Heredity and Evolution Science Notes
Important Points :
- The chromosomes made up of nucleic acids and proteins, present in the nucleus of the cell are the components that carry hereditary characters in living organisms.
- The process of transfer of physical and mental characters from parents to the progeny is called inheritance or heredity.
- DNA molecule is made up of two helical strands consisting of deoxyribose sugar, phosphoric acid and
- pairs of nitrogenous bases.
- These three together is called a nucleotide.
Heredity and hereditary changes :
→ Heredity : The transfer of biological characters from one generation to the next one with the help of genes is called heredity.
→ History of genetics:
|Year||Scientist||Study topic||Discovery / Contribution|
|Pioneer of the modern genetics||Series of experiments on pea plant to explain inheritance of characters.|
|1901||Hugo de Vries||Mutational theory||Reasons behind the sudden changes.|
|1902||Walter, Sutton||Study of chromosomes||Paired chromosomes in grasshopper cells.|
|1944||Oswald Avery, Mclyn McCarthy and Colin MacLeod||DNA||All living organisms have genetic material in the form of DNA (except viruses).|
|1961||Francois Jacob and Jack Monod||Protein synthesis||A model for process of protein synthesis with the help of DNA in bacterial cells.|
→ After the process of protein synthesis was discovered …
- Genetic codes hidden in DNA were understood.
- Field of genetic engineering was developed.
- Emergence of the technique of recombinant DNA technology.
→ The benefits of science of heredity :
- Diagnosis of hereditary disorders.
- Treatment of incurable hereditary disorders.
- Prevention of hereditary disorders.
- Production of hybrid varieties of animals and plants.
- Use of microbes in industrial processes.
→ The purines are of two types, viz. adenine and guanine and pyrimidines are of two types viz, cytosine and thymine. The adenine always pairs with thymine with double hydrogen bonds while cytosine always pairs with guanine with triple hydrogen bond. The helices remain bound due to these hydrogen bonds.
→ DNA: DNA molecule is a double helix consisting of two strands. Each strand of this helix is made up of nucleotides. Each nucleotide is made up of a phosphoric acid, a deoxyribose sugar and a nitrogenous base. Nitrogenous bases are of two types, viz. purines and pyrimidines.
→ RNA : RNA is nucleic acid taving single strand of ribonucleotides. Each ribonucleotide is made up of ribose sugar, phosphate molecules and a nitrogenous base. There are four types of nitrogenous bases, viz. adenine, guanine, cytosine and uracil. RNA is found both in nucleus as well as in cytoplasm.
→ According to the functions RNA is of the following three main types: mRNA, RNA and tRNA
→ mRNA carries the information for protein synthesis from genes on DNA chain in nucleus to the ribosomes in the cytoplasm. Therefore, it is called messenger RNA.
→ rRNA is the component of the ribosome. It helps in protein synthesis.
→ tRNA is present in the cytoplasm. According to the message of the mRNA, it carries the specific amino acid up to the ribosomes as per the message coded on mRNA
→ Genetic disorders are caused due to abnormalities in chromosomes and mutations in genes. Some important causes of genetic disorders are as follows:
- Increase or decrease in number of chromosomes causing numerical change.
- Deletion of any part of the chromosome
- Translocation of chromosomes.
- Sudden change or mutation occurring in a normal gene, turning it into a defective gene.
- Mutations in more than one gene at the same time causing polygenic disorder.
→ Examples: Disorders due to numerical changes in the chromosomes :
- Down’s syndrome.
- Turner’s syndrome
- Klinefelter’s syndrome.
→ Monogenic disorders caused due to mutations: Hutchinson’s disease. Tay-Sachs disease, galactosaemia, phenylketonuria, sickle cell anaemia, cystic fibrosis, albinism, haemophilia, night blindness, etc.
→ Polygenic disorders: Cleft lip, cleft palate, constricted stomach, spina bifida (a defect of the spinal cord), etc. are polygenic disorders. Diabetes, blood pressure, heart disorders, asthma, obesity are also polygenic disorders.
Transcription, Translation and Translocation :
- Synthesis of mRNA as per the nucleotide sequence present on the DNA molecule, is called the process of transcription.
- The nucleotide sequence present in the DNA molecule is called gene. Genes control the structure and functioning of cells of the body.
- Information required for the synthesis of proteins is stored in the genes i.e. in the nucleotide sequences of DNA. The proteins are synthesised according to the need of the body.
- ‘Central Dogma’ : Synthesis of proteins by DNA through the RNA is called central dogma.
- DNA Transcription RNA Translation Protein
- Genetic information is thus used from DNA to RNA, then through RNA for protein synthesis.
