Nucleic Acids: Structure & DNA Replication

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Apuntes de la lección

Nucleotide Structure

Both DNA and RNA are polymers made of repeating units called nucleotides.
Each nucleotide is formed from a pentose sugar, a nitrogen-containing organic base, and a phosphate group.
In a DNA nucleotide, the pentose sugar is deoxyribose (with hydrogen at the 2' position).
In an RNA nucleotide, the pentose sugar is ribose (with a hydroxyl group at the 2' position).
DNA contains the bases adenine (A), cytosine (C), guanine (G) and thymine (T).
RNA contains the bases adenine (A), cytosine (C), guanine (G) and uracil (U); uracil replaces thymine.
The 2' hydroxyl group in ribose makes RNA more susceptible to hydrolysis, so DNA is the storage molecule and RNA is the transport molecule with a shorter lifespan.

The Phosphodiester Bond

Nucleotides join together via condensation reactions between the phosphate group of one nucleotide and the pentose sugar of the next.
A condensation reaction between two nucleotides forms a phosphodiester bond.
A phosphodiester bond consists of a phosphate group and two ester bonds.
Many phosphodiester bonds create a chain of alternating phosphate groups and pentose sugars known as the sugar-phosphate backbone.
The phosphodiester bonds link the 5' carbon of one sugar to the phosphate group, which is linked to the 3' carbon of the next sugar.

DNA Structure

DNA is a polynucleotide made of two polynucleotide strands that run in opposite directions, an arrangement called antiparallel.
Each strand has a sugar-phosphate backbone formed by alternating deoxyribose sugars and phosphate groups joined by phosphodiester bonds.
One DNA strand runs from 5' to 3', and the other runs from 3' to 5'.
The nitrogenous bases project from the backbone towards the interior of the double-stranded molecule.
The two antiparallel strands are held together by hydrogen bonds between complementary bases.
Adenine always pairs with thymine (two hydrogen bonds) and guanine always pairs with cytosine (three hydrogen bonds).
DNA is described as a double helix, referring to its three-dimensional shape.

DNA Function

The function of DNA is to hold or store genetic information.
DNA contains the instructions for the growth and development of all organisms.
DNA is protected in the nucleus, which provides security for the genetic material.
DNA is easily copied and therefore conserved throughout generations of cells and inherited between generations.

RNA Structure and Function

RNA is a polynucleotide made up of many nucleotides linked together in a chain.
RNA molecules are single-stranded and relatively short compared to DNA.
Each RNA strand has a sugar-phosphate backbone of alternating ribose sugars and phosphate groups linked by phosphodiester bonds.
The nitrogenous bases project out sideways from the single-stranded RNA molecule.
Examples of RNA include messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
The function of RNA is to transfer the genetic code from DNA out of the nucleus and carry it to the ribosomes in the cytoplasm.
Ribosomes 'read' the RNA to make polypeptides (proteins) in a process called translation.

Ribosomes

Ribosomes are small organelles that are either free in the cytoplasm or attached to the rough endoplasmic reticulum (in eukaryotic cells).
Ribosomes are the site of protein synthesis and 'read' RNA to make polypeptides in translation.
Ribosomes are formed from RNA and proteins; the RNA is known as ribosomal RNA (rRNA).
The rRNA in ribosomes has enzymatic properties that catalyse the formation of peptide bonds between amino acids.
Each ribosome is composed of a small subunit and a large subunit.
Eukaryotic cells have 80S ribosomes (60S and 40S subunits); prokaryotic cells, mitochondria and chloroplasts have 70S ribosomes (50S and 30S subunits).
The large subunit is the site of translation; mRNA sits between the two subunits and the ribosome moves along it.
Ribosomes are not surrounded by a membrane.

The Origins of Research on the Genetic Code

DNA was first observed in the 1800s by Friedrich Miescher, who named it 'nuclein'.
Many scientists initially doubted that DNA could carry the genetic code because of its relatively simple chemical composition.
DNA was thought to be too simple as it is made of only four repeating nucleotides, each composed of a phosphate group, deoxyribose and a nitrogen-containing base.
It wasn't until the 1940s that the role of DNA in genetic inheritance began to be more fully researched and understood.
By 1953, experiments confirmed that DNA carried the genetic code; the triplet code enables much variation (the code is universal and degenerate).
The location of DNA, protected in the nucleus, enabled the security of the genetic material rather than proteins that are found in the cytoplasm and susceptible to hydrolysis.
1953 was also the year in which Watson and Crick confirmed the double-helix structure of DNA using Rosalind Franklin's X-ray data.

