SAMACHEER CLASS 10 UNIT 18 NOTES

 

UNIT 18

  samacheer SCIENCE
CLASS 10

BIOLOGY – HEREDITY

INTRODUCTION

·     Living organisms – produce offsprings – of their own kinds

·     Offsprings – will have characteristics of their parents

·     Like colour of eye, colour of hair, shape of nose, type of earlobe, etc.,

·     Some of the characteristics – inherited from grand parents

HOW CHARACTERS ARE INHERITED FROM ONE GENERATION TO ANOTHER?

·     Because of the genes – inherited from parents

·     Genes – responsible for – physical outlook & biological functions

·     Branch of biology that deals with genes, genetic variation & heredity of living organism – Genetics

·     HEREDITY – Transmission of characters from one generation to next generation

·     VARIATION – Differences shown by – individuals of same species – & also by the offsprings of same parents

·     These happen – due to chromosomes

GREGOR JOHANN MENDEL – FATHER OF GENETICS

·     Mendel (1822-1884) – Austrian Monk – discovered basic principle of heredity – through his experiments

·     His experiments – foundation for modern genetics

·     Born In 1822 – family of farmers in Selisian of Chechoslovakia

·     At the age of 18 – finished high school – entered Augustinian Monastery – at Brunn – as a priest

·     From there he went to – University of Vienna – for training in Physics, Mathematics & Natural Science

·     Returned to Monastery – 1854 – continued as a priest – & teached in high school

 

·     In his leisure time – started his famous experiments – on garden pea plant

·     9 years – conducted his experiments – in the monastery (1856 – 1865)

·     He worked on – nearly 10,000 pea plants – 34 different varieties

·     He noted – they differ from one another – in many ways

·     Mendel – chose 7 pairs of contrasting characters – for his study

REASONS FOR MENDEL’S SUCCESS

·     He chose pea plant – advantageous in many ways

·     Naturally self-pollinating – and very easy to raise pure-breeding varieties

·     Has short life span (annual) – so it was possible to follow – several generations

·     Easy to cross pollinate

·     Has deeply defined contrasting characters

·     Flowers – bisexual

MONOHYBRID CROSS – INHERITANCE OF ONE GENE

·     Cross – involves – inheritance of only one pair of contrasting characters – Monohybrid cross

·     Eg: Cross between 2 forms of single trait – Tall & dwarf plants

MENDEL’S EXPLANATION OF MONOHYBRID CROSS PARENTAL GENERATION

·     He selected – pure breeding tall plant & pure breeding dwarf plant

F1 GENERATION

·     He crossed the parents

·     From the seeds obtained – he raised first filial generation (F1)

·     Here the plants were – all tall & monohybrids

 

F2 GENERATION

·     Mendel allowed – selfing of F1 monohybrids

·     He obtained tall & dwarf plants – in the ratio 3:1 respectively

·     Actual number of tall & dwarf plants – obtained by Mendel – 787 tall & 277 dwarf

·     External expression – of particular trait – phenotype

·     Phenotypic ratio – 3:1

·     In F2 generation – 3 different types were obtained

·     Tall Homozygous – TT (pure) – 1

·     Tall Heterozygous – Tt – 2

·     Dwarf Homozygous – tt – 1

·     So, Genotypic ratio – 1:2:1

·     Genotype – genetic expression of an organism

MENDEL’S INTERPRETATION ON MONOHYBRID CROSS

·     Based on his observation – Mendel confirmed that – ‘Factors’ are passed from one generation to another

·     Factors – now referred as ‘genes’

·     Tallness & dwarfness – determined by pair of contrasting factors

·     Tall plant – has a pair of factors (represented by T – dominant character)

·     Dwarf plant – has factors of dwarfness (represented by t – recessive character)

·     Factors – occur in pairs

·     May be alike or unlike

·     Pure breeding tall plant (TT) & dwarf plant (tt) – are homozygous

·     Tt – heterozygous

o  2 factors – makes up a pair of Contrasting characters – called alleles or allelomorphs

