A phospholipid segment Hydrophilic head, hydrophobic tail Watson, The Cell

Classes of proteins bound within the lipid bilayer, Watson, the Cell

The DNA double helix contains the information for protein manufacture The four base pairs are made up of : Adenine, (A) Cytosine, (C) Guanine, (G) Thymine, (T)

The base units are always paired in the DNA double helix Adenine always pairs with thymine: A -T or T- A Cytosine always pair with guanine: C - G or G - C

Directional specificity to the DNA stepladder Both DNA and RNA have a directional specificity. At one end of the chain the free phosphate group is attached to the 5’ atom of the first sugar, at the other end to the 3’ atom of the last sugar has no phosphate group. The two strands are antiparallel, they run in opposite directions.

The Double helix The base pairs make the steps of a model organic ladder of the double helix holding the two twist together, while the ribose sugar makes up the outside hand-rails of our model organic ladder.

The double helix step-ladder

Central Dogma of molecular biology Aidley & Stanfield, Ion Channels

The role of mRNA RNA acts as an intermediary between DNA and the protein. RNA codes the complementary base pair of one DNA strand with uracil (U) pairing with adenine in place of thymine. A, C and G pairing with T, G and C.

The central dogma the idea of a genetic code A set of rules that relates groups of bases on mRNA to amino acids which determine the protein. The Rule: three sequential bases (forming a codon) code for each amino acid. There are 64 possible combinations, 4 3. Three triplets are known as stop codons.

mRNA is read sequentially from the 5’ end. Each successive amino acid is added to the developing protein chain. The α-amino group combines with the α- carboxyl group of the previous member to form the peptide linking of the two. The first member of the chain has a free amino acid, the N terminus, and the last has a free carboxyl group, the C terminus.

Not all section of the mRNA are the same. There are transcription control regions as well as the part that forms the primary RNA molecule. The sequence making up a gene contains a number of sections called exons, which encode parts of the protein sequence separated by introns (intervening sequences) which do not.

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Voltage gated channel

Potassium voltage gated channel

The model assume that channels are water filled pores through which ions can pass. Effective size of K + channel is dependent on the hydrated size of the ion.

Hydrated K + is smaller than hydrated Na + While K + is a larger ion than Na +, its ability to hold on to its water cloud is less than that of Na+ making Na + a bigger ion an unable to pass through a K + channel.

Channel selective filter for Na + Hillel’s thesis: Na+ ions bind transiently at an active site as it moves through the channel.

Hillel’s model of a Na + voltage gated channel

Amino acid residues in the wall are active sites for stabilization of the positive charges ion At the binding site the positive charge of the ion is stabilized by a hydrophilic negatively charged amino acid residue lining the channel wall and by a water molecule that is attracted to a second amino acid residue lining the other side of the channel.

K+ cannot move through a Na + channel even though it is a smaller ion because it can not be stabilized by the negative charge of the filter.

Chemical vs electrical synapse

Electrical synapse vis-a-vi Chemical synapse

The Gap junction

Passive channels There are passive channels that are selective to only one species of ion. The ion of interest is K+. These channels are not effected by changing voltages across the membrane. They are always open and are the basis for the resting membrane potential

Active channels Active channels allow ion to flow through the channel dependent upon the state of some physical attribute of the channel, Physical attribute: –Voltage –Ligand –Stretch –2 nd messenger

Active channel can be open, open but deactivated, blocked or closed

K+ voltages-gated channel

4different types of stimuli that opens gated channels

Note here the refractory (deactivated) state

Exogenous ligands (drugs) can bias an ion channel to either open or close

When ion channels open, ions move down there concentration gradient When ions move down their concentration gradients, electrical current flows in the direction of the ion movement. The magnitude of the current and the rate of movement of the current follows a know law (to be discussed next week).