Semi-Conservative Replication

Before a cell divides, it copies its DNA to ensure each daughter cell receives a full copy of the parental DNA.
DNA is copied by semi-conservative replication: each new DNA molecule has one original strand and one newly synthesised strand.
This process ensures genetic continuity between generations of cells and is essential for replacing body cells, growth and development.
DNA replication occurs during the S phase of interphase in the cell cycle.
Helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs, forming two single strands that act as templates.
Free nucleotides are attracted to the exposed bases by complementary base pairing.
DNA polymerase catalyses condensation reactions to join the new nucleotides together, forming the sugar-phosphate backbone.
The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule.
Free nucleotides exist as nucleoside triphosphates (activated nucleotides); the extra phosphate groups provide energy for replication.
DNA polymerase removes two phosphate groups from each nucleotide to release energy used to form phosphodiester bonds.

Calculating the Frequency of Nucleotide Bases

All DNA nucleotides contain the same phosphate group and deoxyribose sugar, but the nitrogenous base can vary.
The bases always pair up in the same way: Adenine pairs with Thymine (A-T) and Cytosine pairs with Guanine (C-G).
The bases in each pair are said to be complementary to one another.
Therefore, the frequency of adenine equals the frequency of thymine, and the frequency of cytosine equals the frequency of guanine.
The frequency of bases can be calculated if the number or percentage of one specific base is known.
For example, if 28% of bases are cytosine, then 28% are guanine (total 56%), leaving 44% for adenine and thymine, so adenine = 22% and thymine = 22%.

The Watson-Crick Model and Meselson & Stahl's Experiment

In 1953, Watson and Crick confirmed the double-helix structure of DNA and proposed the semi-conservative model of replication.
The conservative model suggested that the original DNA remains intact and the new DNA is made entirely of new strands.
Meselson and Stahl tested the theories using bacteria grown in heavy nitrogen (15N) followed by growth in light nitrogen (14N).
Bacteria grown in 15N had DNA containing only heavy nitrogen, which settled near the bottom of a centrifuge tube.
After one round of replication in 14N, if replication were conservative, original DNA would settle at the bottom and new DNA at the top.
If replication were semi-conservative, all DNA molecules would contain both heavy and light nitrogen and settle in the middle of the tube.
The DNA from the second centrifugation settled in the middle, confirming that each DNA molecule contained a mixture of heavy and light nitrogen isotopes.
If more rounds of replication occurred, the ratio of 15N:14N would go from 1:1 after the first round to 3:1 after the second and 7:1 after the third.
This experiment proved Watson and Crick's theory correct, confirming semi-conservative replication.
The experiment was well-controlled and repeatable, but only tested one organism (E. coli) and didn't show how enzymes like helicase and polymerase work.

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Preguntas de práctica

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1.Which of the following correctly lists the three components of a nucleotide?
Easy
- APentose sugar, phosphate group, nitrogen-containing organic base
- BPentose sugar, carboxyl group, nitrogen-containing organic base
- CHexose sugar, phosphate group, amino acid
- DPentose sugar, phosphate group, fatty acid
2.Which sugar is found in RNA nucleotides?
Easy
- ARibose
- BDeoxyribose
- CGlucose
- DFructose
3.Which base is found in RNA but not in DNA?
Easy
- AUracil
- BThymine
- CAdenine
- DGuanine
4.DNA is a polynucleotide made up of many nucleotides joined together.
Easy
True or false?
5.RNA molecules are double-stranded.
Easy
True or false?
6.How many hydrogen bonds form between guanine and cytosine in DNA?
Medium
- A3
- B2
- C1
- D4
7.Which enzyme unwinds the DNA double helix by breaking hydrogen bonds between base pairs?
Medium
- AHelicase
- BDNA polymerase
- CLigase
- DRNA polymerase
8.Which enzyme catalyses the formation of phosphodiester bonds between nucleotides during DNA replication?
Medium
- ADNA polymerase
- BHelicase
- CDNA ligase
- DRNA polymerase

