Protein Synthesis & DNA
– Exam-Ready Biology
Notes
A-Level Notes with Diagrams,
Definitions, and Model Answers
Notes by Iqra Ansari
,PART 1:
FOUNDATIONS OF DNA AND GENE EXPRESSION
1. Introduction to Genetics
Genetics is the branch of biology that deals with the study of heredity and variation. It
explains how traits such as eye colour, height, blood group, and even certain diseases are
passed from parents to offspring. Every living organism carries a set of instructions that
controls how it grows, develops, and functions. These instructions are stored in a chemical
substance called DNA.
Without genetics, life would not be organized or predictable. Cells would not know how to
divide, grow, or perform specific functions. Genetics provides the foundation for
understanding how life continues from one generation to the next.
2. DNA as the Genetic Material
DNA, or deoxyribonucleic acid, is the molecule responsible for carrying genetic
information. Scientists discovered that DNA, and not protein, is the true hereditary material
after several experiments conducted in the twentieth century. These experiments showed
that DNA contains all the information needed to control the structure and activities of cells.
DNA is extremely important because it has three key abilities. First, it can store
information in the form of sequences of bases. Second, it can replicate, meaning it can
make exact copies of itself. Third, it can control protein synthesis, which directly affects
cell structure and function.
3. Relationship Between DNA, Genes, and Chromosomes
DNA does not exist freely inside the cell. Instead, it is organized into structures called
chromosomes. Each chromosome is made up of a long DNA molecule tightly coiled around
proteins. A gene is a specific segment of DNA that contains instructions for making a
particular protein.
In simple terms, chromosomes are made of DNA, and genes are small functional units
located on that DNA. Each gene carries information that contributes to a specific trait or
cellular function.
4. Location of DNA in Cells
The location of DNA depends on the type of cell. In eukaryotic cells, such as plant and
animal cells, DNA is enclosed within the nucleus. This protects the genetic material and
allows complex regulation of gene expression. Small amounts of DNA are also present in
mitochondria and chloroplasts.
,In prokaryotic cells, such as bacteria, DNA is not enclosed in a nucleus. Instead, it is
found in the cytoplasm in a region called the nucleoid. This difference affects how genetic
processes like replication and transcription occur in different organisms.
5. Chemical Composition of DNA
DNA is made up of repeating units called nucleotides. Each nucleotide consists of three
components: a deoxyribose sugar, a phosphate group, and a nitrogenous base. The sugar
and phosphate form the backbone of the DNA molecule, while the bases carry genetic
information.
These nucleotides join together to form long chains, creating the DNA strand. The order of
the bases determines the information stored in DNA.
6. Nitrogenous Bases and Their Types
There are four nitrogenous bases found in DNA: adenine, thymine, guanine, and cytosine.
These bases are divided into two groups based on their structure: purines and pyrimidines.
Adenine and guanine are purines, while cytosine and thymine are pyrimidines.
The specific arrangement of these bases forms the genetic code. Even a small change in base
sequence can lead to significant changes in protein structure and function.
7. Complementary Base Pairing
The nitrogenous bases in DNA pair in a specific way known as complementary base pairing.
Adenine always pairs with thymine, while guanine pairs with cytosine. These pairs are held
together by hydrogen bonds.
This pairing is extremely important because it allows DNA to replicate accurately. When
one strand serves as a template, the complementary strand can be formed easily using base-
pairing rules.
8. Structure of DNA: The Double Helix
Deoxyribonucleic acid (DNA) is the molecule that stores genetic information in all living
organisms. Its structure is often described as a double helix, resembling a twisted ladder.
The “sides” of this ladder are made of sugar-phosphate backbones, while the “rungs”
are composed of pairs of nitrogenous bases.
Each DNA strand has a directionality, with one end labelled 5′ (five prime) and the other 3′
(three prime). The sugar-phosphate backbone is formed by alternating deoxyribose sugar
molecules and phosphate groups, which provide stability and structural support to the
molecule. The backbone is on the outside of the helix, while the bases are on the inside.
, The nitrogenous bases consist of adenine (A), thymine (T), guanine (G), and
cytosine (C). These bases pair specifically: adenine always pairs with thymine via two
hydrogen bonds, and guanine always pairs with cytosine via three hydrogen
bonds. This specific pairing, known as complementary base pairing, ensures accurate
replication of genetic information during cell division.
The two strands of DNA are antiparallel, meaning one strand runs 5′→3′ while the other
runs 3′→5′. This orientation is essential for DNA replication and for enzymes like DNA
polymerase to function correctly.
The double helix is right-handed, with the two strands twisting around each other in a
clockwise direction. The helical structure provides compact storage of genetic material,
protects it from damage, and allows it to interact efficiently with proteins during replication,
transcription, and repair processes.
This diagram (refer to the uploaded image) clearly shows all components:
• The sugar-phosphate backbone on each side
• The hydrogen bonds connecting the bases in the center
• The complementary base pairs (A–T and G–C)
• The antiparallel nature of the two strands (5′ and 3′ ends)
Understanding the detailed structure of DNA is fundamental because it forms the basis for
all subsequent molecular biology processes such as replication, transcription, and
translation.
