Semiconservative DNA Replication: Scientific Notes Online

Introduction

DNA is just one type of genetic information carried by humans and other living beings. The DNA bases known as adenine, guanine, cytosine and thymine have been used to store information, which has been encoded as a code. This is the model that helps define the characteristics of the organism. DNA helps store the information an organism needs to survive, grow, develop and reproduce. The DNA sequence must be translated into a message that will be used to create proteins, which are much more complex molecules that perform most of the body’s functions, in order to carry out all of these functions. There are three different types of DNA, and they are all double-stranded and connected by interactions between complementary base pairs. They are known as B form, A form and Z form of DNA. DNA has three distinct purposes, including those related to immunology, structure, and genetics.

During cell division, a process known as DNA replication It occurs in which DNA is copied or duplicated. This DNA replication process is semiconservative. Initiation, elongation and termination are parts of the DNA replication process. It begins with the identification of the replication site, continues through several steps, and ends with the joining of DNA fragment molecules. By joining the two separate DNA strands into one, double-stranded DNA is created. DNA helps the genetic code to be transferred for the purposes of growth, function and development. It also helps in the reproduction of living cells by preserving their original genetic makeup. This type of nucleic acid exists.

Modes of DNA replication

There are three possible modes of DNA replication.

1. Semiconservative: By breaking hydrogen bonds, the two strands of DNA molecules gradually separate during DNA replication.

2. Conservative: DNA produces two molecules: one that is identical to the original DNA molecule and contains both original DNA strands; the other is made up of two new strands of DNA (with exactly the same sequences as the original molecule).

3. Dispersive: DNA replication generates two DNA molecules that are “hybrids” or mixtures of the daughter’s and father’s DNA.

Semiconservative DNA replication

• “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible mechanism for copying genetic material.” (Watson and Crick, 1955)
• As the two strands separated, they would serve as a template for the synthesis of new complementary strands.
• Each DNA molecule would have a newly synthesized strand once replication is complete. Semiconservative DNA replication was the name given to this scheme.

Experiment demonstrating semiconservative DNA replication.

Matthew Meselson Stahl performed the following experiment in 1958.

• For many generations, they raised Escherichia coli in a medium with NH₄Cl as the only source of nitrogen (N15 is the heavy isotope of nitrogen).
• The cells were then transferred to medium containing regular 14NH.₄Cl samples were taken as the cells multiplied and DNA that was still present as double-stranded helices was extracted.
• Because Escherichia coli cleaves after 20 minutes, one generation after switching from N15 to 14N medium, the DNA extracted from the culture had a hybrid or intermediate density.

Enzymes involved

The semiconservative DNA replication process is carried out by the following enzymes:

• DNA helicase enzyme: DNA helicase helps unwind double-stranded DNA.
• Primase enzyme: addition of a short RNA primer
• DNA polymerase enzyme: prokaryotes
• DNA POL I: remove primers and fill the gap
• DNA POL II: DNA repair
• DNA POL III; primary enzyme for DNA synthesis
• DNA polymerase enzyme: eukaryotes
• DNA POL alpha: initiates replication by synthesizing RNA primers
• DNA POL beta: Repair (DNA POL II)
• DNA POL gamma: Polymerization in mitochondria
• DNA POL delta: Main enzyme for DNA synthesis (DNA POL III)
• DNA POL ephsylan: review and filling of gaps
• Single-strand binding proteins (SSB): SSB checks the coiling of DNA strands.
• Exonucleases Enzyme FEN1 Enzyme and RNase H Enzyme: The initial RNA nucleotides formed are removed
• Ligase enzyme: produces double-stranded DNA by combining the two independent DNA strands into one.
• Topoisomerase/Gyrases: prevents DNA supercoiling

Semiconservative DNA replication mechanism.

There are three steps in the semiconservative DNA replication process.

1. Initiation
2. Elongation
3. Termination

1. Initiation

This is the beginning or initial process of DNA replication. ORIC origins of replication are recognized by the enzyme helicase, which binds to the DNA strand and unwinds or separates the double-stranded DNA molecule. The result of the step is a partial separation of the two double-stranded DNA strands into two separate DNA strands. The replication fork is created, which looks like a fork but is actually a double strand because the other half of the strand is not participating at that point in the positive cycle.

2. Lengthening

Here, the primer modifies the template DNA strand by adding RNA nucleotides. The next enzyme recognizes and binds to a relatively short strand of RNA produced by another enzyme called Primase. The main strand is the 3′ to 5′ strand direction. The term “lagging strand” refers to the direction of the strand from 5′ to 3′. A particular enzyme called DNA polymerase adds the nucleotide that completes the template DNA by adding nucleotides. DNA polymerase can only complement an existing strand; It is incapable of creating or synthesizing new ones. Finally, the exonucleases FEN1 and RNase H will convert the RNA nucleotides into DNA nucleotides. As a result, thymine is added when adenine is present and vice versa for guanine and cytosine.

3. Termination

At some point, the synthesis of new chains must stop. DNA polymerase stops adding nucleotides when the termination point is reached. DNA polymerase cannot join them together by forging a bond between them. By introducing a phosphodiester bond into the DNA molecule, the enzyme ligase joins the strands.

References:

• Lambda Geeks. (2022, February 18). lambdageeks.com. https://lambdageeks.com/semiconservative-dna-replication-process/
• Royal Field. royalpitch.com, April 24, 2022, https://royalpitch.com/sickle-cell-anemia-is-an-example-of-codominance/.
• Studio.Com. Study.com, https://study.com/learn/lesson/codominance-examples-in-humans.html. Accessed August 19, 2022.
• DNA REPLICATION Semiconservative model. (North Dakota).
• Hanawalt, P. C. (2004). Density matters: semiconservative DNA replication. Proceedings of the National Academy of Sciences of the United States of America, 101(52), 17889–17894. https://doi.org/10.1073/pnas.0407539101
• Lambda Geeks. lambdageeks.com, February 13, 2022, https://lambdageeks.com/what-is-conservative-dna-replication/.
• Categorization of conservative, semiconservative and dispersive DNA replication theories (1953-1956) | The embryo project encyclopedia. embri.asu.edu, Oct. 31, 2019, https://embryo.asu.edu/pages/categorization-conservative-semi-conservative-and-dispersive-dna-replication-theories-1953.

Semiconservative DNA replication