Más preguntas

9.Which of the following is a pentose sugar?
Easy
- ADeoxyribose
- BGlucose
- CFructose
- DGalactose
10.In a sample of DNA, 22% of the nucleotides contain guanine. What percentage contain adenine?
Medium
- A28%
- B22%
- C44%
- D56%
11.In a length of DNA 1000 nucleotides long, 382 guanine nucleotides are found in one strand. Why is it impossible to calculate the number of guanine nucleotides in the opposite strand?
Medium
- AThe number of guanine in the opposite strand depends on the number of cytosine in the first strand, which is not known
- BGuanine always pairs with thymine, so the number cannot be predicted
- CThe two strands are identical, so the number must be the same
- DDNA is single-stranded, so there is no opposite strand
12.Which of the following statements about DNA and RNA are correct? (select all that apply)
Medium
- ADNA contains deoxyribose; RNA contains ribose
- BRNA is usually single-stranded
- CDNA contains uracil; RNA contains thymine
- DBoth DNA and RNA contain a phosphate group
- EDNA is a polymer of nucleotides; RNA is not
13.The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases.
Easy
True or false?
14.Ribosomes are surrounded by a membrane.
Easy
True or false?
15.Which of the following statements about the nitrogenous bases in DNA are correct? (select all that apply)
Medium
- AAdenine and guanine are purine bases
- BCytosine and thymine are pyrimidine bases
- CPurine bases always pair with pyrimidine bases
- DAdenine pairs with cytosine via three hydrogen bonds
- EGuanine pairs with thymine via two hydrogen bonds
16.Which of the following are differences between DNA and RNA? (select all that apply)
Medium
- ADNA contains deoxyribose; RNA contains ribose
- BDNA contains thymine; RNA contains uracil
- CDNA is double-stranded; RNA is single-stranded
- DDNA is a polynucleotide; RNA is a polynucleotide
- EDNA is found only in the nucleus; RNA is found only in the cytoplasm
17.Match each nucleic acid component to its correct description.
Medium
- Deoxyribose
- Ribose
- Phosphate group
- Nitrogenous base
- Pentose sugar with hydrogen at the 2' position
- Pentose sugar with a hydroxyl group at the 2' position
- Component of a nucleotide that is negatively charged
- Component of a nucleotide that contains nitrogen
18.Place the steps of semi-conservative DNA replication in the correct order.
Medium
- Helicase unwinds the double helix by breaking hydrogen bonds
- Free nucleotides pair with complementary bases on the template strands
- DNA polymerase catalyses condensation reactions to form phosphodiester bonds
- Two new DNA molecules each containing one original and one new strand are formed
19.In a sample of double-stranded DNA, 30% of the nucleotides contain adenine. What percentage of the nucleotides contain guanine?
Medium
- A20%
- B30%
- C40%
- D60%
20.A sample of DNA contains 2000 nucleotides. If 600 of these contain cytosine, how many contain adenine?
Medium
- A400
- B600
- C800
- D1400
21.Meselson and Stahl grew bacteria in ¹⁵N then transferred them to ¹⁴N for one round of replication. After centrifugation, where did the DNA band appear?
Hard
- AAt the bottom of the tube only
- BAt the top of the tube only
- CIn the middle of the tube only
- DBoth at the bottom and at the top of the tube
22.Which of the following statements about Meselson and Stahl's experiment are correct? (select all that apply)
Hard
- AThey used heavy nitrogen (¹⁵N) and light nitrogen (¹⁴N) isotopes
- BAfter one round of replication, all DNA molecules contained a mixture of heavy and light nitrogen
- CTheir results supported the semi-conservative model of DNA replication
- DThe experiment showed that DNA replication is conservative
- EThe experiment directly demonstrated the action of DNA helicase
23.DNA polymerase catalyses the formation of hydrogen bonds between complementary bases during DNA replication.
Easy
True or false?
24.A DNA nucleotide and an RNA nucleotide are compared. Which statement describes a difference between them?
Medium
- AThe DNA nucleotide contains deoxyribose with hydrogen at the 2' position, while the RNA nucleotide contains ribose with a hydroxyl group at the 2' position.
- BThe DNA nucleotide contains ribose with a hydroxyl group at the 2' position, while the RNA nucleotide contains deoxyribose with hydrogen at the 2' position.
- CThe DNA nucleotide contains uracil, while the RNA nucleotide contains thymine.
