DNA Structure and Replication
Discover the elegant double helix structure of DNA, the rules of complementary base pairing, and how DNA replicates itself with remarkable accuracy before every cell division.
The Structure of DNA
DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. Its structure was famously described by Watson and Crick in 1953 as a double helix -- two strands wound around each other like a twisted ladder.
DNA Double Helix Structure
Sugar-phosphate backbone (sides) with base pairs (rungs) connected by hydrogen bonds (dashed lines)
Nucleotide Components
Each nucleotide consists of three parts:
- Deoxyribose sugar (5-carbon sugar)
- Phosphate group
- Nitrogenous base (A, T, G or C)
Complementary Base Pairing
Adenine (A) always pairs with Thymine (T) -- 2 hydrogen bonds
Guanine (G) always pairs with Cytosine (C) -- 3 hydrogen bonds
Remember: A-T, G-C (or "Apple Tree, Green Car")
DNA Replication
Before a cell divides, it must copy its DNA so each daughter cell receives an identical copy. This process is called semi-conservative replication because each new DNA molecule consists of one original strand and one newly synthesised strand.
Steps of DNA Replication
Unwinding
The enzyme helicase unzips the double helix by breaking hydrogen bonds between base pairs.
Base Pairing
Free nucleotides from the cytoplasm pair with exposed bases on each template strand (A with T, G with C).
Joining
The enzyme DNA polymerase adds nucleotides to the new strand and checks for errors (proofreading).
Result
Two identical DNA molecules are produced, each with one original and one new strand (semi-conservative).
Why semi-conservative? Meselson and Stahl (1958) confirmed that each daughter DNA molecule contains one old and one new strand. This ensures genetic accuracy during cell division.
The Significance of DNA
DNA carries the instructions for building proteins, which carry out most of the work in cells. The sequence of bases along a DNA strand encodes genetic information in units called genes.
Genes
Specific sequences of DNA that code for particular proteins. Humans have approximately 20,000--25,000 genes.
Mutations
Changes in the DNA base sequence. Can be caused by errors in replication, UV radiation or chemicals. May be harmful, beneficial or neutral.
DNA in Chromosomes
In eukaryotes, DNA is tightly coiled around histone proteins and packaged into chromosomes. Humans have 46 chromosomes (23 pairs).
Key Vocabulary
Nucleotide
The monomer (building block) of DNA, consisting of a deoxyribose sugar, a phosphate group and a nitrogenous base.
Complementary Base Pairing
The specific pairing of bases in DNA: adenine with thymine and guanine with cytosine, held together by hydrogen bonds.
Helicase
The enzyme that unwinds and separates the two DNA strands by breaking hydrogen bonds during replication.
DNA Polymerase
The enzyme that synthesises new DNA strands by adding complementary nucleotides and proofreading for errors.
Worked Examples
If one strand of DNA reads ATCGGTA, what is the complementary strand?
Apply base pairing rules: A→T, T→A, C→G, G→C
A-T, T-A, C-G, G-C, G-C, T-A, A-T
Complementary strand: TAGCCAT
In a sample of DNA, 30% of the bases are adenine. What percentage of the bases are cytosine?
Chargaff's rule: %A = %T and %G = %C.
If A = 30%, then T = 30%.
Remaining: 100% - 30% - 30% = 40% (shared between G and C).
C = 20% (and G = 20%).
Explain why DNA replication is described as "semi-conservative".
"Semi" means half and "conservative" means preserved/kept.
After replication, each new DNA molecule consists of one original (parent) strand that is conserved and one newly synthesised strand.
So half of the original molecule is conserved in each copy -- hence "semi-conservative".
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Adenine pairs with which base in DNA?
Question 2
The "backbone" of a DNA strand is made up of alternating:
Question 3
Which enzyme unwinds the DNA double helix during replication?
Question 4
If a DNA sample contains 22% thymine, what percentage is guanine?
Question 5
DNA replication is called "semi-conservative" because each new DNA molecule:
Key Concepts Summary
- ●DNA is a double helix made of two antiparallel strands of nucleotides.
- ●Each nucleotide contains a deoxyribose sugar, phosphate group and nitrogenous base.
- ●Complementary base pairing: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds).
- ●Helicase unzips DNA; DNA polymerase builds new strands.
- ●Replication is semi-conservative: each new molecule has one old and one new strand.