Effigy three).

Practice Question

Figure 3. Pre-mRNA splicing involves the precise removal of introns from the principal RNA transcript. The splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA molecules called snRNAs. Spliceosomes recognize sequences at the five′ and iii′ cease of the intron.

Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors?

[practice-area rows="2″][/practise-area]
[reveal-reply q="454729″]Show Answer[/reveal-respond]
[hidden-answer a="454729″]Think of different possible outcomes if splicing errors occur. Mutations in the spliceosome recognition sequence at each terminate of the intron, or in the proteins and RNAs that make up the spliceosome, may impair splicing. Mutations may too add new spliceosome recognition sites. Splicing errors could lead to introns being retained in spliced RNA, exons being excised, or changes in the location of the splice site.[/hidden-respond]

See how introns are removed during RNA splicing at this website.

Bank check Your Understanding

Answer the question(south) below to meet how well you understand the topics covered in the previous section. This short quiz does non count toward your grade in the class, and you tin retake it an unlimited number of times.

Utilize this quiz to check your understanding and decide whether to (1) written report the previous department further or (2) move on to the next section.

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Source: https://bio.libretexts.org/Courses/Lumen_Learning/Book%3A_Biology_for_Non-Majors_I_(Lumen)/09%3A_DNA_Transcription_and_Translation/9.02%3A_Transcription

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