The Structure of DNA

Watson and Crick’s Double Helix Model

The Double Helix DNA is a thread-like molecule, made up of two long strands of nucleotides that are bound together in the shape of a double helix. If the helix were unwound, the molecule would look something like a ladder.

What are nucleotides? DNA is composed of repeating units referred to as nucleotides. Each nucleotide molecule contains: -Deoxyribose (5-carbon sugar) -Phosphate group -One of 4 nitrogenous bases: -Adenine (A) -Thymine (T) -Guanine (G) -Cytosine (C)

How are nucleotides connected? Nucleotides are joined together to form long chains. The phosphate group of one nucleotide is bonded to the deoxyribose sugar of an adjacent nucleotide.

Components of the Double Helix The sugar and phosphate groups form the sides or backbone of the DNA molecule (“handrails” of the ladder). The nitrogenous bases form the interior of the DNA molecule (“rungs” of the ladder).

How are the two strands of nucleotides connected? The structure of the bases enable them to pair with one another. Purines = adenine and guanine (“AG’s are pure”) -Family of nitrogenous compounds that have a double ring structure Pyrimidines = cytosine and thymine (“pyramids are cut (CT) from stone”) -Family of nitrogenous compounds that have a single ring structure

Purine and Pyrimidine Structure

Continued… The bases are arranged in a way that the distance between the two handrails remains constant. This is accomplished by a purine bonding with a pyrimidine, giving the base pairs a constant width of three rings.

Complimentary Base Pairing In a DNA molecule, adenine (A) can only form a stable bond with thymine (T). Likewise, cytosine (C) can only form a bond with guanine (G). This is referred to as complimentary base pairings.

Continued… Base pairs are joined by weak hydrogen bonds -A to T = two H-bonds -C to G = three H-bonds The two strands of nucleotides are not identical, but are complementary to one another.

http://www.youtube.com/watch?v=qy8dk5iS1f0

The Antiparallel Nature of DNA The nucleotide strands are also antiparallel, that is the phosphate bridges run in opposite directions in each strand. This means that the end of each double-stranded DNA molecule contains the 5΄ end of one strand and the 3΄ end of the other.

RNA (ribonucleic acid) Three differences from DNA: It contains ribose instead of deoxyribose. The nucleotide thymine is not found in RNA. Instead it contains the nucleotide uracil. RNA is single stranded.

DNA Replication Humans have about 1 trillion cells! Each of these cells is genetically identical to the zygote from which they formed. For this to happen: -The genetic material must be copied quickly and accurately

The Process of DNA Replication During DNA replication, two molecules of DNA are made from one. The replication process follows a semi-conservative model. This means that when a molecule of DNA is copied, each new molecule contains one strand of parental DNA and one strand of new DNA.

Three Stages of the Replication Process There are four main stages of DNA replication: Initiation Elongation Termination All of these activities may take place simultaneously on the same DNA molecule.

Initiation A set of enzymes, referred to as helicases, unzip or unwind the DNA. The regions at which the DNA helix is unwound and new strands develop are called replication forks.

Replication Bubble

Elongation During this stage, an enzyme referred to as, DNA polymerase, adds free nucleotides to the unzipped strand of DNA. Before this can begin, a primer is formed by an enzyme called a primase. A primer is a short strand of RNA that consists of a few nucleotides that are complimentary to the DNA template. Once the primer has been constructed, DNA polymerase extends the fragment by adding DNA nucleotides.

Elongation Continued… Recall: -The strands of DNA are antiparallel, which means that the end of each double-stranded DNA molecule contains the 5΄ end of one strand and the 3΄ end of the other.

Elongation Continued… DNA polymerase can only add nucleotides in the 5΄ to 3΄ direction (it adds nucleotides to the 3΄ hydroxyl end of a DNA strand). This conflicts with the observation that both DNA strands are replicated simultaneously. Therefore replication must take place in different ways along each strand of DNA.

Elongation Continued… The Leading Strand -replicated continuously in the 5΄ to 3΄ direction -elongation proceeds in the same direction as the movement of the replication fork

Elongation Continued… The Lagging Strand -elongation takes place in the opposite direction of the replication fork -DNA polymerase builds Okazaki fragments (short fragments of nucleotides) in the 5΄ to 3΄ direction -the Okazaki fragments are joined by an enzyme called DNA ligase. -this strand is manufactured more slowly than the leading strand

Elongation Continued… On the leading strand, only one primer has to be constructed. On the lagging strand, a new primer has to be made for each Okazaki fragment.

http://www.youtube.com/watch?v=teV62zrm2P0

Termination The new DNA molecules twist into double helices. Proofreading and Correction -DNA polymerase checks the new DNA strands for errors (for example, mismatched bases). -Makes the necessary corrections