Answer:
Most pre-mRNA molecules have sections that are removed from the molecule, called introns, and sections that are linked or together to make the final mRNA, called exons.
 
        
             
        
        
        
The movement of substances (as by diffusion) across a cell membrane without the expenditure of energy.
 
        
             
        
        
        
These abnormalities occur in the serotonin system which reduces and/or inhibits control in their movement. This disruption of the neurotransmitter causes them to act maladaptively. Thank you for your question. Please don't hesitate to ask in Brainly your queries. 
        
             
        
        
        
Answer:
20
Explanation:
Amino Acids can be arranged in different ways. The books say there are only 20 amino acids.
 
        
                    
             
        
        
        
The reactions of glycolysis are made up of ten steps, out of the ten step three are irreversible, this is because, these reactions have large negative free energies The three steps that are irreversible are:
1. Phosphorylation of glucose.
2. Phosphorylation of glucose 6 phosphate and 
3. Transfer of phosphate from phosphoenolpyruvate to ADP.