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    name the enzyme that helps in the synthesis of new dna strand during replication


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    Which enzyme catalyses the synthesis of a new strand for a DNA molecule by linking nucleotides to the developing strand?

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    Which enzyme catalyses the synthesis of a new strand for a DNA molecule by linking nucleotides to the developing strand?


    RNA polymerase


    DNA ligase


    DNA polymerase



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    Updated on : 2022-09-05

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    Correct option is C)

    RNA polymerase helps in DNA dependent RNA synthesis. This process is known as transcription. DNA strands are used as a template to synthesize RNA molecules.

    DNA ligase is used to join DNA segments during DNA repair or during replication (to join Okazaki fragments of the lagging strands).

    DNA polymerase works in the pair to synthesis new strand for DNA molecule from deoxyribonucleotides. In other words, it helps in DNA dependent DNA synthesis.

    Topoisomerase enzyme participates in overwinding or unwinding of DNA. During DNA replication or transcription, DNA becomes overwound ahead of the replication fork. It binds to DNA and cut the phosphate backbone of either one or both the DNA strands.

    So, the correct option is 'DNA polymerase'.

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    Major Enzymes

    Major Enzymes


    Identify the major enzymes that play a role in DNA replication

    The process of DNA replication is catalyzed by a type of enzyme called DNA polymerase (poly meaning many, mer meaning pieces, and –ase meaning enzyme; so an enzyme that attaches many pieces of DNA). Observe Figure 1: the double helix of the original DNA molecule separates (blue) and new strands are made to match the separated strands. The result will be two DNA molecules, each containing an old and a new strand. Therefore, DNA replication is called semiconservative. The term semiconservative refers to the fact that half of the original molecule (one of the two strands in the double helix) is “conserved” in the new molecule. The original strand is referred to as the template strand because it provides the information, or template, for the newly synthesized strand.

    Figure 1. By Madprime(wikipedia) (DNA replication split horizontal) CC BY-SA 2.0

    Figure 2. Primer and Template

    DNA replication relies on the double-stranded nature of the molecule. One double stranded DNA molecule, when replicated, will become two double-stranded molecules, each containing one original strand and one newly synthesized strand. You remember that the two strands of DNA run antiparallel: one from the 5′ to the 3′, and the other from the 3′ to the 5′. The synthesis of the new DNA strand can only happen in one direction: from the 5′ to the 3′ end. In other words, the new bases are always added to the 3′ end of the newly synthesized DNA strand. So if the new nucleotide is always added to the 3′ end of an existing nucleotide, where does the first nucleotide come from? In fact, DNA polymerase needs an “anchor” to start adding nucleotides: a short sequence of DNA or RNA that is complementary to the template strand will work to provide a free 3′ end. This sequence is called a primer (Figure 2).

    How does DNA polymerase know in what order to add nucleotides? Specific base pairing in DNA is the key to copying the DNA: if you know the sequence of one strand, you can use base pairing rules to build the other strand. Bases form pairs (base pairs) in a very specific way. Figure 3 shows how A (adenine) pairs with T (thymine) and G (guanine) pairs with C (cytosine). It is important to remember that this binding is specific: T pairs with A, but not with C. The molecular recognition occurs because of the ability of bases to form specific hydrogen bonds: atoms align just right to make hydrogen bonds possible. Also note that a larger base (purine, A or G) always pairs with a smaller base (pyrimidine, C or T).

    Figure 3. DNA chemical structure. Modification of DNA chemical structure by Madeleine Price Ball; CC-BY-SA-2.0


    True/False: DNA replication requires an enzyme.

    Show Answer

    What are the building blocks on DNA?


    Fatty acids Ribonucleotides Amino acids Show Answer

    True/False: DNA replication requires energy.

    Show Answer

    We have the building blocks, a source of energy, and a catalyst. What’s missing? We need instruction about the order of nucleotides in the new polymer. Which molecule provides these instructions?

    Protein DNA Carbohydrate Lipid Show Answer

    There is one more thing required by the DNA polymerase. It cannot just start making a DNA copy of the template strand; it needs a short piece of DNA or RNA with a free hydroxyl group in the right place to attach the nucleotides to. (Remember that synthesis always occurs in one direction—new building blocks are added to the 3′ end.) This component starts the process by giving DNA polymerase something to bind to. What might you call this short piece of nucleic acid?

