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1. Genetic Materials Syllabus

Unit 3: Genetic Materials Notes

3. Genetics
3.1 Genetic Materials: Introduction to genetics and genetic materials, composition, structure and function of DNA and RNA, DNA replication, introduction of genetic code.
3.2 Mendelian genetics: General terminology, Mendel’s experiment and laws of inheritance, gene interactions (incomplete dominance, co-dominance).
3.3 Linkage and crossing over: Concept and types of linkage, sex-linked inheritance (colour blindness in man and eye colour of Drosophila), concept and significances of crossing over.
3.4 Mutation and polyploidy: Concept, type (gene and chromosomal mutation), importance of mutation, polyploidy (origin and significance).

These genetic materials notes follow the NEB Class 12 Biology syllabus (+2 Biology, Nepal), chapter 3.1 of Unit 3 Genetics. Practise the multiple choice questions first, then work through the solved short and long answer questions with their diagrams, and finish with the PDF notes for quick revision.

Micro syllabus: 3. Genetics

Define genetics and describe the composition and structure of DNA and RNA. (this chapter)
Describe the mechanism of DNA replication and the concept of the genetic code. (this chapter)
Explain Mendelian genetics, including complete and incomplete dominance.
Explain linkage, crossing over, and sex-linked inheritance.
Describe mutation, its importance, and the concept of polyploidy.

Read more about genetics, DNA, RNA and the genetic code on Wikipedia. More notes: Class 12 Biology notes, Class 12 Chemistry notes and Class 12 Physics notes.

2. Genetic Materials Notes: Multiple Choice Questions

Question 1. Name the nitrogenous base that is found in RNA but not in DNA?

  • a) Adenine
  • b) Guanine
  • c) Cytosine
  • d) Uracil

Correct Answer: d) Uracil

Reason: Uracil replaces thymine in RNA and pairs with adenine. DNA has thymine.

Question 2. What type of bond connects the sugar and phosphate in the RNA backbone?

  • a) Hydrogen bond
  • b) Ionic bond
  • c) Peptide bond
  • d) Phosphodiester bond

Correct Answer: d) Phosphodiester bond

Reason: The phosphate joins the 3′ carbon of one sugar to the 5′ carbon of the next sugar by a phosphodiester bond. Hydrogen bonds join the base pairs.

Question 3. Which of the following correctly pairs nitrogenous bases in DNA?

  • a) Adenine-Guanine and Cytosine-Thymine
  • b) Adenine-Thymine and Cytosine-Guanine
  • c) Adenine-Cytosine and Guanine-Thymine
  • d) Adenine-Uracil and Cytosine-Guanine

Correct Answer: b) Adenine-Thymine and Cytosine-Guanine

Reason: A pairs with T by two hydrogen bonds and C pairs with G by three. Uracil is not found in DNA.

Question 4. How many nucleotide bases form one codon in the genetic code?

  • a) 1
  • b) 2
  • c) 3
  • d) 4

Correct Answer: c) 3

Reason: The genetic code is a triplet code. Three bases make one codon, which codes for one amino acid.

Question 5. Which of the following statements about the genetic code is true?

  • a) It overlaps in codons
  • b) It is universal for all organisms
  • c) One codon specifies multiple amino acids
  • d) It has punctuation marks

Correct Answer: b) It is universal for all organisms

Reason: The same codon gives the same amino acid in almost all organisms, from bacteria to humans. The code is non-overlapping, unambiguous and punctuationless.

Question 6. Which of the following enzyme unwinds the DNA helix during replication?

  • a) DNA polymerase
  • b) Ligase
  • c) Helicase
  • d) Primase

Correct Answer: c) Helicase

Reason: Helicase breaks the hydrogen bonds between the bases and opens the helix into a replication fork.

Question 7. Which of the following best explains why one strand is synthesized continuously and the other discontinuously during replication?

