DNA Structure

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DNA structure 3.3.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate. A nucleotide is made of the sugar deoxyribose, a base (which can be either adenine, guanine, cytosine or thymine) and a phosphate group. Below is a representation of a nucleotide. 3.3.1 Outline DNA nucleotide structure in terms of a sugar (deoxyribose), base and phosphate

3.3.2 State the names of the four bases in DNA. Adenine, Guanine, Cytosine and Thymine. 3.3.3 Outline how DNA nucleotides are linked together by covalent bonds into a single strand. Below is a diagram showing how nucleotides are linked to one another to form a strand. A covalent bond forms between the sugar of one nucleotide and the phosphate group of another nucleotide. 3.3.3 Outline how the DNA nucleotides are linked together by covalent bonds into a single strand

▪ Nucleotides a linked into a single strand via a condensation reaction ▪ The phosphate group (attached to the 5'-C of the sugar) joins with the hydroxyl (OH) group attached to the 3'-C of the sugar ▪ This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule ▪ Successive condensation reactions between nucleotides results in the formation of a long single strand

3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds. DNA is made up of two nucleotide strands. The nucleotides are connected together by covalent bonds within each strand. The sugar of one nucleotide forms a covalent bond with the phosphate group of another. The two strands themselves are connected by hydrogen bonds. The hydrogen bonds are found between the bases of the two strands of nucleotides. Adenine forms hydrogen bonds with thymine whereas guanine forms hydrogen bonds with cytosine. This is called complementary base pairing. Below is a digram showing the molecular structure and bonds within DNA. 3.3.4 Explain how a DNA double helix is formed using complementary base pairing and hydrogen bonds Two polynucleotide chains of DNA are held together by hydrogen bonds between complementary base pairs ▪ Adenine pairs with thymine (A=T) via two hydrogen bonds ▪ Guanine pairs with cytosine (G=C) via three hydrogen bonds Thymine

Adenine Cytosine

Guanine

In order for bases to be facing each other and thus able to pair, the two strands must run in opposite directions (i.e. they are anti-parallel) As the polynucleotide chain lengthens, the atoms that make up the molecule will arrange themselves in an optimal energy configuration This position of least resistance results in the double-stranded DNA twisting to form a double helix with approximately 10 - 15 bases per twist

3.3.5 Draw and label a simple diagram of the molecular structure of DNA.

DNA replication 3.4.1 Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase. DNA replication is semi-conservative as both of the DNA molecules produced are formed from an old strand and a new one. The first stage of DNA replication involves the unwinding of the double strand of DNA (DNA double helix) and separating them by breaking the hydrogen bonds between the bases. This is done by the enzyme helicase. Each separated strand now is a template for the new strands. There are many free nucleotides around the replication fork which then bond to the template strands. The free nucleotides form hydrogen bonds with their complimentary base pairs on the template strand. Adenine will pair up with thymine and guanine will pair up with cytosine. DNA polymerase is the enzyme responsible for this. The new DNA strands then rewind to form a double helix. The replication process has produced a new DNA molecule which is identical to the initial one. 3.4.1 Explain DNA replication in terms of unwinding of the double helix and separation of the strands by helicase, followed by the formation of the new complementary strands by DNA polymerase Helicase ▪ Unwinds the DNA and separates the two polynucleotide strands by breaking the hydrogen bonds between complementary base pairs ▪ The two separated polynucleotide strands act as templates for the synthesis of new polynucleotide strands DNA Polymerase

▪ Synthesises new strands from the two parental template strands ▪ Free deoxynucleoside triphosphates (nucleotides with three phosphate groups) are aligned opposite their complementary base partner and are covalently bonded together by DNA polymerase to form a complementary nucleotide chain The energy for this reaction comes from the cleavage of the two extra phosphate groups 3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA. Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA. 3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA Each of the nitrogenous bases can only pair with its complementary partner (A=T ; G=C) Consequently, when DNA is replicated by the combined action of helicase and DNA polymerase: ▪ The new strands formed will be identical to the original strands separated from the template ▪ The two DNA molecules formed will be identical to the original molecule DNA Replication is a Semi-Conservative Process

3.4.3 State that DNA replication is semi- conservative. DNA replication is semi-conservative process because when a new double-stranded DNA molecule is formed: ▪ One strand will be from the original molecule One strand will be newly synthesised

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