- DThe DNA nucleotide is single-stranded, while the RNA nucleotide is double-stranded.
25.In a sample of double-stranded DNA, 15% of the nucleotides contain thymine. What percentage of the nucleotides contain guanine?
Medium
- A15%
- B35%
- C70%
- D30%
26.A section of DNA has 1200 nucleotides. If 300 of these contain adenine, how many contain cytosine?
Medium
- A300
- B600
- C150
- D900
27.Which of the following statements about DNA replication are correct? (select all that apply)
Medium
- AHelicase breaks hydrogen bonds between complementary bases.
- BDNA polymerase catalyses the formation of phosphodiester bonds.
- CDNA polymerase catalyses the formation of hydrogen bonds between complementary bases.
- DFree nucleotides are activated by the removal of two phosphate groups.
- EThe original DNA molecule remains completely intact and a new double-stranded molecule is made entirely of new strands.
28.Meselson and Stahl grew bacteria in ¹⁵N, then transferred them to ¹⁴N for two rounds of replication. After centrifugation, what proportion of the DNA molecules contained only ¹⁴N?
Medium
- A1/2
- B1/4
- C3/4
- D0
29.Which of the following is a correct description of the sugar-phosphate backbone of DNA?
Medium
- AIt consists of alternating deoxyribose sugars and phosphate groups joined by phosphodiester bonds.
- BIt consists of alternating ribose sugars and phosphate groups joined by hydrogen bonds.
- CIt consists of nitrogenous bases joined by phosphodiester bonds.
- DIt consists of two antiparallel strands joined by peptide bonds.
30.DNA polymerase removes two phosphate groups from each nucleotide to release energy used to form phosphodiester bonds.
Easy
True or false?
31.In DNA, the two polynucleotide strands are held together by phosphodiester bonds between complementary bases.
Easy
True or false?
32.Match each scientist or model with its correct description.
Medium
- Watson and Crick
- Meselson and Stahl
- Conservative model
- Semi-conservative model
- Proposed the double-helix structure of DNA
- Provided evidence for semi-conservative replication using ¹⁵N and ¹⁴N
- Original DNA remains intact and a new molecule is made entirely of new strands
- Each new DNA molecule contains one original and one new strand
33.In the Meselson and Stahl experiment, bacteria were grown in a broth containing heavy nitrogen (¹⁵N) and then transferred to a broth containing light nitrogen (¹⁴N). After two rounds of DNA replication, what proportion of the DNA molecules contained only ¹⁴N?
Medium
- AOne quarter
- BOne half
- CThree quarters
- DAll of them
34.A sample of DNA was extracted from bacteria grown in ¹⁵N, then transferred to ¹⁴N broth for one round of replication. The DNA was spun in a centrifuge. If conservative replication had occurred instead of semi-conservative replication, what would the centrifuge tube have shown?
Hard
- AA single band in the middle of the tube
- BTwo bands: one near the bottom and one near the top
- CA single band near the bottom of the tube
- DThree bands at the bottom, middle and top
35.Meselson and Stahl used the isotope ¹⁵N rather than ¹⁴N in their experiment. Why was this necessary?
Medium
- A¹⁵N forms more hydrogen bonds between the bases than ¹⁴N
- BDNA containing ¹⁵N is denser and so forms a band at a different position in the centrifuge tube
- C¹⁵N is radioactive and can be detected on photographic film
- D¹⁵N is a heavier isotope that stops DNA replication occurring
36.A piece of mitochondrial DNA (mtDNA) contains 33,140 nucleotides. Replication of the second strand begins only after two-thirds of the first strand has been replicated, and it takes 0.02 seconds to replicate one nucleotide. Which calculation gives the time in seconds to copy this mtDNA?
Hard
- A33,140 × 0.02
- B(33,140 × 0.02) + (33,140 × ⅔ × 0.02)
- C(33,140 × ⅔ × 0.02) + (33,140 × ⅓ × 0.02)
- D33,140 × ⅔ × 0.02
37.Which of the following are strengths of the Meselson and Stahl experiment as evidence for semi-conservative replication? (select all that apply)
Hard
- AThe method produced observable, measurable bands in the centrifuge tube
- BThe experiment showed exactly how helicase and DNA polymerase work
- CThe experiment was well-controlled and could be replicated using other organisms
- DThe results disproved the conservative and dispersive models
- EThe experiment tested every type of organism, so the results apply to all life