  • a) Helicase works only one direction
  • b) DNA polymerase can only add nucleotides in the 5’ to 3’ direction
  • c) One strand has more A-T content
  • d) The lagging strand is longer

Correct Answer: b) DNA polymerase can only add nucleotides in the 5’ to 3’ direction

Reason: The two strands are antiparallel and polymerase works only in the 5′→3′ direction. So one strand is made in one stretch and the other in Okazaki fragments.

Question 8. Study the given figure of types of DNA replication. Identify the types A, B and C.

Class 12 Biology Unit 3 Genetic materials notes - figure for Question 8, types of DNA replication

Figure for Question 8

  • a) A – Conservative, B – Semi conservative, C- Dispersive
  • b) A – Semi conservative, B – Conservative, C – Dispersive
  • c) A – Consevative, B – Dispersive, C – Semi conservative
  • d) A – Dispersive, B – Semi conservative, C – Dispersive

Correct Answer: a) A – Conservative, B – Semi conservative, C- Dispersive

Reason: A gives one fully old and one fully new helix, B gives two hybrid helices (one old and one new strand each), and C gives mixed patches of old and new DNA.

Question 9. If a DNA molecule has 2000 base pairs, how long will the molecule be approximately?

  • a) 680 nm
  • b) 340 nm
  • c) 2000 nm
  • d) 34 nm

Correct Answer: a) 680 nm

Reason: Adjacent base pairs are 0.34 nm apart. So the length is 2000 × 0.34 nm = 680 nm.

Question 10. If a drug is developed that strengthens the hydrogen bonds between DNA base pairs, what cellular process would be most directly affected?

  • a) Formation of the sugar-phosphate backbone
  • b) Transcription and DNA replication
  • c) Transport of mRNA
  • d) Digestion of nucleotides in the cytoplasm

Correct Answer: b) Transcription and DNA replication

Reason: Both processes need the two strands to separate by breaking hydrogen bonds. The backbone is held by phosphodiester bonds, which the drug does not change.

3. Genetic Materials Notes: Short and Long Answer Questions

Short Answer Questions

Q.1 Define heredity and variation.

Heredity is the transmission of characters from one generation to the next generation, for example from parents to offspring. A dog always gives birth to a dog, and a watermelon seed always grows into a watermelon plant.

Variation is the difference shown by the individuals of a species, and also by the offspring of the same parents. Brothers and sisters look alike, but they are never exactly the same. Variations arise from mutation and from the recombination of genes in sexual reproduction.

The branch of biology that deals with the study of heredity and variation is called genetics.

Q.2 What are the three components of a nucleotide?

A nucleotide is the basic unit of the nucleic acids DNA and RNA. It has three components.

Nucleotide = Pentose sugar + Nitrogenous base + Phosphate group
Pentose sugar: a 5-carbon sugar. It is deoxyribose in DNA and ribose in RNA.
Nitrogenous base: a purine (adenine, guanine) or a pyrimidine (cytosine, thymine, uracil).
Phosphoric acid: the phosphate group that joins two nucleotides by a phosphodiester bond.

A sugar joined to a base is called a nucleoside. When a phosphate group is added, it becomes a nucleotide.

Q.3 Differentiate between purine and pyrimidine bases.
PurinePyrimidine
Has two rings in its structure (a 6-membered ring fused with a 5-membered ring).Has one ring in its structure (a 6-membered ring).
Larger in size.Smaller in size.
Adenine (A) and guanine (G).Cytosine (C), thymine (T) and uracil (U).
Both bases are present in DNA and RNA.Cytosine is present in both. Thymine is present in DNA only and uracil in RNA only.
Always pairs with a pyrimidine: A with T (or U), two hydrogen bonds.Always pairs with a purine: C with G, three hydrogen bonds.
Q.4 Compare the leading and lagging strands during DNA replication.
Leading strandLagging strand
Made continuously in one stretch.Made discontinuously, as short pieces called Okazaki fragments.
Synthesized in the 5′→3′ direction, towards the replication fork.Synthesized in the 5′→3′ direction, away from the replication fork.
Its template runs in the 3′→5′ direction.Its template runs in the 5′→3′ direction.
Needs only one RNA primer.Needs many RNA primers, one for each fragment.
DNA ligase is not needed to join pieces.DNA ligase joins the fragments after the primers are replaced by DNA.
Class 12 Biology Unit 3 Genetic materials notes - semi-conservative mode of DNA replication diagram

Figure: Genetic materials notes: semi-conservative mode of DNA replication

Q.5 If the DNA template strand is TACGGA, what will be the RNA sequence formed?

