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Sense Strand and Antisense Strand – What is the Difference?

Sense Strand and Antisense Strand – What is the Difference?

Do you guys get confused between the terms sense and antisense strand or what is coding and non-coding strand, which is plus and minus strand? Is it the template or non-template strand? Well, then get ready to clear your doubts and have crystal clear concept about these terms.

Let’s start with template strand and non-template strand because I am sure we are all familiar with these two terms for sure. DNA is double stranded which has one strand running in 5’->3’ direction and other one in 3’->5’ direction.

  • Template strand and Non-template strand –

Template strand is the strand of DNA which serves as template to give the mRNA which means it is the strand that is going to get transcribed to mRNA. Now RNA polymerase which synthesizes mRNA can read the DNA in 3’->5’ direction (newly synthesized mRNA is in 5’->3’ direction) so this means the 3’->5’ direction DNA strand is the template strand. If 3’->5’ direction is template strand clearly the opposite strand i.e. 5’->3’ direction is non-template strand.

  • Sense strand and Antisense strand –

Now look at the newly formed mRNA (5’->3’ direction), it is the copy of non-template strand (because it is transcribed from the template strand which is complementary to non-template strand). The only difference would be mRNA would have uracil instead of thymine. So by looking at non-template strand we can actually predict what would be the expected mRNA and thus the 5’->3’ direction strand is called sense strand. This is one point I use to get confused. Sense doesn’t mean it is going to get transcribed but it just means it makes sense, when we read it we know the expected mRNA J and of course the other strand (3’->5’ direction) is called antisense strand.

  • Coding strand and Non-coding strand –

When we read the 5’->3’ direction strand or which is also called sense or non-template strand as we saw above, it gives us the expected codon sequence we would get in mRNA right? That means we can predict all the codons from the 5’->3’ direction strand and because of this it is called coding strand and the other strand is called non-coding strand (because it doesn’t show us codon sequence).

  • Plus strand and Minus strand –

When we read mRNA (5’->3’ direction), it is going to be the same as codon strand (5’->3’ direction) and because they both are running in same direction having the same nucleotides (only difference is of uracil in mRNA instead of thymine) it is called the plus strand. The other strand is called minus strand.

I hope this helped 🙂

Watch this video to understand this topic more in details.

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Antibody – Structure Overview

Antibody – Structure Overview

In this post we will discuss the structure of antibody. We will try and understand all the terms related to antibody structure and why are they named so. Let’s begin 🙂

Antibody = is a globular protein which is synthesized by plasma B-cells in respond to foreign antigen.

Because antibody is globular protein with immune function, it is also known as Immunoglobulin.

So when we talk about the structure of antibody, this Y looking figure (figure a) comes in our mind. This figure (a) is the typical structure of an antibody. Now we will see each of the labels one by one and see why are they named so.

Figure a – Structure of Antibody

  1. Heavy chain and Light chain – antibody is made up of two heavy chains and two light chains.Why are they named so – because of their molecular weight. That means heavy chain is more in molecular weight than light chains. Each heavy chain weighs 50 kD (kilodalton) whereas each light chain weighs 25 kD (kilodalton).Therefore, molecular weight of one single molecule of antibody = 150 kD (50 kD + 50 kD + 25 kD + 25 kD, because as mentioned before each antibody is made up of two heavy chains and two light chains). This is for one single molecule of antibody which means if you talk about IgA or IgM, molecular weight will be more because IgA is a dimer and IgM is pentamer.Types of heavy chains are µ, γ, α, δ and ε and light chains are κ and λ. Any of these heavy and light chains will be present in different types of antibody. The class of antibody is determined based on type of heavy chain present in it.That means heavy chain µ gives IgM, heavy chain γ gives IgG, α gives IgA, δ gives IgD and ε gives IgE. Let me tell you guys how to easily remember which heavy chain gives what antibody. Just look at the first letter of each heavy chain and there is the answer.Therefore, µ – Mu –IgM,γ – Gamma – IgG,α – Alpha – IgA,δ – Delta – IgD,ε – Epsilon – IgE
  2. Variable and Constant region –
    As you can see in figure (a) the top most subunits in heavy as well light chains are labeled as VH and VL respectively.
    Why are they named so – because of the high variability of amino acid sequence present in these regions. Therefore, VH is variable heavy chain and VL is variable light chain. Similarly, other subunits are labeled as CH and CL meaning constant heavy chain and constant light chain because there is no much change in the amino acid sequence.
  3. Antigen Binding site –
    It is present in the variable region. It is the specific amino acid sequence present in the variable region which can recognize the antigen and are called Complementarity Determining Region (CDR). CDR is also called Paratope. You must have read in immunology books that paratope of antibody binds to an epitope of antigen (epitope is part of antigen which can be recognized by paratope).
  4. Glycosylation site –
    There is no clear reason known for sites of glycosylation in antibody but one of the possible reasons says that as we know antibody is protein compound and it might contain hydrophobic amino acids which decreases its solubility in blood plasma. And we know that glycosylation increases the protein solubility so antibody solubility increases in blood plasma.
  5. Fab and Fc region
    Why are they named so – when the antibody structure was studied, it was digested by different enzymes which digested the antibody near hinge region giving two subunits. Upper subunit was named as Fab – Fragment antigen binding because this was the portion of antibody which binds to an antigen. Lower subunit was named as Fc – Fragment crystalizable and it is named so because when this fragment was stored it started crystalizing.
  6. Hinge region
    It gives the flexibility to antibody so that it can bind with an antigen in the best way possible.

So, these are the points we should know when we talk about the structure of antibody.

I hope this blog was helpful 🙂

 
Watch a video on this topic here.