38.Meselson and Stahl's experiment confirmed the pattern of DNA replication but did not show how enzymes such as DNA helicase and DNA polymerase work.
Medium
True or false?
39.Which statement best describes the advantage of DNA being double-stranded, other than for replication?
Medium
- AThe second strand allows the molecule to be translated directly into a polypeptide
- BThe second strand protects the bases from chemical attack and allows damage to one strand to be repaired using the other
- CThe second strand provides the energy needed to form phosphodiester bonds
- DThe second strand allows DNA to leave the nucleus and reach the ribosomes
40.Ribosomes are formed from which biological molecules?
Medium
- ADNA and protein
- BRNA and protein
- CPhospholipid and protein
- DRNA and DNA
41.Complete the sentence about the bond that joins nucleotides in a polynucleotide strand.
Easy
The bond that joins the phosphate group of one nucleotide to the pentose sugar of the next is called a ____ bond.
42.Complete the sentence about the structure of DNA.
Medium
The two polynucleotide strands of DNA run in opposite directions and are therefore described as ____.
43.A piece of mitochondrial DNA contains 33,140 nucleotides. It takes 0.02 seconds to replicate one nucleotide. Calculate how long it would take to copy this mtDNA to the nearest whole second.
Medium
44.A cell's DNA replicates by semi-conservative replication. After 3 full cycles of replication, what fraction of the original DNA (not copies) will be present in a great-granddaughter cell? Give your answer as a decimal.
Hard
45.Name the enzyme that unwinds the DNA double helix by breaking hydrogen bonds between base pairs.
Medium
46.Name the enzyme that catalyses the formation of phosphodiester bonds between nucleotides during DNA replication.
Medium
47.A piece of DNA contains 33,140 nucleotides. It takes 0.02 seconds to replicate one nucleotide. Calculate the time in seconds to copy this DNA to the nearest whole second.
Medium
48.In a section of DNA, 28% of the nucleotides contain cytosine. Calculate the percentage of nucleotides that contain adenine.
Medium
49.A piece of mitochondrial DNA contains 33,140 nucleotides and it takes 0.02 seconds to replicate one nucleotide. Calculate how long it would take to copy this mtDNA to the nearest whole second.
Medium
50.Complete the sentences about the bases in DNA.
Easy
In DNA, adenine always pairs with ____ and guanine always pairs with ____. The base found in RNA but not in DNA is ____.
51.Complete the sentence about DNA replication.
Medium
During DNA replication, the enzyme ____ unwinds the double helix by breaking ____ bonds between the base pairs.
52.Name the bond that joins nucleotides together to form the sugar-phosphate backbone of DNA.
Medium
53.Describe the process of semi-conservative replication of DNA. In your answer give details of any molecules, bonds or enzymes involved.
Hard
54.Compare the structures and functions of DNA and RNA.
Medium
55.State two features of DNA and explain how these features are important in the process of semi-conservative replication of a cell’s DNA.
Medium
56.Describe the structure of a DNA molecule.
Medium
57.Explain why scientists initially concluded that DNA was unlikely to carry the genetic code.
Hard
58.A piece of mitochondrial DNA contains 33,140 nucleotides. It takes 0.02 seconds to replicate one nucleotide. Calculate how long it would take to copy this mtDNA to the nearest whole second.
Medium
- A597 seconds
- B663 seconds
- C729 seconds
- D795 seconds
59.A cell's DNA replicates by semi-conservative replication. After 3 full cycles of replication, what fraction of the original DNA (not copies) will be present in a great-granddaughter cell? Give your answer as a decimal.
Hard
- A0.125
- B1.125
- C2.125
- D-0.875
60.A piece of DNA contains 33,140 nucleotides. It takes 0.02 seconds to replicate one nucleotide. Calculate the time in seconds to copy this DNA to the nearest whole second.
Medium
- A729 seconds
- B663 seconds
- C597 seconds
- D795 seconds
61.In a section of DNA, 28% of the nucleotides contain cytosine. Calculate the percentage of nucleotides that contain adenine.
Medium
- A26 %
- B22 %
- C24 %
- D20 %
62.A piece of mitochondrial DNA contains 33,140 nucleotides and it takes 0.02 seconds to replicate one nucleotide. Calculate how long it would take to copy this mtDNA to the nearest whole second.
Medium
- A795 s
- B597 s
- C729 s
- D663 s

