Reproduction

விளையாடிக் கற்றுக்கொள்ளுங்கள்

ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.

கல்வியாளர்களுக்கு: Reproduction (Science, Biology)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.

பாட குறிப்புகள்

Sexual Reproduction

  • Sexual reproduction involves the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell), producing offspring that are genetically different from each other.
  • Fertilisation is the fusion of gamete nuclei; because each gamete comes from a different parent, there is variation in the offspring.
  • A gamete is a sex cell: in animals, sperm and egg (ovum); in plants, pollen nucleus and egg (ovum).
  • Gametes contain half the number of chromosomes of normal body cells, so they have a haploid nucleus (one copy of each chromosome).
  • In humans, a normal body cell contains 46 chromosomes, but each gamete contains 23 chromosomes.
  • When male and female gametes fuse, the zygote contains the full 46 chromosomes (half from each parent) and is described as diploid.

During sexual reproduction, a sperm fertilizes an egg.

During sexual reproduction, a sperm fertilizes an egg.

Asexual Reproduction

  • Asexual reproduction produces genetically identical offspring from one parent.
  • It does not involve gametes or fertilisation, so there is no mixing of genetic information.
  • The offspring are clones – genetically identical to the parent and to each other.
  • Many plants reproduce asexually.
  • Bacteria produce exact genetic copies of themselves by a type of asexual reproduction called binary fission.

Comparing Sexual and Asexual Reproduction

  • Sexual reproduction involves two parents; asexual reproduction involves one parent.
  • Sexual reproduction produces offspring that are genetically different; asexual reproduction produces genetically identical offspring.
  • Sexual reproduction involves gametes and fertilisation; asexual reproduction does not.
  • Sexual reproduction creates genetic variation in offspring; asexual reproduction creates no genetic variation (except mutations).
  • Asexual reproduction can produce many offspring quickly; sexual reproduction is generally slower and produces fewer offspring.

The Role of Meiosis

  • Cells in reproductive organs divide by meiosis to form gametes (sex cells).
  • Meiosis is a reduction division: the chromosome number is halved from diploid to haploid.
  • The chromosome number must be halved so that after fertilisation the zygote has the correct diploid number.
  • Meiosis involves two divisions, producing four haploid daughter cells.
  • Meiosis produces genetic variation by forming new combinations of maternal and paternal chromosomes in each gamete.
  • When gametes fuse randomly at fertilisation, each offspring is genetically different from any other.

Meiosis

Meiosis

DNA and the Genome

  • The genome is the entire set of genetic material of an organism.
  • The Human Genome Project was completed in 2003 after a 13-year project to sequence the whole human genome.
  • A gene is a section of DNA that codes for a particular sequence of amino acids, which form proteins.
  • DNA (deoxyribonucleic acid) is found in the nucleus of a cell and is a polymer made of two strands coiled into a double helix.
  • DNA is made of repeating subunits called nucleotides; each nucleotide has a common sugar and phosphate group with one of four bases attached.
  • The four bases are Adenine (A), Cytosine (C), Thymine (T) and Guanine (G).
  • Bases pair up by complementary base pairing: A–T and C–G, held together by weak hydrogen bonds.

DNA, genes and chromosomes: genes are sections of chromosomes, and chromosomes are made of DNA.

DNA, genes and chromosomes: genes are sections of chromosomes, and chromosomes are made of DNA.

Protein Synthesis

  • A gene is a section of DNA with a particular sequence of bases that codes for a particular sequence of amino acids.
  • A sequence of three bases codes for a single specific amino acid.
  • The order of bases controls the order of amino acids joined together in a polypeptide chain, which folds to form a protein.
  • Transcription occurs in the nucleus: DNA unwinds, and RNA polymerase makes a complementary mRNA copy of the gene.
  • Translation occurs in the cytoplasm: mRNA attaches to a ribosome, which reads the code in groups of three bases (codons).
  • tRNA molecules bring specific amino acids to the ribosome; their anticodons pair with complementary codons on mRNA.
  • Peptide bonds form between amino acids until a stop codon is reached, completing the polypeptide chain.

Genetic Variants and Mutations

  • A mutation is a random change to the DNA base sequence.
  • Genetic variants are different versions of genes caused by mutations.
  • Mutations in non-coding DNA can affect phenotype by influencing the binding of RNA polymerase, altering the quantity of protein produced.
  • Mutations in coding DNA may change the sequence of amino acids, altering the shape and function of the protein.
  • Insertions, deletions and substitutions are types of mutation; insertions and deletions have a knock-on effect on later base triplets.
  • Mutations can be caused by ionising radiation (gamma rays, X-rays, UV) and certain chemicals (e.g. tar in tobacco).
  • Sickle cell anaemia is caused by a gene mutation that changes haemoglobin, making red blood cells stiff and sickle-shaped.

