Unit 6: Topic 3 - Transcription and RNA Processing

Question 1

List the three types of RNA molecules and explain each of their functions.

Answer 1

1. mRNA: This type of RNA is used to carry the genetic information used to code for particular proteins from the DNA in the nucleus to the ribosomes.
2. tRNA: This type of RNA is used in the process of translation to bring in amino acids from the cytoplasm. Each tRNA is assigned to a particular amino acid, and the tRNAs are recruited to the ribosome based off of the anticodon sequence they have (which matches up with the sequence on the mRNA).
3. rRNA: This type of RNA is one of the crucial components of the ribosome, and it plays a big role in catalyzing the formation of peptide bonds between amino acids.

Question 2

Explain what is meant by the Central Dogma of biology.

Answer 2

The Central Dogma states that in the cell, genetic information flows from DNA to mRNA, which is then used by the ribosomes to synthesize proteins.

Question 3

Explain the process of transcribing a molecule of mRNA.

Answer 3

The transcription of a molecule of mRNA has 3 main steps: initiation, elongation, and termination. In the process of initiation, transcription factors call over RNA polymerase to promoter region (which is directly upstream of the gene being transcribed). Next, RNA Polymerase starts reading DNA in the 3’ to 5’ direction, synthesizing the mRNA molecule in the 5’ to 3’ direction. The mRNA molecule is synthesized based off of only one of the two strands of DNA (known as the template, noncoding, minus, or antisense strand), which is chosen based off of the gene being transcribed. This process of extending the mRNA molecule is called elongation. Elongation terminates when RNA polymerase reads a specific stop sequence in the DNA, cause the RNA polymerase to release both the DNA and the newly synthesized pre-mRNA molecule (called pre-mRNA because it is yet to be processed).

A diagram depicting this process: https://www.researchgate.net/publication/332329904/figure/fig3/AS:746165347827712@1554911090731/Steps-in-transcription-process-This-image-was-taken-from-42.jpg

Question 4

List and explain the three main types of modification the pre-mRNA molecule undergoes while it is being processed.

Answer 4

1. Splicing: During the process of splicing, noncoding regions in the pre-mRNA molecule (meaning that these regions do not code for proteins), called introns, are excised, and exons (coding regions) are joined back together. The main purpose for the presence of introns is for alternative splicing to be able to occur (will be explained in a later flashcard).
   Diagram showing this in more detail: https://upload.wikimedia.org/wikipedia/commons/3/37/Process_of_RNA_splicing.png
2. Addition of the poly-A tail: The poly-A tail (a series of adenosines) is added to the 3’ end of the mRNA in order to protect the coding region of the mRNA from degradation. It also assists in exporting the mRNA from the nucleus to the cytoplasm.
3. Addition of the GTP cap: The GTP cap (made of a methylated guanine) is added to the 5’ end. Its main function is to help with ribosomal recognition of the mRNA molecule.

Question 5

Explain what is meant by alternative splicing.

Answer 5

During the process of splicing, exons can be joined together in different ways (certain exons can be spliced out with the introns), resulting in different mRNA molecules. This process is known as alternative splicing, and it means that different proteins can be created based off of a single gene.

A diagram depicting this: https://www.ncbi.nlm.nih.gov/Class/MLACourse/Modules/MolBioReview/images/alternative_splicing.gif