Question

During which stage of interphase do cells copy their chromosomes?

Answer 1

Answer:

The S phase (Synthesis phase)

• DNA replication takes place during the S-phase of interphase, creating two identical sets of DNA. The DNA coils and condenses into units known as chromosomes during the start of mitosis. The two identical sister chromatids that make up each chromosome are joined by a centromere.

How does DNA replication work?

The biological process of creating two identical copies of DNA from a single original DNA molecule is known as DNA replication.

A double helix of two complementary strands makes up DNA. These strands are split apart in the replication process. The method of semi-conservative replication uses each strand of the original DNA molecule as a template to create its counterpart.

Several enzymes that carry out this process are given below, along with their activities.

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1. DNA Dependent DNA POLYMERASE is a class of enzymes that is responsible for all organisms' DNA replication processes. It accelerates DNA replication in the 5′ to 3′ range.
2. The enzyme known as TOPOISOMERASE is involved in the over- or under-winding of DNA.
3. HELICASE—Using the energy from ATP hydrolysis, which is characterized by the breakdown of hydrogen bonds between annealed nucleotide bases, helicases are frequently employed to separate the strands of a DNA double helix or a self-annealed RNA molecule.
4. DNA GYRASE, a particular kind of topoisomerase, relieves strain caused by DNA helicase's unwinding action.
5. DNA LIGASE- Joins the OKAZAKI FRAGMENT of the lagging strand and re-anneals the semi-conservative strands.
6. PRIMASE - Primase catalyzes the production of a short primer, which is a ssDNA template-specific piece of RNA (or DNA in other species).

REPLICATION FORK -(Attachment #1)

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REPLICATION PROCESS -

1. INITIATION - The beginning of DNA replication takes place at certain locations in the DNA molecule known as ori-sites or regions. Enzymes often recognize these locations and initiate the process. As opposed to C-G base pairs, which produce three hydrogen bonds, A-T base pairs only form two hydrogen bonds, making them easier to strand split. As a result, these sites are typically "AT-rich" (rich in adenine and thymine bases). Following the discovery of the origin, enzymes create a pre-replication complex that unzips the double-stranded DNA. The enzyme topoisomerase is involved in this process.
2. Elongation - The 5'-3' activity of the DNA polymerase enzyme. Now, a new strand was created by DNA polymerase enzymes in the direction of 5′ to 3′ (note: the DNA template is read in 3' to 5' direction). RNA primers were simultaneously created by the primase enzyme and utilized by the polymerase to create new strands of RNA.

As DNA polymerase enzyme has 5′ - 3′ direction so two types of strand are form -

LEADING STRAND - A DNA polymerase with high processivity that has the direction 5′ - 3′ (template strand 3′-5′) extends the continuous strand from the primer in the same direction as the replication fork that is expanding. One RNA primer is applied to the leading strand.

LAGGING STRAND - It is the discontinuous strand whose synthesis is going in the opposite direction of the replication fork that is developing. Each primer extends the lagging strand intermittently to create Okazaki fragments. A low processivity DNA polymerase that is different from the replicative polymerase enters to fill the gaps after RNase eliminates the priming RNA pieces. Multiple primers are applied to the lagging strand, and then the Okazaki fragments are joined by the ligase enzyme, filling in the gaps.

TERMINATION - Replication forks meet and end at several locations throughout the chromosome because eukaryotes start DNA replication at various locations; it is unknown how these are controlled. Termination necessitates the halting or blocking of the DNA replication fork's forward motion. When termination occurs at a particular locus, it includes the interaction of two elements: (1) a DNA sequence known as a termination site sequence, and (2) a protein that binds to this region to actually cease DNA replication.

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