- mRNA is produced according to the sequence of nucleotides on DNA.
- During transcription only one of the two strands of DNA is used.
- The sequence of nucleotides present on DNA strand gets copied in mRNA. Hence there is always complementary sequence produced on the new mRNA molecule.
- RNA molecules have uracil instead of thymine present in DNA.
- Thus by transcription mRNA molecule which is complementary to DNA is produced.
→ Role of different RNA in protein synthesis :
Triplet codon :
→ The code for each amino acid consisting of three nucleotides, is called ‘triplet codon’.
mRNA formed in nucleus brings the coded message from DNA when it comes in cytoplasm. This message contains the codes for amino acids.
→ Dr. Har Govind Khorana, made an important contribution in discovery of triplet codons for 20 amino acids. He was awarded the Nobel Prize in 1968 for this work, along with two other scientists.
→ Thousands of triplet codons are present in each mRNA molecule. As per the message on mRNA amino acids are supplied by the tRNA.
→ Translation: As per the codon on mRNA, the tRNA molecule with complementary ‘anticodon’ is brought near mRNA, this process is called translation’.
→ Formation of peptide bonds: Every tRNA brings specific amino acid. These individual amino acids are joined together by peptide bonds with the help of rRNA.
→ Translocation: The ribosome keeps on moving from one end of mRNA to other end by the distance of one triplet codon. This process is ‘translocation’.
→ Many such polypeptide chains come together to form different and complex proteins. The proteins are essential for controlling various functions of body of living organisms.
→ Sudden change occurring in the genetic material is known as mutation.
→ Due to transmission of parental genes to offspring, there is remarkable similarity between parents and their offspring. But if there is mutation in any nucleotide then there are changes in the characters of the offspring. Mutations are of two types, viz. minor and major. Minor mutations can also bring about considerable changes. E.g.
→ Genetic disorders like sickle cell anaemia is caused due to mutation. Mutation is an everlasting process which leads to the process of evolution. It also offers proof for Darwin’s theory of natural selection. Appendix of human digestive system is a functionless or vestigial organ, Wisdom teeth are not useful for chewing the food.
→ Dinosaur could not adapt to the environment in which they were trying to survive. There was beginning of ice age which resulted in scarcity of food. The starvation was one of the reasons for extinction. According to one theory, a meteor collided with the earth and this resulted in catastrophic death of the dinosaurs.
→ There is continuous and rapid change in the environment which is causing threat to the natural habitats of the animals and birds. Pollution, climate change, increasing urbanization etc. are the factors causing depletion of food and shelter of animals and birds. Moreover, hunting and poaching has also resulted in extinction of many species.
- Evolution : The gradual change occurring in living organisms over a long duration is called evolution. Evolution results in the development of organisms.
- Transformations ranging from changes in the stars and planets, to the changes in biosphere on the earth are studied in field of evolution.
- Due to evolution, there is formation of new species. Due to natural selection, there are continuous changes in specific characters of several generations of living organisms.
- Approximately 3.5 billion years ago, life was not present on the Earth.
→ The living organisms have been developed according to the following phases:
- Simple elements
- Organic and inorganic compounds
- Complex Organic compounds such as proteins and nucleic acids
- Mixture of different types of organic and inorganic compounds.
- First primitive type of cells
- Processes to take up surrounding chemicals developed leading to growth of cells in numbers →
- First living organism those organisms that could adjust and adapt to the surrounding conditions, survived and grew.
- This is according to the principle of natural selection.
- Some of them that could not adjust, perished.
- Ranging diversity in animals : From unicellular amoeba and paramoecium to giant whale and man.
- Ranging diversity in plants : From unicellular Chlorella to the huge banyan tree.
- Existence of life on Earth: From equator t both the poles on the entire earth, living organisms are seen in air, water, land, rock, etc.
- Theory of “Gradual development of living organisms’ is accepted worldwide.
→ Theory of Evolution :
- First living material is the protoplasm which was formed in the ocean.
- This gave rise to unicellular organisms in gradual time.
- Over the span of approximately 300 crore years, very gradual changes occurred in the unicellular organisms which slowly evolved into larger and more complex life forms.
- All round and multi-dimensional changes occurred which resulted in evolution of different types of organisms.
- Organizational evolution : The progressive development of plants and animals over a very large period of time is called organizational evolution.
- Evolution : Development of living organisms from the ancestors having different structural and functional organization is called evolution,
Evidences of evolution :
→ Evolution is an everlasting process of changes. The proofs to support this process are called evidences of evolution which are of following types:
→ Morphological Evidences :
- Similarities in the external and visible features are called morphological evidences.
