Question

11. What are the post-translational modifications that take place in eukaryotes? Give examples.

12. Compare and contrast gene expression (transcription and translation) in prokaryotes and eukaryotes. Highlight the main similarities and differences.

13.   List and compare the different types of point mutations that could affect the coding sequence of a gene? Can mutations in non-coding regions (such as introns, promoter, etc.)  affect gene expression? Explain.

14.   What many cyclins and cyclin-dependent-kinases are present in mammals? Give one example for each phase of the cell cycle.

15.   Search for 2 examples of genes associated with genetic traits that fit each of these categories:

a.       Pleitropy

b.      Codominance

c.       Incomplete dominance

d.      Epistasy

Answer 1

Q. 11.

Post transitional modification are important component in cell signalling and is a biochemical mechanism in which amino acid in proteins are modified covalently. It can occur on the side chain of the amino acid at C or N terminus. Proteins are synthesized by ribosome translating mRNA into polypeptide chains that undergo PTM and mature protein product formed. Post translational modification are

            1. Acetylation

            2. Glycosylation: the process carbohydrate molecule attachment

            3. Methylation: introduction of methyl group

            4. Phosphorylation: introduction of phosphate

            5. Hydroxylation: process of introduction of hydroxyl group

            6. Lipidation: process of attachment of lipid molecule

            7. Formation of disulfide bonds from cysteine residue

Q. 12.

In prokaryotes, expression of genes is mainly controlled in clusters and are also known as operon. Expression of a region mainly depends upon the operator, promoter, RNA polymerase and regulatory protein. Translation of the polycistronic mRNA run simultaneously with transcription which suggest that expression of mRNA always leads to protein. However, in eukaryotes, operons are rare and expression of RNA occur in nucleus and RNA desires to be processed and translocated to cytoplasm of the eukaryotic cell as mRNA for translation. This is a translational level control in eukaryotes in comparison to prokaryotes which generally not have such control. Transcription of gene in eukaryotes mostly follows positive control i.e. RNA pol requires an accessory protein to initiate transcription. In summary, expression in prokaryotes is negatively regulated, exceptions might be possible. while, in eukaryotes it is positively regulated.

One main difference between gene expression in eukaryotes and prokaryotes is that in eukaryotes transcription and translation are spatially and temporally separated. mRNA transcripts produced in the nucleus must make it out of the nucleus to the ribosomes in the cytoplasm before translation. This affords an opportunity for regulation at the mRNA level, where the mRNA can be destroyed, differentially spliced, bound by proteins that alter its interaction with ribosomes, and so forth, prior to getting to the ribosomes, and thus altering the ultimate output of translation.

In prokaryotes, transcription and translation mainly takes place simultaneously. The ribosomes frequently start translating an mRNA sequence into protein even before the whole rRNA has been completely transcribed from the DNA. Thus gene expression cannot be regulated as much at the mRNA level, and the primary regulation of gene expression occurs at the DNA level.

Q. 13.

A point mutation is a type of mutation in DNA or RNA or the genetic material, in which one single nucleotide base is added, deleted or modified.

Types of Point Mutations

a. Substitution: A substitution mutation occurs when one base pair is substituted in place of another. Subtypes of Substitution mutation are Nonsense, Misense and Silent.

b. Insertion and Deletion: An insertion mutation occurs when an extra base pair is inserted to a sequence of bases. A deletion mutation is the opposite to insertion, and occurs when a base pair is deleted from a sequence of bases.

Introns are the major regions involved in the regulation of expression and promoter is a region of DNA that facilitates transcription of a particular gene when a transcription factor binds to it.

The promoter sequence contains the binding patterns for the polymerase and for additional transcription factors or repressors. Undoubtedly, when these patterns are affected also the transcriptional activity will be affected.

The introns may contain sequences that bind additional transcriptional enhancers or silencers, so mutations in these regions can also have an affect on the transcription.

Q. 14.

Cyclin-dependent kinase are family of protein serine/theonine kinase whose activity depends on association with a non catalytic regulatory subunit called a cyclin. They are involved in regulation of transcription, mRNA processing and differentiation of nerve cells.

Phase of cell cycle, cyclins and CDks

Phase	Cyclin	CDK
G0	C	CDK3
G1	D, E	CDK4, CDK2, CDK6
S	A,E	CDK2
G2	A	CDK2, CDK1
M	B	CDK1

Q. 15.

a: Examples of genes associated with genetic traits that fit Pleiotropy are phenylketonuria (inherited disorder that affects the level of phenylalanine in the body), albinism, sickle cell anemia.

b: Examples of genes associated with genetic traits that fit Codominance are Horse color, Flower colors, AB Blood Type.

c: Examples of genes associated with genetic traits that fit Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants.

d: Examples of genes associated with genetic traits that fit Epistasy are gene for total baldness is epistatic to the gene for brown hair, height, pigmentation.