Chapter 14 Flashcards
Exon
Coding sequence
Intron
Noncoding sequence
Draw and label the structure of a mature mRNA
Untranslated regions with coding sequences between them
- List the 3 main steps in pre-mRNA processing
Capping at the 5’ end
Addition of a polyA tail at the 3’ end. and
Splicing to remove introns
- Know the type of cells that pre-mRNA processing occurs as well as the location within the cell
- Occurs in eukaryotic cells within the nucleus
Describe how the 5’ cap is added as well as its function
- it is a guanine nucleotide added backwards (5’ to 5’ bond) to the 5’ end of the mRNA
- then methyl groups are added to the mRNA
- Function:
1) assists with binding of ribosome to mRNA for translation
2) stabilizes mRNA
Describe how the poly(A) tail is added as well as its function
* How? 2 steps
- 11-30 nucleotides cleaved downsteam of AAUAA near 3’ end
- Many adenines added
Define splicing
Removal of noncoding sequences from pre-mRNA and connecting remaining pieces to form mature mRNA
- Define spliceosome as well as its makeup
Spliceosome is a large complex where splicing occurs
* Consists of several snRNPs (‘snurps’)
* snRNP = 1 snRNA + proteins
each snRNP plays a different role in the splicing process but we will not worry about those details
Name the 3 consensus sequences required for splicing
- 5’ splice site
- 3’ splice site
- branch poin
Describe the process of splicing
- Cutting out non-coding sequences (introns) from newly synthesized pre-mRNA.
- Joining the coding regions (exons).
- Creating different variations of the same mRNA through alternative splicing.
- Explain alternative splicing
Cutting out different coding sequences, making different proteins
- Explain multiple 3’ cleavage sites
* Fig 14.13 shows how the calcitonin gene can undergo alternative splicing and it also has multiple 3’
cleavage sites—
different molecules split at sifferent cleavbage sites, menaing different molecules (and therefore proteins) are produced from the same DNA strand
- Describe the basic structure of tRNA and know the tRNA processing
Clover structure with amino acid at the end
1. The 5′ end of the pre-tRNA, called the 5′ leader sequence, is cleaved off.
- The 3′ end of the pre-tRNA is cleaved off.
- In all eukaryote pre-tRNAs, but in only some bacterial and archaeal pre-tRNAs, a CCA sequence of nucleotides is added to the 3′ end of the pre-tRNA after the original 3′ end is trimmed off. Some bacteria and archaea pre-tRNAs already have the CCA encoded in their transcript immediately upstream of the 3′ cleavage site, so they don’t need to add one. The CCA at the 3′ end of the mature tRNA will be the site at which the tRNA’s amino acid will be added.
- Multiple nucleotides in the pre-tRNA are chemically modified, altering their nitorgen bases. On average about 12 nucleotides are modified per tRNA. The most common modifications are the conversion of adenine (A) to pseudouridine (ψ), the conversion of adenine to inosine (I), and the conversion of uridine to dihydrouridine (D). But over 100 other modifications can occur.
- A significant number of eukaryotic and archaeal pre-tRNAs have introns that have to be spliced out. Introns are rarer in bacterial pre-tRNAs, but do occur occasionally and are spliced out.
- Define rRNA
RNA component of ribosomes- molecular machines that catalyze protein synthesis
- Describe the structure of a ribosome as below
Large and small subunit (each made of rRNA and proteins)
* occurs in both eukaryotes and prokaryotes
Why add poly a tail?
1) Increases stability of mRNA
2) Required for ribosome binding to 5’cap