- In animals, similarities in the structures such as mouth, nostrils and ear pinnae, position of eyes, thickly distributed body fur or hair.
- In plants, similar characters such as leaf shape, leaf venation, leaf petiole, etc.
- These similar features indicate that origin of different animal or plant groups must be the same having a common ancestor.
→ Anatomical Evidences :
- Human hand, forelegs of bull, flipper of whale and patagium of bat show difference in their features.
These organs also have different uses.
- However, the structure of bones and joints in organs of each of those animals show similarity.
- This is an indication that all these animals originated from a common ancestor Higher organisms show different body organs and systems that carry out specific functions.
- Each system consists of definite organs.
- In the thoracic cavity, there are lungs and heart. In skull, brain is located. In abdominal cavity there is stomach, intestine, liver, kidney, etc.
→ Each important vital organ performs specific function which is necessary for survival. Brain controls and coordinates all the vital activities. Heart is responsible for the circulation of blood throughout the body. Lungs perform respiration.
→ Kidneys filter out nitrogenous waste products from the blood. In this way, each organ performs its own function. Only few organs such as appendix do not perform any function.
→ Vestigial Organs :
- Vestigial organs : The degenerated or underdeveloped organs which cannot perform any function are known as vestigial organs.
- Existing organs undergo gradual changes during evolutionary process. Thus, they form new tissues, organs, etc.
- Under certain environmental conditions such changes are necessary. However, some structures become useless in newly changed conditions.
- Due to natural selection, such structures undergo degeneration and after a very long time span they vanish.
- Appendix may be useful for grazing, ruminating herbivores but it is vestigial for man.
- Other vestigial organs in human body are tail bone (coccyx), muscles of ear pinna, wisdom teeth, and body hairs.
Paleontological Evidences :
→ Fossil: Fossils are remnants of living organisms or their impressions which are preserved deep down in the earth’s surface. Various natural calamities buried organisms in this way. Fossils throw light on the evolutionary process as they offer direct evidence of evolution.
→ Carbon dating : Carbon dating is a technique to understand the age of a fossil. When alive, animals and plants consume carbon continuously but this process ends after their death. Later their body carbon in the form of C-14 undergoes continuous decaying process. C-12 is not radioactive. Therefore, the ratio between C-14 and C-12 changes continuously.
→ By calculating the following three aspects, the age of the fossil can be determined by knowing I. The time passed since the death of a plant or animal II. Measurement of the radioactivity of C-14. III. The ratio of C-14 to C-12 present in the body.
→ Uses of carbon dating :
- Study of palaeontology
- Understanding anthropology
- Determining the age of human fossils and manuscripts.
- Calculating the age of fossils and placing them in geological time scale
- Deducing the information about other former organisms. E.g. Invertebrates gave rise to vertebrates gradually during evolution.
→ Willard Libby : Willard Libby developed the technique of carbon dating method which is based upon the radioactive decay of naturally occurring C-14. For this contribution he was awarded the Nobel Prize in 1960. In the journal ‘Radio Carbon’ the data about the ages of the materials determined is compiled and published.
- Connecting Links : Some organisms share morphological characters of two groups and hence they are called ‘connecting links’.
- Peripatus : It shows characters of Annelida and Arthropoda and thus it is the connecting link between these two phyla.
- Annelid characters : Segmented body, thin cuticle, and parapodia-like organs.
- Arthropod characters : Tracheal respiration and open circulatory system.
- Duck billed platypus : Duck billed platypus is the connecting link between Reptiles and Mammals.
- Reptilian characters : Egg laying habit, scales on body.
- Mammalian characters : Presence of mammary glands and hair.
- Lung fish : Lung fish is the connecting link between fishes and amphibians.
- Fish characters : Fish like body.
- Amphibian characters : Respiration with lungs. The above connecting links prove that arthropods evolved from annelids, amphibians evolved from fish and mammals evolved from reptiles.
- Embryological Evidences : All vertebrate embryos show extreme similarities during initial stages of development. These similarities disappear gradually in later development.
- This initial similarity indicates that there may be a common origin of all these animals.
Darwin’s theory of natural selection :
- Charles Darwin : (1809-1882) proposed the theory of natural selection. This theory is an important milestone in the study of evolution.
- Theory of natural selection : “The survival of fittest’, i.e. organisms which are fit for survival, evolve while those that are not perish. The natural selection thus acts to produce new species.
- Theory of natural selection is given in the book, Origin of Species’ which is written by Darwin,
- For this study of evolution, Darwin had collected and observed innumerable specimens of plants and animals.
