What is DNA?

What is DNA?

• DNA is a polymer, long chain, of nucleotides. These molecules contain the genetic information of the cell. DNA is a nucleic acid.
• For DNA, each nucleotide contains three parts
• A five-carbon sugar
• Deoxyribose in DNA
• A nitrogenous base
• Purines: 2 rings
• Adenine and Guanine
• Pyrimidines: 1 ring
• Thymine and Cytosine
• Adenine pairs with Thymine
• There are held by 2 hydrogen bonds
• Guanine pairs with Cytosine
• They are held by 3 hydrogen bonds
• Phosphate group

 Important features of DNA:

• Typically recognized as a double helix shape
• The double helix has a constant diameter
• It is right-handed
• DNA strands are complementary
• This means that each DNA strand has the information to construct the other strand.
• DNA is anti-parallel
• One strand runs in the 3’ to 5’ direction and the other strand runs in the 5’ to 3’ direction.
• Follows a semi-conservative replication pattern

What is DNA Replication?

In DNA Replication, the double helix unwinds and separates into two template strands. Then new nucleotides are added to the new strand at the 3’ position. This sequence will depend on the complementary base pairing.

DNA Replication Process

• The process begins at the origin of replication, which is a specific DNA sequence where enzymes can start separating the double helix

• An enzyme called helicase unwinds the DNA double helix and separates it into two strands.
• A Y-shaped replication fork will form as the double unwinds
• Single-stranded DNA binding proteins help keep the strands separate and prevent the strands from getting back together

• DNA polymerase is an enzyme that assembles a new DNA strand that is complementary to the template strand. Primers are needed.
• A primer, which is a short single strand of RNA, is needed in order for the DNA replication to start. Primers are synthesized by primase
• Primers bind to the origin of replication in order to initiate the synthesis of new strands
• Then the DNA polymerase begins adding nucleotides to the 3’ end of the primer
• DNA polymerase can only add a nucleotide to the 3’ end

• DNA polymerase continues to move along the template strand and add new nucleotides to the growing or complement strand until the entire genome is replicated.

• Remember, replication occurs in both directions because the two DNA strands are anti-parallel
• DNA polymerase can only add a nucleotide to the 3’ end
• The leading complementary strand is assembled continuously
• The lagging complementary strand is assembled in short fragments called Okazaki fragments.
• The lagging complementary strand is synthesized in discontinuously or in segments
• The lagging strand is in the opposite direction of the leading strand
• Each Okazaki fragment requires a primer
• DNA ligase joins the Okazaki fragments together and the primers are replaced by DNA nucleotides

When Primers are replaced by DNA nucleotides, the DNA polymerase can’t continue to add nucleotides without an existing strand. However, chromosomes have telomeres, which are special nucleotide sequences that do not contain genes.   

• Over time, telomeres start to get removed each time the cell replicates.
• Some cells contain telomerase, which catalyzes the addition of telomeres.

Cell can have several DNA polymerases as they can be involved in different roles, such as:

• DNA Replication
• DNA Repair
• Primer Removal

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