During transcription, RNA polymerase makes an RNA strand complementary to the DNA template strand. Adenine pairs with uracil (not thymine), and guanine pairs with cytosine.

DNA templateTACGGA
RNA formedAUGCCU

Answer: the RNA sequence is AUGCCU. The textbook gives the same rule: a DNA template CAATGA makes the RNA GUUACU.

Q.6 A scientist extracts a nucleic acid and finds that it contains uracil but no thymine, and ribose sugar is present. Identify the molecules. In which organisms might this serve as the genetic material?

Identification: the nucleic acid is RNA (ribonucleic acid). Uracil and ribose are found in RNA only, and DNA has thymine and deoxyribose.

Organisms: DNA is the genetic material of all organisms except RNA viruses. RNA serves as the genetic material in some viruses, for example the tobacco mosaic virus (TMV), influenza virus, SARS-CoV-2 and the retrovirus HIV.

Q.7 A student is given a nucleotide containing ribose sugar, adenine and phosphate. Is it a component of DNA or RNA? Justify your answer.

It is a component of RNA.

Justification: adenine and phosphate are present in both DNA and RNA, so the sugar decides. RNA has ribose, which has an OH group on the 2′ carbon. DNA has deoxyribose, which has only H at that place (Figure 3.1.1 of the textbook). A nucleotide of ribose, adenine and phosphate is adenosine monophosphate (AMP), a ribonucleotide.

Q.8 Study the given diagram and answer the following questions:

a) What is shown in the figure?

The figure shows DNA replication at a replication fork. The parental duplex is opened, the fork moves forward, and one new strand is made continuously while the other is made in pieces.

b) Label A, B and C.

A: leading strand. It is made continuously, towards the fork.
B: RNA primer (the short green segment at the 5′ end of a new piece).
C: lagging strand, the strand made in short pieces (Okazaki fragments).

c) Name the enzymes that add DNA nucleotide to the template.

DNA polymerase (DNA polymerase III in bacteria). It adds nucleotides to the 3′ end of the growing strand, so it works only in the 5′→3′ direction.

d) What is okazaki fragment?

Okazaki fragments are the short pieces of DNA made discontinuously on the lagging strand. Each one starts from an RNA primer. The primers are later replaced by DNA, and DNA ligase joins the pieces into one continuous strand.

Class 12 Biology Unit 3 Genetic materials notes - DNA replication fork figure for Q.8

Figure for Q.8

Long Answer Questions

Q.1 Describe the structure and function of DNA with necessary diagram.

DNA (deoxyribonucleic acid) is the genetic material in all organisms except RNA viruses. In prokaryotic cells it is found in the cytoplasm. In eukaryotic cells it is found mainly in the nucleus, and in small amounts in mitochondria and plastids. It is a big molecule made of many deoxyribonucleotides joined by phosphodiester bonds.

Structure of DNA. J. D. Watson (American biologist) and F. H. C. Crick (English chemist) designed the double helix model of DNA in 1953. The main characteristics are:

DNA has two strands that twist around each other to form a double helix.
The two strands run in opposite directions (antiparallel): one runs 5′ to 3′ and the other 3′ to 5′.
The double helix has a constant diameter of about 2 nm.
It has a major groove and a minor groove. One major and one minor groove make one complete turn.
One complete turn is about 3.4 nm long.
The sides of the helix are made of alternating deoxyribose sugar and phosphate groups. This is the backbone, and the bases project inside.
The bases of the two strands are paired by hydrogen bonds. Adenine pairs with thymine by two hydrogen bonds (A=T), and cytosine pairs with guanine by three hydrogen bonds (C≡G).
The distance between two base pairs is about 0.34 nm, so there are about 10 base pairs in one complete turn.
The amount of A equals T, and the amount of G equals C. So the total purine equals the total pyrimidine, (A+G)/(T+C) = 1. This is Chargaff’s rule.