·     One member of each pair – comes from one parent

o  when 2 factors for alternate traits brought together by fertilization (Tt)

·     Only one expresses (Tallness) – masking the other (dwarfness)

·     Character that expresses – called Dominant condition

·     Character that is masked – called Recessive condition

o  Factors – always pure

·     When gametes are formed – factors segregate – so each gamete gets one of the two alternate factors

·     Eg: Factors for tallness (T) & deafness (t) – separate entities

·     In a gamete – either T or t – present

·     When F1 hybrids are self crossed – 2 entities separate – then unite independently – forming tall & dwarf plants

INFO BITS

·     Punnett Square – checked board form – devised by – R.C. Punnett (British Geneticist) – for the study of genetics

·     Graphical representation – to calculate – probability of possible genotypes – of offsprings – in a genetic cross

DIHYBRID CROSS – INHERITENCE OF TWO GENES & LAW OF INDEPENDENT ASSORTMENT

·     Dihybrid cross – involves inhesitance of 2 pairs of contrasting characteristics (or contrasting traits) at the same time

·     Mendel chose – shape & colour of seeds – Round yellow seeds & Wrinkled green seeds

·     When these 2 pea plant were crossed – Mendel observed the following

·     Mendel crossed – pure breeding pea plants – with round-yellow seeds & wrinkled-green seeds respectively

·     He found in F1 generation – only round-yellow seeds were produced

 

·     No wrinkled-green seeds in F1 generation

·     He concluded – round shape & yellow colour of seeds – dominant traits

·     Wrinkled shape & green colour of seeds – recessive traits

·     When hybrids of F1 generation – having round-yellow seeds – cross bred by self pollination – 4 types of seeds – having different combination of shape & colour – obtained in F2 generation

·     Round-yellow, round-green, wrinkled-yellow & wrinkled-green seeds

·     Phenotypic ratio of F2 generation – 9:3:3:1 – known as Dihybrid ratio

CONCLUSION

·     The factors for each character – remains independent – and maintain their identity in gametes

·     Factors – independent to each other – and pass to the offsprings – through gametes

RESULTS OF DIHYBRID CROSS

·     4 types of plants – dihybrid cross produced – 4 types of offsprings in F2 generation

·     Ratio – 9:3:3:1

o  9 – with 2 dominant traits

o  3 – with 1 dominant & 1 recessive trait

o  3 – with another dominant & another recessive trait

o  1 – with 2 recessive traits

·     New combination – 2 new combinations of traits – round green & wrinkled yellow – appeared in F2 generation of Dihybrid cross

MENDEL’S LAWS

·     Based on his experiments (monohybrid & dihybrid cross) – Mendel proposed – 3 important laws – now called – Mendel’s Laws of Heredity

LAW OF DOMINANCE

·     When 2 homozygous individuals (with one or more contrasting characters) – crossed – characters that appear in F1 generation – dominant – characters that don’t appear in F1 – recessive

LAW OF SEGREGATION OR LAW OF PURITY OF GAMETES

·     When a pair of contrasting factors – brought together – in a heterozygote or hybrid – 2 members of the pair – remain together without mixing

·     When gametes are formed – the 2 separate out – only one enters each gamete

LAW OF INDEPENDENT ASSORTMENT

·     In case of inheritance of 2 or more pairs of characters simultaneously – factors or genes of one pair – assort out independently – of the other pair

MORE TO KNOW

·     T.H. Morgan – awarded Nobel Prize (1993) – for determining – role of chromosomes in heredity

 

CHROMOSOMES, DNA & GENES

·     Human body – million of cells

·     Each cell has a nucleus

·     Nucleus – contains thin thread like structures – chromosomes

·     Term ‘chromosomes’ – first named by Waldeyer (1888)