→ Important explanation of Theory of natural selection :
- All living organisms reproduce prolifically,
- There is competition with each other or struggle for survival.
- Organisms that show essential modifications for survival, sustain while remaining perish.
- Survival of the fittest and elimination of misfit is the natural selection’.
- Well adapted, sustaining organisms reproduce more such offspring and in turn produce new species having specific characters.
→ Objections to Darwin’s theory :
- Some more factors are responsible for evolution and not just the natural selection.
- Any explanation about useful and useless modifications was not provided by Darwin.
- Causes of slow and abrupt changes was not explained by Darwin.
- Jean-Baptiste Lamarck (1744-1829): Lamarck proposed principle of ‘use or disuse of organs and ‘theory of inheritance/ancestry of acquired characters’.
- His concepts are known as Lamarckism.
- Every living organism tries to evolve. During its lifetime, it acquires certain characters through adaptations and modification.
- These characters are passed to the next generation.
- Depending upon the activities or inactivity of that organism these changes occur.
→ Examples of acquired characters :
- For several generations, giraffe extended the neck for browsing on leaves from heights. Due to such extension of neck , giraffe became long-necked.
- Due to frequent hammering movements, shoulders of the ironsmith became very strong.
- Flightless birds like emu and ostrich have weak wings as they do not fly.
- Due to wading and swimming in water the hind limbs of aquatic birds such as swan and duck became fitter for such mode.
- Due to burrowing habit, snakes lost the limbs. According to Lamarck such acquired characters are inherited by the next generation.
→ Objections to Lamarckism :
- The view that unused organs degenerate and used ones evolve was accepted but inheritance of such characters was not agreed by the scientists.
- Modifications formed are not transferred to the next generation.
- Ancestry/Inheritance of acquired characters : This concept states that the living organism is able to transfer the characters which it has acquired, to the next generation.
- Due to evolution, new species of plants and animals are formed.
- Species : The group of organisms that can produce fertile individuals through natural reproduction is called species.
- Speciation: The process of formation of new species from earlier ones is called speciation.
- Each species possesses specific characters. Each species differs in its geographical conditions, food preference, habitat, reproductive ability and period, etc. Therefore, the specific characters are retained
→ Speciation depends upon following factors :
- Genetic variation
- Geographical changes
- Reproductive changes
- Geographical or reproductive isolation for a long period.
Human Evolution :
→ Stages of human evolution :
→ Around 2 Crore years agd some species of apes started evolving in different direction.
- Changing climate, dry environment, loss of forest cover,
- Apes descended from trees and started living on land. (Arboreal → terrestrial transition)
- Increasing use of hands for feeding and working.
|Approximately||Africa and some||Walking on two legs, instead of four.||First human-like animal:|
|2 crores years ago.||parts of India||Development of lumbar bones. Walking semierect||Ramapithecus|
|When?||Where ?||What happened?||Who were extinct? Who evolved?|
|Approximately 40 lakh years ago.||South Africa||Ape became larger and more intelligent.|
|Approximately 15 lakh years ago.||China and Indonesia in Asia||Skillful man. Man who could walk upright.||Human like animals evolved. Rise of genus ‘Homo’.|
|Approximately 1 lakh years ago.||Africa, Asia and Europe||Upright walking human. Enlargement of brain||First member of Wise man (Homo sapiens neanderthalensis) Neanderthal man was very intelligent|
|Well-developed brain. Use of fire.|
|Approximately 50,000 years ago.||Africa, Asia and Europe||Evolution at very fast rate.||Cro-Magnon man|
|10,000 years ago||On entire earth||Invention of agriculture.Rearing of cattle. Establishing cities and civilizations. Development of culture.||Homo sapiens sapiens (Wise man) ’|
|5000 years ago||On entire earth||Invention of art of writing and beginning of historical periods.||Modern man|
|400 years ago||On entire earth||Emergence of Modern sciences||Cultural evolution of man|
|200 years ago||On entire earth||Rise of industrial society|
- Important: First record of human-like animal: ‘Ramapithecus
- First example of wise-man: Neanderthal man
→ Stages of human evolution and its time :
|Stage of human evolution||Time period|
|1. Ancient animals like Lemur||7,00,00000 years ago|
|2. Egyptopithecus||4,00,00000 years ago|
|3. Dryopithecus||2,50,00000 years ago|
|4. Ramapithecus||1,00,00000 years ago|
|5. Australopithecus||40,00000 years ago|
|6. Skilled Human||20,00000 years ago|
|7. Man with erect posture||15,00000 years ago|
|8. Neanderthal||1,50000 years ago|
|9. Cro-Magnon man||50000 years ago|