Structure of DNA and how it copies itself
DNA is a double helix of two strands made of nucleotides. Before a cell divides, the DNA unwinds and each strand gets a new partner: this is DNA replication.
The DNA molecule
A DNA molecule is a long chain of small units called nucleotides, so it is a polynucleotide (poly means many). Each nucleotide has three parts: a nitrogenous base, a deoxyribose sugar and a phosphate. DNA has four bases: adenine, guanine, thymine and cytosine, so it has four kinds of nucleotides. Different orders of these nucleotides along a stretch of DNA make different genes. In shape, DNA is a double helix: two strands twisted around each other like a spiral staircase. Many books add that the bases on the two strands pair up: A with T, and G with C. So each old strand can guide an exact new partner.
Why DNA must copy itself
A dividing cell gives two identical cells, and each must receive a full, identical set of chromosomes. So, before division, every chromosome must hold two identical DNA molecules, one for each new cell. The process that makes this exact copy is DNA replication (also called DNA duplication).
DNA replication: three main steps, then cell division
- 1Unwinding: certain enzymes unwind the double helix and open it up, exposing the two strands.unwinding enzymes
- 2★Building new strands: the enzyme DNA polymerase builds a new (daughter) strand along each old strand. Each new strand forms a double helix with one parent strand.DNA polymerase
- 3★Two identical molecules: there are now two DNA molecules, each with one old and one new strand. They become the two chromatids of the chromosome, joined at the centromere.
- 4During cell division, the two chromatids separate and become two chromosomes, one passing into each daughter cell.
★ key step
Draw it: DNA replication
Each new DNA molecule keeps one strand from the parent molecule.
How to draw DNA replication
- Draw two wavy lines twisted round each other at the bottom: label it parent DNA (double helix).
- Above it, draw the two lines opening out like a Y: label unwinding.
- Along each opened strand, draw a new strand in dotted lines or another colour: label new (daughter) strand and DNA polymerase.
- At the top, draw two double helices, each with one solid (old) and one dotted (new) strand: label two identical DNA molecules.
- Write A, T, G, C on a few cross-links between the strands to show the bases.
Your skin, blood and gut lining make huge numbers of new cells every day, and each needs a full copy of your DNA. When a cut on your hand heals, DNA replication is working in the cells around it.
1 marks: “Name the enzyme that builds a new DNA strand during replication.” DNA polymerase.
3 marks: “What is a nucleotide? Name its three parts and the four bases found in DNA.” A nucleotide is the unit of DNA. Parts: nitrogenous base, deoxyribose sugar, phosphate. Bases: adenine, guanine, thymine, cytosine.
5 marks: “Before a skin cell divides, its DNA must make an exact copy of itself. Explain why, and describe how the copy is made, with a labelled diagram.” Each daughter cell needs identical chromosomes. Steps: enzymes unwind the double helix; DNA polymerase builds a new strand on each old strand; two identical molecules, each one old + one new strand, form two chromatids joined at the centromere. Diagram with parent strand, new strand, polymerase labelled.
Unzip, copy, keep one old: unwind, DNA polymerase builds, each new DNA keeps one parent strand. Bases: All Girls Take Care.