·     Chromosomes – carrier of genetic material – contains heredity information

 

·     Chromosomes – highly condensed – coiled chromatin fibres – packed with DNA

·     DNA – Deoxyribonucleic Acid – forms genetic material

·     Genes – segments of DNA – responsible for inheritance – of a particular phenotypic character

·     Each gene – present at specific position on chromosome – called locus

·     During cell division – genetic information in genes – passed from one generation to another

STRUCTURE OF CHROMOSOME

·     Chromosomes – thin, long & thread like structure

·     Consists of 2 identical strands – called sister chromatids

·     Chromatids – held together by – centromere

·     Each chromatid – made of spirally coiled thin structure – called chromonema

·     Chromonema – has number of bead-like structure – along its length – called chromomeres

·     Chromosomes – made of DNA, RNA, chromosomal proteins (histones & non-histones) & certain metallic ions

·     These proteins – provide structural support to chromosome

REGIONS OF CHROMOSOME

PRIMARY CONSTRICTION

·     Two arms of chromosome – meet at a point – called primary constriction or centromere

·     During cell division – spindle fibres attach to chromosome – at the centromere

SECONDARY CONSTRICTION

·     Some chromosome – have secondary constriction – at any point

·     Known as nuclear zone or Nucleolar organizer (formation of nucleolus in nucleus)

 

TELOMERE

·     End of the chromosome – Telomere

·     Each extremity of chromosome – has polarity – prevents from joining adjacent chromosome

·     It maintains & provides stability – to the chromosomes

SATELLITE

·     Some chromosome – have an elongated knob-like appendage – at one end – Satellite

·     Chromosomes with Satellite – called Sat-chromosomes

DO YOU KNOW?

·     Telomere – acts as ageing clock – in every cell

·     Telomeres – protective sequences of nuceotides – in chromosomes

·     As cell divides – every time – they become shorter

·     Telomeres – become too short – to do their job – causing aging in cells

TYPES OF CHROMOSOMES BASED ON THE POSITION OF CENTROMERE

·     Based on position of centromere – chromosomes are of 4 types

o  Telocentric

o  Acrocentric

o  Submetacentric

o  Metacentric

 

TELOCENTRIC

·     Centromere – on proximal end

·     Rod shaped chromosomes

ACROCENTRIC

·     Centromere – found very close to one end

·     One end – short arm; other end – long arm

·     Rod shaped chromosomes

SUBMETACENTRIC

·     Centromere – found near the centre of chromosome

·     Forms 2 unequal arms

·     J shaped or L shaped chromosomes

METACENTRIC

·     Centromere – found in the centre

·     Forms 2 equal arms

·     V shaped chromosomes

TYPES OF CHROMOSOMES – BASED ON FUNCTION

·     Eukaryotic chromosomes – classified into

o  Autosome &

o  Allosomes

AUTOSOMES

·     Contain genes – that determine somatic (body) characters

·     Male & female – have equal number of autosomes

 

ALLOSOMES

·     Allosome – chromosomes – responsible for determining – sex of an individual

·     Also called sex chromosomes or hetero chromosomes

·     2 types of sex chromosomes – X and Y chromosomes

·     Human Male – one X & one Y chromosome

·     Human Female – 2 X chromosome

KARYOTYPE

·     Number of chromosomes – in any living organism (animal / plant) – constant

·     Human – each cell normally contains – 23 pairs of chromosomes

·     Out of 23 pairs – 22 pairs are autosomes; 23^rd pair – allosome or sex chromosome

·     In body cells of sexually reproducing organisms – chromosomes occur in pairs

·     This condition – called diploid (2n)

·     Gametes – produced by organisms – contain single set of chromosome

·     Gametes – are haploid (n)

·     Karyotype – number, size & shape of chromosomes – in cell nucleus of an organism

