Is DNA Replication Antiparallel? 5 Facts Beginners Should Know


The replication of DNA is the process by which an organism copies its genetic material to pass it from one generation to another.

Yes, DNA replication occurs in an antiparallel manner. DNA replication is characterized by 2 points- its semi-conservative nature and the antiparallel direction.

If asked is DNA replication is antiparallel? the answer would be that the 2 strands in the DNA  helix are antiparallel to each other, where one strand goes from 5’ to 3’ while the opposite goes from 3’ to 5’.

How is DNA replication antiparallel?

DNA replication is itself not parallel but designated so.

The two strands of the DNA double helix run in antiparallel directions, hence to form a double strand each strand is also replicated in an antiparallel fashion.

is-dna-replication antiparallel
Representation of the replication fork showing the antiparallel strands
Image: Wikipedia

A DNA molecule is composed of 2 strands running in opposite directions. They are joined together by the Hydrogen bonds between the complimentary Nitrogenous bases in the nucleotides.

Adenine compliments Thymine, while Cytosine compliments Guanine. The two strands are represented as one 3’ to 5’ and the opposite one running from 5’ to 3’. Both the strands are replicated individually thereby conserving one strand in each new DNA double helix from the original DNA.

How does antiparallel affect replication?

The only reason why replication can occur is that the two DNA strands are antiparallel.

The two strands are arranged in an antiparallel fashion which ensures that the complementary base pairs can bind to each other in the correct configuration.

Because the DNA strands run in opposite directions, the replication fork can designate them as leading and lagging strands. Since the leading strand produces a continuous complement, the lagging strand is complemented by the Okazaki fragments.

How the DNA helix is unzipped and replicated
Image: Wikipedia

This helps to achieve the semi-conservative nature of DNA replication. Without the above measure, the two complementary strands could intertwine or bind with each other.

Semi-conservative DNA replication means that in the daughter DNA helices one of the strands comes from the mother helix, while the other one is made by the process of replication.

How is an antiparallel strand replicated?

DNA strands are replicated by the formation of a replication fork and using enzymes.

The two strands get unwounded and separated to form the replication fork after which the two strands are individually replicated. After this process, the daughter DNA double strands again wind themselves up.

Steps by which DNA polymerase makes a new complimentary strand
Image: Wikipedia

Step 1:

A total of four enzymes are involved with the antiparallel replication of DNA- helicase, primase, DNA polymerase and ligase. Since Hydrogen bonds bind the 2 strands of the DNA together the first step is to unwind the twist and open the two chains. This function is performed by the enzyme helicase. Small proteins called SBBs or Single-Strand Binding proteins attach to the single strands to stop the work done by the helicase.

Step 2:

Once the 2 strands are separated the Nitrogen bases are exposed to the environment and are free, hence the DNA polymerase can come and attach to them and start building a new DNA strand with a complimentary strand which is also antiparallel in the case of the leading strand which has the 3’ end.

Replication complex showing the enzymes involved and Okazaki fragments
Image: Wikipedia

Step 3:

The other strand with the 5’ end at the replication fork is called the laggings strand, and the complementary strand for this stand is not continuous but rather fragmented. These pieces of di-nucleotide chains are called Okazaki fragments.

Step 4:

Once DNA polymerase finishes the replication process, the enzyme Ligase comes and joins the Okazaki fragments to form a continuous di-nucleotide chain. After the two strands are completed the enzyme again joins and twists the 2 pairs of DNA strands to form 2 DNA helices.

What would happen if DNA strands were parallel?

If the DNA strands were parallel, the process of replication would not be possible.

The two DNA strands have complementary nitrogenous bases, which allows them to bind to each other via hydrogen bonds and form the ladder structure. This is not possible if the 2 are parallel.

This is because the two nitrogen bases can only bind if one is facing the right way, and the other is mirrored or rotated. In parallel strands this would not be possible, meaning it would not be possible for the complementary Nitrogenous bases to bind to each other.

This means that the DNA helix itself would not exist, so there is no way that it can be unwound or replicated.

Conclusion:

DNA replication is semi-conservative and antiparallel. Without the antiparallel nature of the individual DNA strands, neither the DNA helix nor replication of the DNA helix would be possible.

Trisha Dey

I am Trisha Dey, a postgraduate in Bioinformatics. I pursued my graduate degree in Biochemistry. I love reading .I also have a passion for learning new languages. Let’s connect through linked in: https://www.linkedin.com/in/trisha-dey-183482199

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