Gene expression and histone methyltransferase
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Gene expression and histone methyltransferase
Array
(
[passage] => WP_Post Object
(
[ID] => 560288
[post_author] => 12815
[post_date] => 2025-01-12 10:09:59
[post_date_gmt] => 2025-01-12 15:09:59
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Genetic inheritance extends beyond the simple transmission of nucleic acid sequences from parent to offspring, encompassing epigenetic regulation through histone structure. One crucial aspect of this epigenetic regulation is the post-translational modification of histone proteins, which can modulate chromatin architecture and, consequently, gene expression. These modifications include methylation, acetylation, phosphorylation, ubiquitination, and other modifications.
Histone methyltransferases (HMTs) are a class of enzymes that catalyze the transfer of methyl groups to specific residues on histone proteins, predominantly arginine and lysine residues, which contain amine groups on their side chains. The primary cofactor for HMTs is S-adenosylmethionine (SAM), which donates methyl groups in these reactions. The addition of one, two, or three methyl groups (mono-, di-, or tri-methylation) to these residues can have distinct effects on chromatin structure and function, leading to either the activation or repression of gene transcription.
For instance, methylation of lysine 4 on histone H3 (H3K4) is generally associated with transcriptional activation, while methylation of lysine 27 on histone H3 (H3K27) is linked to transcriptional repression. The specific outcome of histone methylation is determined by the context of other histone modifications and the recruitment of additional regulatory proteins, such as transcription factors, chromatin remodelers, and histone code readers (e.g., bromodomains and chromodomains).
Histone methylation modulates chromatin structure in several ways:- - Direct Structural Changes: The addition of methyl groups can directly alter the physical properties of histone tails, affecting their interactions with DNA and other histones, thus impacting nucleosome stability and positioning.
- - Recruitment of Effector Proteins: Methylated histones serve as binding sites for effector proteins that possess methyl-binding domains. These proteins can further recruit chromatin remodeling complexes, histone acetyltransferases (HATs), or histone deacetylases (HDACs), leading to changes in chromatin accessibility and gene expression.
- - Polycomb and Trithorax Complexes: Histone methylation plays a critical role in the recruitment of Polycomb and Trithorax group proteins, which are involved in maintaining repressive and active chromatin states, respectively, across cell divisions, thereby contributing to cellular memory and identity.
The HMT-catalyzed reaction creates which functional group in a lysine residue on histone proteins?
[value] => Array ( [answer] => 1 [description] =>Reason for the Correct Answer:
Histone Methyltransferases (HMTs) catalyze the transfer of methyl groups to the nitrogen atoms in the side chains of lysine residues on histone proteins.
This process results in the formation of methylated lysine, which can exist as mono-, di-, or trimethylated forms, as in the reaction shown here.
These modifications effectively create a more substituted amine group, where the nitrogen atom is bonded to additional methyl groups. This methylation alters the chemical properties of the lysine residues and affects chromatin structure and gene expression.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. A more substituted amine
) [1] => Array ( [each_answer] =>B. A hydroxyl group
) [2] => Array ( [each_answer] =>C. An acetyl group
) [3] => Array ( [each_answer] =>D. An amide group
) ) ) [1] => Array ( [quiz_unique_key] => 3873426850 [question] =>How would the rate of a reaction catalyzed by histone methyltransferase change if you were to increase the concentration of SAM?
[value] => Array ( [answer] => 2 [description] =>Reason for the Correct Answer:
Histone methyltransferases (HMTs) catalyze the transfer of a methyl group from SAM to specific amino acids on histone proteins. SAM acts as a cofactor that donates a methyl group to the substrate (histone proteins) during the methylation reaction.
SAM is a reactant (not a product) in the reaction.
Increasing the concentration of a reactant increases the rate of the reaction.
The higher the rate of a reaction, the closer it is to Vmax.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. The rate of the reaction would decrease, bringing the reaction velocity further from Vmax.
) [1] => Array ( [each_answer] =>B. The rate of the reaction would increase, bringing the reaction velocity closer to Vmax.
) [2] => Array ( [each_answer] =>C. The rate of the reaction would increase, bringing the reaction velocity further from Vmax.
) [3] => Array ( [each_answer] =>D. The rate of the reaction would decrease, bringing the reaction velocity closer to Vmax.
) ) ) [2] => Array ( [quiz_unique_key] => 83407773 [question] =>Is it possible for histone proteins to be methylated in the absence of HMTs?
