Tutor profile: Alisha B.

Describe covalent, ionic, and hydrogen bonds. Which bond is the strongest and which is the weakest?

Covalent bonds occur when two non-metal atoms share electrons. An example of a covalent bond is when two hydrogen atoms bind with oxygen to form water, or H2O. A covalent bond is the strongest because it allows the outer shell of electrons on an atom to be filled. This makes the bond stable and thus it requires a lot of energy to break a covalent bond. An ionic bond occurs between two oppositely-charged ions, such as sodium (Na+) and chlorine (Cl-) which bond to form salt (NaCl). In this bond, the outer electron from the sodium atom is transferred to the chlorine atom so that the chlorine atom has a full outer electron shell. This bond is moderately strong falling between covalent and hydrogen bonds. Hydrogen bonds occur in molecules that have covalent bonds, such as water. For the example of water, the electrons are not shared equally between the hydrogen and the oxygen since the oxygen is more electronegative, meaning that it attracts the shared pairs of electrons more strongly than hydrogen. This causes the hydrogen to have a partly positive charge and the oxygen to have a partly negative charge within the water molecule. The negative charge of the oxygen from one water molecule can be attracted to the positive charge of the hydrogen of another water molecule forming a hydrogen bond. Hydrogen bonding is an intermolecular force as it allows for interactions between molecules and is the weakest bond of the three.

How do neurons communicate?

Neurons communicate through electrical and chemical signals. The electrical signal is called an action potential, which occurs when the voltage across the membrane changes due to ions flowing into or out of the neuron. Once the action potential reaches the axon terminal, the electrical signal becomes a chemical signal as the presynaptic neuron releases neurotransmitters into the synapse, which is the small space between two neurons. The neurotransmitters travel across the synaptic cleft to the dendrites of the postsynaptic neuron. When the neurotransmitters bind to the postsynaptic neuron, they can cause the neuron to be more or less likely to fire an action potential.

How does CRISPR/Cas9 work?

In order to target a genetic sequence in DNA, a guide RNA (gRNA) must be created. The gRNA binds to the Cas9 enzyme and has a short sequence which binds to the target sequence of the DNA in the genome. Once the gRNA is bound, the Cas9 enzyme cuts the DNA. Once the DNA is cut, endogenous cell machinery can be used to repair the DNA by adding or deleting base pairs or a customized DNA sequence could be inserted at the cut site.