9. Hydrogen Bonds and Stability of DNA
– Exam-Ready Biology
Notes
A-Level Notes with Diagrams,
Definitions, and Model Answers
Notes by Iqra Ansari
,PART 1:
FOUNDATIONS OF DNA AND GENE EXPRESSION
1. Introduction to Genetics
Genetics is the branch of biology that deals with the study of heredity and variation. It
explains how traits such as eye colour, height, blood group, and even certain diseases are
passed from parents to offspring. Every living organism carries a set of instructions that
controls how it grows, develops, and functions. These instructions are stored in a chemical
substance called DNA.
Without genetics, life would not be organized or predictable. Cells would not know how to
divide, grow, or perform specific functions. Genetics provides the foundation for
understanding how life continues from one generation to the next.
2. DNA as the Genetic Material
DNA, or deoxyribonucleic acid, is the molecule responsible for carrying genetic
information. Scientists discovered that DNA, and not protein, is the true hereditary material
after several experiments conducted in the twentieth century. These experiments showed
that DNA contains all the information needed to control the structure and activities of cells.
DNA is extremely important because it has three key abilities. First, it can store
information in the form of sequences of bases. Second, it can replicate, meaning it can
make exact copies of itself. Third, it can control protein synthesis, which directly affects
cell structure and function.
3. Relationship Between DNA, Genes, and Chromosomes
DNA does not exist freely inside the cell. Instead, it is organized into structures called
chromosomes. Each chromosome is made up of a long DNA molecule tightly coiled around
proteins. A gene is a specific segment of DNA that contains instructions for making a
particular protein.
In simple terms, chromosomes are made of DNA, and genes are small functional units
located on that DNA. Each gene carries information that contributes to a specific trait or
cellular function.
4. Location of DNA in Cells
The location of DNA depends on the type of cell. In eukaryotic cells, such as plant and
animal cells, DNA is enclosed within the nucleus. This protects the genetic material and
allows complex regulation of gene expression. Small amounts of DNA are also present in
mitochondria and chloroplasts.
,In prokaryotic cells, such as bacteria, DNA is not enclosed in a nucleus. Instead, it is
found in the cytoplasm in a region called the nucleoid. This difference affects how genetic
processes like replication and transcription occur in different organisms.
5. Chemical Composition of DNA
DNA is made up of repeating units called nucleotides. Each nucleotide consists of three
components: a deoxyribose sugar, a phosphate group, and a nitrogenous base. The sugar
and phosphate form the backbone of the DNA molecule, while the bases carry genetic
information.
These nucleotides join together to form long chains, creating the DNA strand. The order of
the bases determines the information stored in DNA.
6. Nitrogenous Bases and Their Types
There are four nitrogenous bases found in DNA: adenine, thymine, guanine, and cytosine.
These bases are divided into two groups based on their structure: purines and pyrimidines.
Adenine and guanine are purines, while cytosine and thymine are pyrimidines.
The specific arrangement of these bases forms the genetic code. Even a small change in base
sequence can lead to significant changes in protein structure and function.
7. Complementary Base Pairing
The nitrogenous bases in DNA pair in a specific way known as complementary base pairing.
Adenine always pairs with thymine, while guanine pairs with cytosine. These pairs are held
together by hydrogen bonds.
This pairing is extremely important because it allows DNA to replicate accurately. When
one strand serves as a template, the complementary strand can be formed easily using base-
pairing rules.
8. Structure of DNA: The Double Helix
Deoxyribonucleic acid (DNA) is the molecule that stores genetic information in all living
organisms. Its structure is often described as a double helix, resembling a twisted ladder.
The “sides” of this ladder are made of sugar-phosphate backbones, while the “rungs”
are composed of pairs of nitrogenous bases.
Each DNA strand has a directionality, with one end labelled 5′ (five prime) and the other 3′
(three prime). The sugar-phosphate backbone is formed by alternating deoxyribose sugar
molecules and phosphate groups, which provide stability and structural support to the
molecule. The backbone is on the outside of the helix, while the bases are on the inside.
, The nitrogenous bases consist of adenine (A), thymine (T), guanine (G), and
cytosine (C). These bases pair specifically: adenine always pairs with thymine via two
hydrogen bonds, and guanine always pairs with cytosine via three hydrogen
bonds. This specific pairing, known as complementary base pairing, ensures accurate
replication of genetic information during cell division.
The two strands of DNA are antiparallel, meaning one strand runs 5′→3′ while the other
runs 3′→5′. This orientation is essential for DNA replication and for enzymes like DNA
polymerase to function correctly.
The double helix is right-handed, with the two strands twisting around each other in a
clockwise direction. The helical structure provides compact storage of genetic material,
protects it from damage, and allows it to interact efficiently with proteins during replication,
transcription, and repair processes.
This diagram (refer to the uploaded image) clearly shows all components:
• The sugar-phosphate backbone on each side
• The hydrogen bonds connecting the bases in the center
• The complementary base pairs (A–T and G–C)
• The antiparallel nature of the two strands (5′ and 3′ ends)
Understanding the detailed structure of DNA is fundamental because it forms the basis for
all subsequent molecular biology processes such as replication, transcription, and
translation.
9. Hydrogen Bonds and Stability of DNA
