Semi-conservative DNA Replication

Semi-conservative DNA Replication – Online Science Notes

Source: https://old-ib.bioninja.com.au/standard-level/topic-2-molecular-biology/27-dna-replication-transcri/semi-conservative.html

Introduction

DNA is only one type of genetic information that is carried by humans and other living things. The bases of the DNA known as adenine, guanine, cytosine, and thymine have been used to store the information, which has been encoded as code. This is the blueprint that aids in defining the characteristics of the organism. DNA aids in the storage of the information that 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 carry out the majority 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 referred to as DNA’s B-form, A-form, and Z-form. DNA serves three distinct purposes, including those related to immunology, structure, and genetics.

During cell division, a process known as DNA replication occurs in which DNA copies itself or duplicates itself. This process of DNA replication is semiconservative. Initiation, elongation, and termination are all parts of the DNA replication process. It begins with the identification of the replication site, proceeds through a number of 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 aids in the genetic code being transferred for the purposes of growth, functioning, and development. It also aids in the reproduction of living cells by preserving their original genetic makeup. This kind 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 of the original DNA strands; the other one is made up of two new DNA strands (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 parent’s DNA.

Semiconservative DNA Replication

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

Experiment demonstrating semi-conservative DNA replication

Matthew Meselson Stahl performed the following experiment in 1958.

For many generations, they raised E. coli in a medium with NH₄Cl as the only source of nitrogen (N15 is the heavy isotope of nitrogen).
The cells were then transferred into a medium containing regular 14NH₄Cl, samples were taken as the cells multiplied, and the DNA that was still present as double stranded helices was extracted.
Because E. coli divides 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: The double stranded DNA is helped to unwind by the DNA helicase.
Primase Enzyme: Addition of a short RNA primer
DNA Polymerase Enzyme : Prokaryotes
DNA POL I : Remove primers and fills the gap
DNA POL II : DNA repair
DNA POL III; primary enzyme for DNA synthesis
DNA Polymerase Enzyme : Eukaryotes
DNA POL alpha : Initiate replication by synthesizing RNA primers
DNA POL beta : Repairing (DNA POL II)
DNA POL gama : Polymerization in mitochondria
DNA POL delta : Main enzyme for DNA synthesis (DNA POL III)
DNA POL ephsylan: Proofreading & Filling gap
Single stranded binding Proteins(SSB) : SSB checks the winding of DNA strands.
Exonucleases Enzyme FEN1 Enzyme and RNase H Enzyme: The initial RNA nucleotides formed are removed
Enzyme Ligase: Make double-stranded DNA by combining the two independent DNA strands into one.
Topoisomerase/Gyrases: prevents supercoiling of DNA

Mechanism of semiconservative DNA replication

There are three steps in the semiconservative DNA replication process.

Initiation
Elongation
Termination

1. Initiation

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

2. Elongation

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 that is produced by another enzyme called Primase. The leading strand is the direction of the strand from 3′ to 5′. 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 when adding nucleotides. The DNA polymerase can only supplement an existing strand; it is unable to create or synthesize 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, new strand synthesis must stop. The DNA polymerase stops nucleotide addition when the termination point is reached. The DNA polymerase is unable to link them together by forging a bond between them. By introducing a phospho-di-ester bond into the DNA molecule, the enzyme ligase joins the strands together.

References:

Lambda Geeks. (2022, February 18). lambdageeks.com. https://lambdageeks.com/semiconservative-dna-replication-process/
Royal Pitch. royalpitch.com, 24 Apr. 2022, https://royalpitch.com/sickle-cell-anemia-is-an-example-of-codominance/.
Study.Com. study.com, https://study.com/learn/lesson/codominance-examples-in-humans.html. Accessed 19 Aug. 2022.
DNA REPLICATION Semi Conservative Model. (n.d.).
Hanawalt, P. C. (2004). Density matters: The semiconservative replication of DNA. 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, 13 Feb. 2022, https://lambdageeks.com/what-is-conservative-dna-replication/.
Categorization of Conservative, Semi-Conservative, and Dispersive DNA Replication Theories (1953–1956) | The Embryo Project Encyclopedia. embryo.asu.edu, 31 Oct. 2019, https://embryo.asu.edu/pages/categorization-conservative-semi-conservative-and-dispersive-dna-replication-theories-1953.