Apuntes de repaso

## Nucleotide Structure & the Phosphodiester Bond - Both DNA and RNA are **polynucleotides** – polymers made of many repeating **nucleotides**. - Each nucleotide is formed from: - A **pentose sugar** (carbons numbered 1′, 2′, 3′, 4′, 5′) - A **nitrogen-containing organic base** - A **phosphate group** - **DNA nucleotide**: deoxyribose sugar (H at 2′), phosphate, one of A, C, G, T. - **RNA nucleotide**: ribose sugar (OH at 2′), phosphate, one of A, C, G, U. - The 2′ hydroxyl group makes RNA more susceptible to hydrolysis → DNA is the storage molecule, RNA is the transport molecule with a shorter lifespan. - Nucleotides join via **condensation reactions** between the phosphate of one nucleotide and the pentose sugar of the next, forming a **phosphodiester bond**. - A phosphodiester bond consists of a phosphate group and two ester bonds. - The chain of alternating phosphate groups and pentose sugars is the **sugar-phosphate backbone**. ## DNA: Structure & Function - DNA consists of **two polynucleotide strands** that are **antiparallel** (one runs 5′→3′, the other 3′→5′). - Strands held by **hydrogen bonds** between complementary bases: - **A pairs with T** – two hydrogen bonds - **G pairs with C** – three hydrogen bonds - This is **complementary base pairing**; the pairs are **DNA base pairs**. - DNA is a **double helix** – a three-dimensional shape. - **Function**: to hold or store genetic information – the instructions for growth and development of all organisms. ## RNA: Structure & Function - RNA is a **single-stranded** polynucleotide; chains are relatively short compared to DNA. - Contains **ribose** instead of deoxyribose; contains **uracil (U)** instead of thymine (T). - Sugar-phosphate bonds are **phosphodiester bonds** (covalent). - Examples: **mRNA** (transcript copy of a gene), **tRNA**, **rRNA**. - **Function**: to transfer the genetic code from DNA out of the nucleus to ribosomes in the cytoplasm for **translation**. ## Ribosomes - Small organelles, free in cytoplasm or attached to rough ER (eukaryotes). - Site of **protein synthesis**; they ‘read’ RNA to make polypeptides in **translation**. - Formed from **RNA and proteins**; the RNA is **rRNA**. - rRNA has enzymatic properties that catalyse formation of **peptide bonds** between amino acids. - Composed of a **small subunit** and a **large subunit**; not surrounded by a membrane. - **80S** ribosomes (60S + 40S) in eukaryotes; **70S** (50S + 30S) in prokaryotes, mitochondria and chloroplasts. ## The Origins of Research on the Genetic Code - DNA first observed in the 1800s by Friedrich Miescher (named ‘nuclein’, 1869). - Many doubted DNA carried the genetic code because of its relatively simple chemical composition (only simple repeating nucleotides). - By 1953, experiments confirmed DNA carried the genetic code; the triplet code is universal and degenerate. - DNA’s location protected in the nucleus provides security; proteins in the cytoplasm are susceptible to hydrolysis. - 1953: Watson and Crick confirmed the double-helix structure using Rosalind Franklin’s X-ray data. ## The Process of Semi-Conservative Replication - Before a cell divides, DNA is copied so each daughter cell receives a full copy. - **Semi-conservative replication**: each new DNA molecule has one original strand and one newly synthesised strand. - Ensures genetic continuity; essential for replacing body cells and for growth and development. - Occurs during the **S phase of interphase**. - Steps: 1. **Helicase** unwinds the double helix by breaking hydrogen bonds between base pairs. 2. Two single polynucleotide strands act as templates. 3. Free nucleotides are attracted to exposed bases by base pairing. 4. **DNA polymerase** joins new nucleotides by catalysing condensation reactions. 5. Original and new strands join via hydrogen bonding between base pairs. - Free nucleotides exist as **nucleoside triphosphates** (activated nucleotides); extra phosphate groups provide energy. DNA polymerase removes two phosphate groups from each nucleotide to release energy used to form phosphodiester bonds. ## Calculating the Frequency of Nucleotide Bases - A pairs with T; C pairs with G. - Therefore frequency of adenine = frequency of thymine; frequency of cytosine = frequency of guanine. - Worked example: if 28% of bases are cytosine, then 28% are guanine (total 56%). This leaves 44% for A and T, so 22% adenine and 22% thymine. ## The Watson Crick Model - Watson and Crick proposed **semi-conservative replication**: each new DNA molecule contains one original and one newly synthesised strand, based on complementary base pairing. - The **conservative model** suggested the original DNA remains intact and the new DNA is made entirely of new strands. - **Meselson and Stahl** tested the theories using bacteria grown in heavy nitrogen (¹⁵N) then light nitrogen (¹⁴N). - After one round of replication, all DNA molecules contained both heavy and light nitrogen and settled in the middle of the centrifuge tube – supporting semi-conservative replication. - If conservative replication had occurred, original DNA would settle at the bottom and new DNA at the top. - Ratio of ¹⁵N:¹⁴N goes from 1:1 after first round, to 3:1 after second, to 7:1 after third. - The experiment proved Watson and Crick’s theory correct. - Evaluation: method could be replicated, well-controlled, produced observable bands, disproved conservative and dispersive models. However, only tested one organism (E. coli) and didn’t show how enzymes like helicase and polymerase work.