    A solvent A primer A converter A sealant Show Answer

    Now that you understand the basics of DNA replication, we can add a bit of complexity. The two strands of DNA have to be temporarily separated from each other; this job is done by a special enzyme, helicase, that helps unwind and separate the DNA helices (Figure 4). Another issue is that the DNA polymerase only works in one direction along the strand (5′ to 3′), but the double-stranded DNA has two strands oriented in opposite directions. This problem is solved by synthesizing the two strands slightly differently: one new strand grows continuously, the other in bits and pieces. Short fragments of RNA are used as primers for the DNA polymerase.

    Figure 4. By Mariana Ruiz (DNA replication) Public Domain


    Which of these separates the two complementary strands of DNA?

    DNA polymerase helicase RNA primer

    single-strand binding protein

    Show Answer

    Which of these attaches complementary bases to the template strand?

    DNA polymerase helicase RNA primer

    single-strand binding protein

    Show Answer

    Which of these is later replaced with DNA bases?

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    Molecular mechanism of DNA replication (article)

    Roles of DNA polymerases and other replication enzymes. Leading and lagging strands and Okazaki fragments.


    Molecular mechanism of DNA replication

    Roles of DNA polymerases and other replication enzymes. Leading and lagging strands and Okazaki fragments.

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    Key points:

    DNA replication is semiconservative. Each strand in the double helix acts as a template for synthesis of a new, complementary strand.

    New DNA is made by enzymes called DNA polymerases, which require a template and a primer (starter) and synthesize DNA in the 5' to 3' direction.

    During DNA replication, one new strand (the leading strand) is made as a continuous piece. The other (the lagging strand) is made in small pieces.

    DNA replication requires other enzymes in addition to DNA polymerase, including DNA primase, DNA helicase, DNA ligase, and topoisomerase.


    DNA replication, or the copying of a cell's DNA, is no simple task! There are about

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    base pairs of DNA in your genome, all of which must be accurately copied when any one of your trillions of cells divides

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    The basic mechanisms of DNA replication are similar across organisms. In this article, we'll focus on DNA replication as it takes place in the bacterium E. coli, but the mechanisms of replication are similar in humans and other eukaryotes.

    Let's take a look at the proteins and enzymes that carry out replication, seeing how they work together to ensure accurate and complete replication of DNA.

    The basic idea

    DNA replication is semiconservative, meaning that each strand in the DNA double helix acts as a template for the synthesis of a new, complementary strand.

    This process takes us from one starting molecule to two "daughter" molecules, with each newly formed double helix containing one new and one old strand.

    Schematic of Watson and Crick's basic model of DNA replication.

    DNA double helix.

    Hydrogen bonds break and helix opens.

    Each strand of DNA acts as a template for synthesis of a new, complementary strand.

    Replication produces two identical DNA double helices, each with one new and one old strand.

    In a sense, that's all there is to DNA replication! But what's actually most interesting about this process is how it's carried out in a cell.

    Cells need to copy their DNA very quickly, and with very few errors (or risk problems such as cancer). To do so, they use a variety of enzymes and proteins, which work together to make sure DNA replication is performed smoothly and accurately.

    DNA polymerase

    One of the key molecules in DNA replication is the enzyme DNA polymerase. DNA polymerases are responsible for synthesizing DNA: they add nucleotides one by one to the growing DNA chain, incorporating only those that are complementary to the template.

    Here are some key features of DNA polymerases:

    They always need a template

    They can only add nucleotides to the 3' end of a DNA strand

    They can't start making a DNA chain from scratch, but require a pre-existing chain or short stretch of nucleotides called a primer

    They proofread, or check their work, removing the vast majority of "wrong" nucleotides that are accidentally added to the chain

    The addition of nucleotides requires energy. This energy comes from the nucleotides themselves, which have three phosphates attached to them (much like the energy-carrying molecule ATP). When the bond between phosphates is broken, the energy released is used to form a bond between the incoming nucleotide and the growing chain. [See the polymerization reaction]

    In prokaryotes such as E. coli, there are two main DNA polymerases involved in DNA replication: DNA pol III (the major DNA-maker), and DNA pol I, which plays a crucial supporting role we'll examine later.

    Starting DNA replication

    How do DNA polymerases and other replication factors know where to begin? Replication always starts at specific locations on the DNA, which are called origins of replication and are recognized by their sequence.

    E. coli, like most bacteria, has a single origin of replication on its chromosome. The origin is about

    245 245 245

    base pairs long and has mostly A/T base pairs (which are held together by fewer hydrogen bonds than G/C base pairs), making the DNA strands easier to separate.

    Specialized proteins recognize the origin, bind to this site, and open up the DNA. As the DNA opens, two Y-shaped structures called replication forks are formed, together making up what's called a replication bubble. The replication forks will move in opposite directions as replication proceeds.

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