In this chapter, we learn how characteristics are passed from parents to offspring and how living organisms have changed slowly over millions of years. This chapter connects biology with real life by explaining why we look like our parents and how humans evolved. It is an important chapter for the SSC board exam because it includes definitions, theory questions, and long answers. If you understand the concepts clearly, this chapter becomes logical and interesting.
Table of Contents
Important Concepts of the Chapter
Heredity and Hereditary Changes
Heredity is the transfer of biological characters from parents to their offspring. These characters are transferred through genes.
Genes are present on chromosomes inside the nucleus of a cell and control traits like height, skin colour, eye colour, etc.
Sometimes, sudden changes occur in genes. These changes are called hereditary changes and they play an important role in evolution.
Genetic Material and DNA
DNA is the genetic material present in most living organisms.
It carries all the information required for the structure and functioning of the body.
In simple words, DNA acts like an instruction manual that decides how proteins are formed in the body and how traits are expressed.
Central Dogma: Transcription, Translation and Translocation
Central Dogma
The flow of genetic information in the cell is called the Central Dogma.
It states that:
DNA → RNA → Protein
- These proteins control the structure and functioning of the body.
- DNA stores the information for protein synthesis.
- Proteins are synthesized by DNA through RNA.
1) Transcription
The process of synthesis of RNA using DNA as a template is called transcription.
Explanation:
- Genes are present in the form of DNA.
- During transcription, mRNA (messenger RNA) is produced as per the sequence of nucleotides on DNA.
- Only one of the two strands of DNA is used for RNA synthesis.
- The nucleotide sequence of mRNA is complementary to the DNA strand used.
- In RNA, uracil (U) is present instead of thymine (T) of DNA.
👉 Thus, transcription results in the formation of mRNA carrying the coded message for protein synthesis.
2) Translation
The process of synthesis of protein with the help of mRNA is called translation.
Explanation:
- The mRNA formed in the nucleus enters the cytoplasm.
- It carries the coded message from DNA.
- The message contains codes for amino acids.
- Each amino acid is coded by a group of three nucleotides, called a triplet codon.
Triplet Codon:
- Each codon consists of three nucleotides.
- Each codon specifies one amino acid.
- Dr. Har Gobind Khorana discovered the triplet codons for 20 amino acids.
- He was awarded the Nobel Prize in 1968 for this work.
Role of tRNA and rRNA:
- tRNA (transfer RNA) brings specific amino acids to the ribosome.
- tRNA has an anticodon, which has a complementary sequence to the codon on mRNA.
- Amino acids are joined together by peptide bonds.
- This bonding is helped by rRNA (ribosomal RNA) present in ribosomes.
👉 In this way, amino acids form a polypeptide chain, which later forms a protein
3) Translocation
The movement of ribosome on mRNA from one triplet codon to the next during protein synthesis is called translocation.
Explanation:
- During translation, the ribosome moves from one end of mRNA to the other end.
- It moves by a distance of one triplet codon at a time.
- This step-by-step movement is called translocation.
- As the ribosome moves, more amino acids are added to the growing polypeptide chain.
Mutation
Mutation is a sudden change in the nucleotide sequence of a genes.
Some mutations are harmless, while others can cause genetic disorders like sickle cell anaemia.
Mutations create variation, which is essential for evolution.
Evolution
Evolution is the gradual change occurring in living organisms over a long period of time.
It results in the formation of new species from existing ones.
Evolution explains the diversity of plants and animals present on Earth today.
Evidences of Evolution
Evolution is supported by several evidences:
Morphological Evidences
Similarity in external structures (like mouth, eyes, nostrils, ears and hairs) of organisms indicates common ancestry.
Anatomical Evidences
Similarity in internal structures, such as bones of forelimbs in humans, whales, and bats, suggests common origin.
Vestigial Organs
Vestigial organs are underdeveloped or useless organs. Examples include appendix, wisdom teeth, and tail bone in humans.
Palaeontological Evidences (Fossils)
Fossils are preserved remains of ancient organisms. They help us understand evolutionary history.
Embryological Evidences
Early embryos of different vertebrates show similarities, indicating common ancestry.
Connecting Links
Organisms showing characteristics of two different groups are called connecting links, such as duck-billed platypus.
Darwin’s Theory of Natural Selection
According to Charles Darwin, organisms produce more offspring than required. Due to limited resources, there is a struggle for survival.
Only those organisms that are best adapted survive and reproduce. This principle is called survival of the fittest.
Although some objections were raised, Darwin’s theory is a milestone in evolutionary biology.
Lamarckism
Jean-Baptiste Lamarck proposed that organisms acquire characters during their lifetime due to use or disuse of organs, and these acquired characters are passed on to the next generation.
For example, Lamarck explained the long neck of giraffe based on continuous stretching.
However, inheritance of acquired characters was later disproved.
Speciation
Speciation is the formation of new species due to genetic variation, geographical isolation, and reproductive isolation.
Over time, isolated populations change so much that they become a new species.
Human Evolution
Human evolution explains how modern humans evolved from primitive ancestors over millions of years.
Important stages include Ramapithecus, Australopithecus, Neanderthal man, and Homo sapiens.
Development of erect posture, use of hands, and increase in brain size were major steps in human evolution.
Important Laws / Principles
Principle of Natural Selection
Nature selects organisms that are best suited to their environment. These organisms survive, reproduce, and pass on their traits.
This principle explains adaptation and evolution.
Important Definitions
- Heredity – Transfer of characters from parents to offspring.
- Gene – Functional unit of heredity present on chromosomes.
- Mutation – Sudden change in genetic material.
- Evolution – Gradual change in organisms over a long period.
- Vestigial Organs – Useless or reduced organs present in organisms.
- Speciation – Formation of new species from existing ones.
Numerical / Process Explanation
This chapter does not include numerical problems, but students should clearly understand:
- Steps of protein synthesis
- Difference between transcription and translation
- How mutations lead to variation
- Logical flow of evolution from variation to speciation
These concepts are often asked as process-based theory questions.
Important Points for SSC Board Exam
- Definitions carry 1–2 marks
- Central dogma is frequently asked
- Differences between Darwinism and Lamarckism are important
- Fossils and vestigial organs are common long-answer topics
- Human evolution is asked as a descriptive question
- Diagrams related to DNA and evolution may be asked
Quick Revision Summary
- Heredity transfers traits through genes
- DNA is the genetic material
- Transcription forms mRNA
- Translation forms proteins
- Mutation causes variation
- Evolution is gradual and continuous
- Fossils prove evolution
- Natural selection leads to survival of the fittest
- New species are formed by speciation
Understanding heredity and evolution helps students connect biological concepts with real-life examples and prepares them well for SSC board examinations.