·     Idiogram – diagrammatic representation of karyotype

·     Consists of – all metaphasic chromosomes – arranged in homologous pair – according to – decreasing length, thickness, position of centromere, shape, etc.,

·     Sex chromosomes – placed at the end

STRUCTURE OF DNA

·     DNA – Heredity material – as it contains genetic information

·     Most important constituent – of chromosome

·     Most widely accepted model of DNA – double helical structure – of James Watson & Francis Crick

·     They proposed – 3  dimensional model of DNA – on the basis of X-ray diffraction studies of DNA – by Rosalind Franklin & Maurice Wilkins

·     Watson, Crick & Wilkins – awarded Nobel Prize for medicine (1962)

CHEMICAL COMPOSITION OF DNA MOLECULE

·     DNA – large molecule – consists of millions of nucleotides – called Polynucleotide

·     Each nucleotide – has 3 components

·     a sugar molecule – Deoxyribose sugar

·     a nitrogenous base

·     2 types of nitrogenous bases in DNA

o  Purines (Adenosine & Guanine)

o  Pyrimidines (Cytosine & Thymine)

·     a phosphate group

NUCLEOSIDE & NUCLEOTIDE

·     Nucleoside = Nitrogen base + sugar

·     Nucleotide = Nucleoside + Phosphate 

·     Nucleotides – formed according to purine & pyrimidine present in them

WATSON & CRICK MODEL OF DNA

·     DNA molecule – has 2 polynucleotide chains

·     The 2 chains – form a double helix structure – with 2 strands

·     2 strands run anti-parallel to one another

·     Nitrogenous bases – in the centre – linked by sugar-phosphate units – form the backbone of DNA

·     Pairing of nitrogenous bases – specific – always purine & pyrimidine pairs – linked by hydrogen bonds

·     Adenine (A) links Thymine (T) – with 2 hydrogen bonds (A=T)

·     Cytosine (C) links Guanine (G) – with 3 hydrogen bonds (C=G)

·     This is called – complementary base pairing

·     Hydrogen bonds – between nitrogenous bases – make DNA stable

·     Each turn of DNA double helix – 34 A (3.4 nm)

·     10 base pairs – in one turn

·     Nucleotides in a helix – joined together by – phosphodiester bonds 

MORE TO KNOW

·     Chargaff rule of DNA base pairing

·     Erwin Chargaff states – in DNA – proportion of adenine = thymine & proportion of cytosine = guanine

DNA REPLICATION

·     DNA replication – basic process – occurs within a cell

·     During replication – DNA molecule – produces exact copies – of its own structure

·     2 strands of DNA molecule – have complementary base pairs

·     Nucleotide of each strand – provide information needed – to produce a new strand

·     2 resulting daughter cells – contain exactly same genetic information – as the parent cell (during cell division)

STEPS INVOLVED IN DNA REPLICATION

(1)                  ORIGIN OF REPLICATION

·     Specific points on DNA – where replication begins – site of origin of replication

·     2 strands – open & separate at this point – forming replication fork

(2) UNWINDING OF DNA MOLECULE

·     Enzyme helicase – bind to origin of replication site

·     Helicase – separates the 2 strands of DNA

·     Enzyme topoisomerase – separates double helix – above the replication fork – removes the twists formed during unwinding

·     Each separated DNA strand – functions as template

(3) FORMATION OF RNA PRIMER

·     RNA Primer – short segment of RNA nucleotides

·     Primer – synthesized by DNA template – close to origin of replication site

(4) SYNTHESIS OF NEW COMPLEMENTARY STRAND FROM THE PARENT STRAND

·     After the formation of RNA primer – nucleotides added – with the help of an enzyme – DNA polymerase

·     New complementary strand of DNA – formed from – each parent strand (template)

·     Synthesis – is unidirectional

·     In one strand – daughter strand is continuous – called leading strand

·     Other strand – short segments of DNA synthesized – called lagging strand

·     Short segments of DNA – called Okazaki fragments

·     The fragment – joined together by – DNA ligase enzyme

·     Replication stops – when replication fork of the 2 sides – meet terminus site – situated opposite to origin of replication site

