Gene Expression

Question 1

Discuss why every cell does not express every gene.

Answer 1

Every somatic cell has the same genetic content, but gene regulation controls which genes are expressed. This is what dictates whether a cell becomes an eye cell or a lung cell.

It is the different gene expression patterns that give rise to complex organisms. In other words, the key to multicellularity is the specialization of cells due to gene regulation.

Question 2

Describe the components of a prokaryotic operon and how each contributes to gene regulation.

Answer 2

-promoter- RNA polymerase binds to this

-operator- Regulator binds to this, can block RNA polymerase

-structural genes- Genes coding for something but not involved in the regulation process

-regulator gene: This is separate and has its own promoter. It encodes for the regulator protein than affects gene regulation.

Question 3

Compare and contract positive and negative regulators.

Answer 3

Positive regulator protein is an activator and triggers a higher level of expression of an operon through transcription.

Negative regulator protein is a repressor and prevents the expression of operon through transcription.

Question 4

Compare and contrast inducible and repressible gene regulation.

Answer 4

Inducible System:
Transcription goes from low to high.

Repressible System:
Transcription goes from high to low.

Question 5

Describe the processing of eukaryotic pre-mRNA.

Answer 5

1: Addition of 5’ methyl cap
-Protects mRNA from degradation
-Involved in the initiation of translation

2: Addition of 3’ poly-A tail
-Created by poly-A polymerase
-Protects mRNA from degradation
-Part of the sequence gets recognized and leads to cleavage site that enzymes can recognize
-Polyadenylation takes place at 3’ end
-This poly-A tail (consisting of adenine containing nucleotides) does not encode for anything so it doesn’t matter if it degrades

3: Removal on noncoding sequences (introns)
-Pre-mRNA splicing done by spliceosomes.
-RNA splicing removes introns and joins exons.

Question 6

Recognize the effects of alternative splicing.

Answer 6

-A single primary transcript can be spliced into different mRNAs by different combinations of exons. This is why there are more mRNA sequences than gene sequences.

-15% of human genetic disorders are due to altered splicing. BUT 35-59% of human genes exhibit some form of altered splicing.

Question 7

Relate post-translational modification to gene regulation.

Answer 7

Proteins are modified in several ways after they are released from the ribosome.

-cleavage or trimming of the protein
-attachment of carbohydrates (such as glycoproteins)
-phosphorylation of serines or tyrosines
-complexing with metals

Question 8

Exons

Answer 8

The parts of the code that actually get translated into protein sequences are called exons. They are eventually expressed.

Question 9

RNA Splicing

Answer 9

RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence. Thus, the mature mRNA contains less material than the DNA; It contains exons and some untranslated 3’ and 5’ regions, and the stop codons, but no introns. Therefore, mRNA is an expression of the DNA information that is relevant to making protein.

Question 10

Spliceosomes

Answer 10

In some cases, RNA splicing is carried out by spliceosomes.

Spliceosomes consist of a variety of proteins including a protein called small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites.

Question 11

Operon

Answer 11

A group of prokaryotic genes with a single promoter (transcribed as a single mRNA). The genes in an operon encode proteins that all function in a given process.

-promoter
-operator
-structural genes

Ex: the lac operon

Question 12

What could cause a dominant gene to not be expressed?

Answer 12

If the c gene is epistatic to a, the c gene can prevent the dominant a gene from being expressed

Question 13

Function of regulator gene and regulator protein

Answer 13

Regulator gene encodes for the regulator protein.

Regulator protein binds DNA at the operator.

Question 14

In a negative repressible operon, the
regulator protein is synthesized as an

Answer 14

inactive repressor

Question 15

Co-activator

Answer 15

Positive inducible (0 to 1):

Molecule (substrate) that interacts with the activator to make it functional (allowing transcription)

Question 16

Co-repressor

Answer 16

Negative repressible (1 to 0)

Molecule (product) interacts with the activator to make it functional (preventing transcription)

Question 17

In the presence of allolactose (the inducer),
the regulator protein known as the lac
repressor

Answer 17

cannot bind to the operator

Question 18

Given that the the trp operon is a negative
repressible operon, what happens to the trp
repressor in the absence of the tryptophan
co-repressor?

Answer 18

It cannot bind to the operator and transcription takes place.

Question 19

What is the central dogma of biology and what are the different levels of gene regulation during this process?

Answer 19

The central dogma of biology is that genetic information flows from DNA to RNA to protein through transcription and translation.

-Epigenetics (alteration of structure that can prevent transcription from occurring)

-Transcriptional Regulation (in prokaryotes)

-mRNA Processing (in eukaryotes)

-Post Translational Modification

Question 20

Why can eukaryotes regulate gene expression more closely than prokaryotes?

Answer 20

Eukaryotes can regulate gene expression more closely than prokaryotes because they can regulate transcription and translation separately since transcription occurs in the nucleus and translation in the cytoplasm.

Question 21

Describe the process of epigenetics

Answer 21

Alteration of DNA structure that can prevent transcription from occurring.

Nucleosomes consist of DNA coiled tightly around histone proteins. The positive histone tails react with the negatively charged DNA phosphate group to prevent transcription.

Acetylation of the tails can weaken their interaction and help some transcription factors to occur.

Histone components of a nucleosome are dynamic (changing) but can be inherited.

Question 22

Heterochromatin

Answer 22

Tightly packaged chromatin that does not allow access to the DNA sequence.

Question 23

Euchromatin

Answer 23

Loosely packaged chromatin that allows spaces for transcriptional machinery.

Question 24

Positive Repressible

Answer 24

-Activator regulator protein can trigger higher expression of operon.

-Since it’s repressible, it starts at some level of transcription and turns off.

-Feedback Repression: Activator will be turned off by product and become nonfunctional.

Question 25

Positive Inducible

Answer 25

-Activator regulator protein can trigger a higher expression of operon.

-Since it’s inducible though, it starts off with a nonfunctional gene product, and then turns on.

-It needs a co-activator to turn it on.

Question 26

Negative Repressible

Answer 26

-The repressor regulator protein can prevent expression of the operon.

-HOWEVER, since it is repressible, transcription goes from high to low.

-That means that the protein must start off as nonfunctional and be turned on to repress the system.

-The product produced from the operon is a co-repressor that can make the regulator protein nonfunctional.

Ex: trp operon
Regulator protein can’t bind to operon without the co-repressor.

Question 27

Negative Inducible

Answer 27

-The repressor regulator protein prevents expression of the operon. (Think of it as creating a roadblock on the operator so that the RNA polymerase is stuck on the promoter.)

-Since it’s inducible, however, the a substrate could make the repressor inactive.

Ex: The lac operon.
When lactose binds as allolactose to the repressor protein, it inactivates the repressor protein unblocking the operator. This is called substrate induction. It makes sense, because the genes for breaking down lactose would be unnecessary if there wasn’t any lactose. But when they are necessary, substrate induction will always be possible.

Question 28

Exons

Answer 28

The parts of the genetic code that actually get translated into protein sequences.

Question 29

Introns/Intervening sequences

Answer 29

Genes often have long “noncoding” stretches of nucleotides called intervening sequences.