[value] => Array ( [answer] => 3 [description] =>Reason for the Correct Answer:
Reactions will occur as long as they are energetically favorable.
Reactions require substrates but do not require enzymes to occur.
The reaction can occur without the enzyme, however it will proceed much more slowly due to a higher activation energy.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. No, all biochemical reactions require an enzyme in order to proceed.
) [1] => Array ( [each_answer] =>B. Yes, only the molecule SAM is needed to catalyze the reaction.
) [2] => Array ( [each_answer] =>C. Yes, though the reaction would occur at a much slower rate.
) [3] => Array ( [each_answer] =>D. No, SAM cannot transfer a methyl group to a histone without the appropriate enzyme.
) ) ) [3] => Array ( [quiz_unique_key] => 2377279144 [question] =>If histone X were to be methylated, the expression of gene Y would be increased. If gene Y codes for protein Z, what would happen to the levels of protein Z if the concentration of SAM were to decrease?
[value] => Array ( [answer] => 2 [description] =>Reason for the Correct Answer:
Decreasing the concentration of SAM will decrease the amount that histone X will be methylated.
The methylation of histone X leads to increased expression of gene Y and synthesis of protein Z.
With a lower concentration of SAM, histone X will be less methylated, leading to a decrease in expression of gene Y and synthesis of protein Z.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. The concentration of protein Z would increase, as the expression of gene Y is increasing.
) [1] => Array ( [each_answer] =>B. The concentration of protein Z would decrease, as the expression of gene Y is decreasing.
) [2] => Array ( [each_answer] =>C. The concentration of protein Z would increase, as the expression of gene Y is decreasing.
) [3] => Array ( [each_answer] =>D. The concentration of protein Z would decrease, as the expression of gene Y is increasing.
) ) ) [4] => Array ( [quiz_unique_key] => 2261298308 [question] =>If histone X were to be methylated, the expression of gene Y would be increased. If gene Y codes for protein Z, what would happen to the levels of protein Z if the expression of histone methyltransferases were increased?
[value] => Array ( [answer] => 1 [description] =>Reason for the Correct Answer:
Increasing the expression of histone methyltransferase will increase the amount that histone X will be methylated.
The methylation of histone X will lead to increase expression of gene Y and synthesis of protein Z.
With a higher expression of histone methyltransferase, histone X will be more methylated, leading to an increase in expression of gene Y and synthesis of protein Z.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. The concentration of protein Z would increase, as the expression of gene Y is increasing.
) [1] => Array ( [each_answer] =>B. The concentration of protein Z would increase, as the expression of gene Y is decreasing.
) [2] => Array ( [each_answer] =>C. The concentration of protein Z would decrease, as the expression of gene Y is increasing.
) [3] => Array ( [each_answer] =>D. The concentration of protein Z would decrease, as the expression of gene Y is decreasing.
) ) ) [5] => Array ( [quiz_unique_key] => 2495868854 [question] =>Why does the recruitment of HATs usually activate genes?
[value] => Array ( [answer] => 3 [description] =>Reason for the Correct Answer:
HATs are histone acetyltransferases, as discussed in the passage.
All the answer options suggest that acetylation modifies lysine residues, which is true.
Lysine normally has a positive charge, which allows it to interact strongly with the negatively charged phosphate backbone of DNA.
When acetyl groups are added, this positive charge is neutralized, reducing the interaction between histones and DNA. This weakening of the histone-DNA interaction results in a more relaxed chromatin structure, making the DNA more accessible to the transcriptional machinery, thereby activating gene expression.
A. The addition of acetyl groups to lysine residues increases their positive charge, enhancing the interaction between histones and DNA, which loosens the chromatin structure.
) [1] => Array ( [each_answer] =>B. The addition of acetyl groups to lysine residues creates binding sites for repressors that compact the chromatin structure.
) [2] => Array ( [each_answer] =>C. The addition of acetyl groups to lysine residues neutralizes their positive charge, reducing the affinity of histones for DNA.
) [3] => Array ( [each_answer] =>D. The addition of acetyl groups to lysine residues leads to the phosphorylation of histones, which directly activates transcription.
) ) ) ) [total_question] => 6 [correct_answers] => Array ( [560288|1] => A [560288|2] => B [560288|3] => C [560288|4] => B [560288|5] => A [560288|6] => C ) [hide_display_feedback_settings] => [hide_solutions] => )