Nucleotide: The repeating unit of DNA and RNA, formed from a pentose sugar, a nitrogen-containing organic base and a phosphate group.
Phosphodiester bond: The bond formed by a condensation reaction between the phosphate group of one nucleotide and the pentose sugar of the next; it consists of a phosphate group and two ester bonds.
Sugar-phosphate backbone: The chain of alternating phosphate groups and pentose sugars produced by many phosphodiester bonds in DNA or RNA.
Antiparallel: The arrangement of the two DNA polynucleotide strands running in opposite directions (one 5′→3′, the other 3′→5′).
Complementary base pairing: The specific pairing of bases: adenine with thymine (two hydrogen bonds) and guanine with cytosine (three hydrogen bonds).
Double helix: The three-dimensional shape formed by DNA molecules, consisting of two antiparallel polynucleotide strands held together by hydrogen bonds.
Semi-conservative replication: The process of DNA replication where each new DNA molecule contains one original strand and one newly synthesised strand.
Helicase: The enzyme that unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide strands.
DNA polymerase: The enzyme that synthesises new DNA strands from the two template strands by catalysing condensation reactions to form the sugar-phosphate backbone and removing two phosphate groups from each nucleotide to release energy.
Ribosome: A small organelle that is the site of protein synthesis; formed from RNA and proteins, composed of a small and a large subunit, and not surrounded by a membrane.
rRNA: Ribosomal RNA; a specific type of RNA that forms part of the structure of ribosomes and has enzymatic properties that catalyse the formation of peptide bonds between amino acids.
Pentose sugar: A sugar with five carbon atoms; deoxyribose and ribose are both pentose sugars.
Polynucleotide: A polymer made up of many nucleotides joined together in a long chain; DNA and RNA are polynucleotides.
Nucleoside triphosphates: Activated nucleotides that exist as free nucleotides in the nucleus; the extra phosphate groups activate the nucleotides and provide energy for DNA replication.
Meselson and Stahl: Two scientists who tested theories of DNA replication using bacteria grown in heavy nitrogen (¹⁵N) followed by growth in light nitrogen (¹⁴N); their results supported the semi-conservative model.

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