Answer: acetyl group, methyl group, or phosphate group
Explanation  Basically different modification can occur to  the tail regions of histone .This includes addition of the groups listed above 
 
        
                    
             
        
        
        
Answer: Light goes into the eye via the cornea, it then pass through the pupil, the lens, the vitreous humor and finally forms an image on the retina.
Explanation:
Light goes into the eye via the cornea. The cornea is a clear, dome-shaped surface that covers the front of the eye.
From the cornea, the light passes through the pupil. The pupil regulates the amount of light passing through.
From the pupil, , light hits the lens. The lens is the clear structure inside the eye. It focuses light rays onto the retina.
Subsequently, light passes through the vitreous humor. A clear, jelly-like substance that fills the center of the eye. It helps to keep the eye round in shape.
Finally, the light reaches the retina where the image is formed the image is usually inverted. The retina is a light-sensitive nerve layer that is situated at the back of the eye. 
The main function of the optic nerve is to carry the signals to the visual cortex of the brain. The visual cortex turns the signals into images.
 
        
             
        
        
        
When two lions compete for the same habitat, shelter, and food
        
             
        
        
        
Neurotransmitters are the chemical molecules, which help in transfer of the signals from one neuron to another. Inside the neurons, the signals are transferred as electrical signals, but at junction of two neuron, which is known as synapse, the signals are transferred in chemical forms. 
These neurotransmitters have a definite shape and are recognized by the receptors present in the receptor site of the succeeding neuron. The neurotransmitters from the synapse binds to the receptor site of the receiving neuron. binding of the neurotransmitter to the receptor causes excitation of the receiving neuron, which also known as postsynaptic neuron.
Hence, Like a key in a lock, the shape of the neurotransmitter must bind with the receptors of the receiving neuron.