Key Definitions in Inheritance

  • A gene is a short length of DNA on a chromosome that codes for a particular characteristic.
  • Alleles are different versions of the same gene; we inherit two alleles for each gene, one from each parent.
  • The genotype is the combination of alleles an organism has for a characteristic.
  • The phenotype is the observable characteristics of an organism.
  • A dominant allele only needs to be inherited from one parent for the characteristic to show.
  • A recessive allele must be inherited from both parents for the characteristic to show.
  • Homozygous means two identical alleles for a gene; heterozygous means two different alleles.

Predicting Genetic Inheritance

  • Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene.
  • A Punnett square shows the possible combinations of alleles in offspring and can be used to work out ratios and probabilities.
  • In a cross between two heterozygous parents (Tt × Tt), the expected ratio is 3:1 (dominant : recessive).
  • The probability of an offspring showing the dominant phenotype is 75%; the recessive phenotype is 25%.
  • Family pedigree diagrams trace the inheritance of a characteristic through generations and can be used to work out the probability of inheriting a genetic disorder.
  • In pedigree diagrams, males are squares and females are circles; affected individuals are often shaded.

Inheritance of Sex and Other Patterns

  • Sex is determined by an entire chromosome pair: females have XX and males have XY.
  • The father determines the sex of the child because only he can pass on a Y chromosome.
  • Sperm cells determine the sex of the offspring.
  • Codominance occurs when both alleles in a genotype are expressed in the phenotype (e.g. blood group inheritance).
  • Sex-linked characteristics are controlled by alleles on the sex chromosomes; most are on the X chromosome.
  • Males are more likely to show sex-linked recessive conditions (e.g. red-green colour blindness, haemophilia) because they have only one X chromosome.
  • Polygenic inheritance involves characteristics controlled by more than one gene, giving a wide range of phenotypes (e.g. eye colour).

Variation

  • Variation is the differences between individuals of the same species.
  • Continuous variation shows many small degrees of difference and can be measured on a scale (e.g. height, mass); graphs give a smooth bell curve.
  • Discontinuous variation shows distinct differences with no in-betweens (e.g. blood group, sex, tongue rolling); graphs give a step-like shape.
  • Variation can be caused by genes, the environment, or a combination of both.
  • Genetic variation arises from meiosis and the random fusion of gametes at fertilisation.
  • Environmental variation is caused by factors outside the organism, such as climate, diet, lifestyle and accidents.
  • Discontinuous variation is usually caused by genetic variation alone; continuous features often result from genetic and environmental causes together.

A group of dogs of different breeds, illustrating genetic variation within a species.

A group of dogs of different breeds, illustrating genetic variation within a species.

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பதிவு செய்க — இலவசம்உள்நுழைக

பயிற்சி கேள்விகள்

இலவச முன்னோட்டம் — 56-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.

  1. 1.How many chromosomes are contained in a human gamete?

    Easy
    • A46
    • B48
    • C24
    • D23

  2. 2.When the gametes fuse together at fertilisation, the new cell that is produced is genetically different to each of the parents' cells. What is the new cell called?

    Easy
    • AEmbryo
    • BZygote
    • CGamete
    • DClone

  3. 3.In 1953, scientists Watson and Crick published their work on the structure of DNA. What term is used to describe the shape of DNA?

    Easy
    • ADouble helix
    • BSingle strand
    • CPolypeptide
    • DNucleotide

  4. 4.What is the definition of the word allele?

    Medium
    • AA section of DNA that codes for a protein
    • BThe physical appearance of an organism
    • CA variation of a gene
    • DThe combination of alleles in an organism

  5. 5.What is a genotype?

    Medium
    • AThe physical appearance of an organism
    • BA section of DNA that codes for a protein
    • CThe alleles present in an organism for a particular gene
    • DA change in the DNA sequence

  6. 6.Which statement correctly describes asexual reproduction?

    Easy
    • AIt involves the fusion of gamete nuclei
    • BIt produces genetically identical offspring from one parent
    • CIt always requires two parents
    • DIt produces offspring that are genetically different from each other

  7. 7.Which of the following statements about meiosis are correct? (Select all that apply)

    Medium
    • AIt produces four daughter cells
    • BIt produces gametes
    • CIt produces genetically identical cells
    • DThe daughter cells contain half the number of chromosomes as the parent cell
    • EIt occurs during asexual reproduction

  8. 8.Which of the following are advantages of sexual reproduction? (Select all that apply)

    Medium
    • AIt produces genetic variation in the offspring
    • BIt requires only one parent
    • CIt produces offspring that may be better adapted to a changing environment
    • DIt is a fast method of reproduction
    • EIt produces genetically identical offspring

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