Functions of DNA

It carries hereditary characters from parents to the young ones.
It copies its genetic information (replication) to pass it to the cells of the next generation.
Crossing over in DNA produces genetic recombination.
Changes in the components of DNA create variations and new species.
Transcription of DNA makes the different RNAs for cell functions.
It regulates cell metabolic reactions through RNA and protein synthesis (central dogma).
Class 12 Biology Unit 3 Genetic materials notes - structure of DNA diagram (Watson and Crick model)

Figure: Genetic materials notes: structure of DNA (Watson and Crick double helix)

Q.2 Describe the structure and function of RNA with necessary diagram.

RNA (ribonucleic acid) is present in all living cells. It is made in the nucleus but is found in the nucleolus, cytoplasm, mitochondria, plastids and ribosomes. It is a polymer of ribonucleotides.

Structure of RNA

Each ribonucleotide has ribose sugar, a nitrogenous base and phosphoric acid.
The bases are adenine, guanine, cytosine and uracil. Uracil is present instead of thymine.
RNA is single stranded. The single strand can fold on itself and coil into a pseudohelix, with A-U and G-C pairs in the folded part.
Like DNA, RNA has direction. One end is the 5′ end and the other is the 3′ end, according to the sugar-phosphate backbone.
The bases can be in any order, which is complementary to the DNA template. For example, a DNA template CAATGA gives the RNA GUUACU.

Functions of RNA

mRNA carries genetic information from DNA to the ribosomes for protein synthesis.
tRNA brings amino acids to the ribosome to build the protein according to the mRNA.
rRNA forms a part of the ribosome, where proteins are made.
RNA acts as the genetic material in certain viruses.
Some RNA molecules work as enzymes (ribozymes), which catalyse biochemical reactions such as the cleavage of RNA.
Class 12 Biology Unit 3 Genetic materials notes - Structure of RNA

Figure: Structure of RNA

Q.3 What are the three types of RNA? Describe them.

The three types of RNA are messenger RNA, ribosomal RNA and transfer RNA. All three are made from DNA by transcription and work together in protein synthesis.

Messenger RNA (mRNA): it carries the genetic information, as a sequence of codons, from DNA in the nucleus to the ribosomes in the cytoplasm. It is about 5 % of the cell RNA, and it is the longest but the shortest-lived RNA.
Ribosomal RNA (rRNA): a stable RNA linked with the ribosomes. It joins with proteins to form the two subunits of the ribosome and helps in protein synthesis. It is the most abundant RNA, about 80 % of the cell RNA.
Transfer RNA (tRNA): it acts as an adapter and carries amino acids to the site of protein synthesis. It is small (70 to 90 nucleotides) and folded into a clover-leaf shape. Its anticodon loop reads the mRNA codon. It is about 15 % of the cell RNA.
mRNArRNAtRNA
Carries the genetic message from DNA.Part of the ribosome.Carries amino acids.
Has codons.Gives the site for protein synthesis.Has an anticodon.
About 5 %.About 80 %.About 15 %.
Q.4 Define genetic code. Mention the characteristics of it.

The DNA carries genetic information from cell to cell and from generation to generation, and this information is expressed as protein. Proteins are made of 20 kinds of amino acids. A sequence of three bases codes for one amino acid, and it is called a codon.

Definition: the specific order of the nitrogenous bases in RNA that translates the message of DNA into amino acids is called the genetic code. It is a set of instructions that cells use to make proteins.