SIGNIFICANCE OF DNA

·     Responsible for – transmission of hereditary information – from one generation to next generation

·     Contains information – required for formation of proteins

·     Controls the developmental process & life activities of an organism

SEX DETERMINATION

·     Formation of zygote – into male & female sex – during development – called sex determination

·     Sex – determined by chromosomes of an individual

SEX DETERMINATION IN HUMAN

·     Humans have – 23 pairs of chromosomes

·     Out of 23 – 22 pairs are autosomes & 1 pair (23^rd pair) – sex chromosome

·     Female gametes (eggs) – have similar chromosome type – 22+X

·     Therefore, human females – homogametic

·     Male gametes (sperms) – are of 2 types

·     They are produced in equal proportions

·     Sperms with 22+X & 22+Y chromosomes

·     Human males – heterogametic

·     It is a chance of probability – as to which category of sperm – fuse with egg

·     If the egg (X) – fused with sperm (X) – XX individual (female) is produced

·     If egg (X) – fused with sperm (Y) – XY individual (male) – produced

·     Sperm – produced by father – determines the sex of the child

·     Mother – not responsible in determining the sex of the child

 

HOW CHROMOSOMES TAKE PART?

·     Fertilization of the egg (22+XX) – with sperm (22+X) – produce female child (44+XX)

·     Fertilization of the egg (22+X) – with sperm (22+Y) – produce male child

MUTATION

·     Term Mutation – introduced by – Hugo De Vries (1901)

·     He observed phenotypic changes – in Primrose plant, Oenothera lamarckiana

·     Mutation – inherited sudden change – in DNA (genetic material) of an organism

·     Mutations – 2 main types

o  Chromosomal mutation

o  Gene mutation

CHROMOSOMAL MUTATION

·     Sudden change in the structure or number of chromosomes – chromosomal mutation

Changes in structure of chromosomes

·     Structural changes – usually occurs – due to errors in cell division

·     Changes in number & arrangement of genes takes place – due to – deletion, duplication, inversion & translocation – in chromosomes

Changes in number of chromosomes

·     Involves addition or deletion – in the number of chromosomes – in cell – called ploidy

·     2 types of ploidy

§  Euploidy

§  Aneuploidy

EUPLOIDY

·     Condition in which individual bears – more than the usual number of diploid chromosomes (2n)

·     Individual with – 3 haploid sets of chromosomes – Triploidy (3n)

·     Triploid plants & animals – sterile

·     If it has 4 haploid sets of chromosomes – tetraploidy (4n)

·     Tetraploid plants – advantageous – often results in – increased fruit & flower size

ANEUPLOIDY

·     Loss or gain of – one or more chromosomes in a set

·     3 types

o  Monosomy (2n – 1)

o  Trisomy (2n + 1)

o  Nullisomy (2n – 2)

·     In man, Doesn’t syndrome – commonly known aneuploid condition

DOWN’S SYNDROME

·     Condition first identified by – Dr. Langdon Down (1866)

·     Genetic condition – there is an extra copy of chromosome 21 (trisomy 21)

·     Associated with – mental retardation, delayed development, behavioural problems, weak muscle tone, vision & hearing disability – seen in children with Down’s syndrome

GENE OR POINT MUTATION

·     Gene mutation – changes occurring in nucleotide sequence of a gene

·     Involves – substitution, deletion, insertion or inversion – of single or more than 1 nitrogenous base

·     Gene alternation – results in abnormal protein formation – in an organism

DO YOU KNOW?

·     SICKLE CELL ANAEMIA – caused by mutation of a single gene

·     Alternation in gene – changes the structure of protein part of haemoglobin molecule

·     As protein molecule changes – RBC that carries haemoglobin is sickle shaped