Characteristics of the genetic code

Triplet nature: three bases make a codon, and each codon specifies one amino acid. For example, AUG codes for methionine.
Non-overlapping: adjacent codons do not share any base. ACUAAG is read as ACU and AAG only.
Punctuationless: the code is continuous, with no punctuation marks between codons. CCUCAGC… is read as CCU, CAG, C…
Universal: a codon gives the same amino acid in almost all organisms, from bacteria to humans.
Degeneracy: there are 64 codons but only 20 amino acids, so many amino acids have more than one codon. Glycine has four codons: GGU, GGC, GGA and GGG.
Nonsense (terminator) codons: three of the 64 codons (UAA, UAG and UGA) do not code for any amino acid. They stop protein synthesis.
Initiation (start) codons: AUG and GUG begin the synthesis of protein.
Colinearity: the sequence of bases in DNA and mRNA is in the same order as the sequence of amino acids in the polypeptide. The DNA code words are complementary to the mRNA code words. For example, ATG GGA CTT in DNA is transcribed into AUG GGA CUU in mRNA.
Q.5 Describe the semi-conservative mode of DNA replication with appropriate diagram.

DNA replication is the process by which a double stranded DNA makes two identical copies of itself. It is needed in cell division, so that each new cell receives an exact copy of the genetic material.

Types of DNA replication

Conservative: one DNA molecule has only the parental strands and the other has only new strands.
Semi-conservative: each DNA molecule has one original strand and one new strand.
Dispersive: both molecules have patches of old and new DNA.
Class 12 Biology Unit 3 Genetic materials notes - types of DNA replication diagram

Figure: Genetic materials notes: types of DNA replication (conservative, semi-conservative, dispersive)

The semi-conservative mode is the correct one, proved by Meselson and Stahl (1958). Each daughter DNA is a hybrid, which keeps one parental chain and one new chain. The steps are as follows.

1. Initiation

The double helix unwinds at a fixed point called the origin of replication (ori site), and the separation then goes slowly to the other end. Viral and prokaryotic DNA usually has one origin, and eukaryotic DNA has many.
The enzyme helicase breaks the hydrogen bonds between the base pairs. Each strand now works as a template. The opened DNA has a Y shape called the replication fork.
The enzyme primase makes a short RNA segment, the RNA primer, on each strand. It gives the starting point for DNA polymerase.

2. Elongation

DNA polymerase adds DNA nucleotides to the 3′ end of the primer, following the base pairing rule. It works only in the 5′→3′ direction, because the two strands are antiparallel.
On one template, the new strand is made continuously towards the fork. This is the leading strand.
On the other template, the new strand is made in short pieces, the Okazaki fragments, in the direction away from the fork. This is the lagging strand.
The RNA primers are removed and replaced by DNA by DNA polymerase. The gaps are sealed by DNA ligase.

3. Termination. The replication ends when it reaches the end of the DNA molecule. Two DNA molecules are formed, each with one old and one new chain, and the new DNA coils into a double helix automatically.

Class 12 Biology Unit 3 Genetic materials notes - semi-conservative mode of DNA replication diagram

Figure: Genetic materials notes: semi-conservative mode of DNA replication

4. Genetic Materials Notes PDF

PDF Notes 1: Genetics (Unit 3)

PDF Notes 2: Genetic Materials (Chapter 3.1)

5. Genetic Materials: Frequently Asked Questions

What are genetic materials? (Genetic materials notes in simple words)
Genetic materials are the substances that pass the characters from parents to offspring. They are DNA in almost all organisms and RNA in some viruses. Both are large molecules made of a chain of nucleotides, and each nucleotide has a pentose sugar, a nitrogenous base and phosphoric acid.
Is genetic material DNA?
Yes, in all organisms except RNA viruses the genetic material is DNA. In some viruses, such as the tobacco mosaic virus, influenza virus, SARS-CoV-2 and HIV, RNA is the genetic material.
What are genetic materials examples and genetic materials list?
The genetic materials list has two nucleic acids. DNA (deoxyribonucleic acid) is the genetic material of bacteria, plants, animals and most viruses. RNA (ribonucleic acid) is the genetic material of RNA viruses like TMV, influenza virus and HIV. The three types of RNA, mRNA, tRNA and rRNA, work in protein synthesis and are not genetic material in cells.
What is the genetic material structure?
The genetic material DNA is a double helix of two antiparallel polynucleotide strands. The backbone is made of deoxyribose sugar and phosphate, and the bases are paired inside by hydrogen bonds: A with T (two bonds) and C with G (three bonds). The helix is about 2 nm wide, and one turn of 3.4 nm has about 10 base pairs. RNA is mostly a single strand with ribose sugar and uracil in place of thymine.
Where are the genetic materials in humans and in the cell?
In humans and other eukaryotes, DNA is found mainly in the nucleus, where it forms chromosomes, and a small amount is present in the mitochondria. In plant cells a small amount is also in the plastids. In prokaryotic cells, DNA is found in the cytoplasm. RNA is made in the nucleus and is found in the nucleolus, cytoplasm and ribosomes.
What are genetic materials pdf and genetic materials ppt?
The genetic materials pdf of this chapter is in the PDF section of this page, and you can read it online. For a genetic materials ppt, the diagrams and answers on this page can be used to make your own slides for study.
What is genetics in biology (genetics biology) and what are genetics traits?
Genetics is the branch of biology that deals with heredity and variation. A trait is a character that is inherited from parents, for example eye colour, height, hair type and blood group. Genetics examples include a dog giving birth to a dog, a watermelon seed growing into a watermelon plant, and siblings who look similar but not the same.
What is the importance of genetics and genetics in medicine?
The importance of genetics includes: understanding how traits pass from parents to children, finding the cause of genetic diseases, genetic testing and counselling, making medicines and vaccines, improving crop and animal breeds, forensic identification with DNA, and studying evolution. In medicine, genetics is used to diagnose inherited disorders and to design treatment for the individual patient.
Where can I get genetics pdf, genetics ppt and a genetics website?
This page has two PDF notes that you can read online: the PDF notes of Unit 3 Genetics and the PDF notes of the chapter Genetic Materials. For a genetics ppt, you can use the diagrams and answers of this page to make slides. For more notes of the subject, the Class 12 Biology page of this website is the genetics website to use for the NEB syllabus.
What is the difference between DNA and RNA?
DNA has deoxyribose sugar, the bases A, G, C and T, two strands in a double helix, and stores the genetic information. RNA has ribose sugar, the bases A, G, C and U, usually one strand, and helps in protein synthesis. RNA is of three types: mRNA, tRNA and rRNA.
What is the genetic code and how is DNA replicated?
The genetic code is the order of bases in RNA that decides the order of amino acids in a protein. Three bases make a codon, and the code is triplet, non-overlapping, punctuationless, universal and degenerate. DNA replicates by the semi-conservative method: helicase opens the helix, primase makes RNA primers, DNA polymerase adds nucleotides, and DNA ligase joins the Okazaki fragments of the lagging strand.
Where can I get Class 12 Biology Book PDF Nepal and Class 12 Biology Book PDF Asmita Publication?
The Class 12 Biology book of Asmita Publication is a textbook of its publisher, and this page does not claim to host it. These notes follow the NEB Class 12 Biology syllabus of Unit 3 Genetics, solve the exercise of the chapter Genetic Materials and add diagrams, so you can use them together with the textbook.
Where can I get Class 12 biology notes, Class 12 biology pdf and Class 12 Biology Notes NEB for this chapter?
This page gives the Class 12 biology notes NEB of the chapter Genetic Materials (chapter 3.1 of Unit 3): syllabus, micro syllabus, solved multiple choice questions, short and long answers and the notes PDF. The Class 12 biology notes pdf of the other units are on the Class 12 Biology page of the website.
Where can I get Class 12 Biology notes in the style of Note Mandir?
Students who look for notes in the style of Note Mandir will find this chapter in the same way: syllabus, micro syllabus, solved exercise with diagrams and PDF notes. These are NEB Class 12 Biology notes for Unit 3, chapter Genetic Materials.

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