IGCSE Biology: Chapter 17 - Inheritance (Part 1)

Introduction to Inheritance

• Inheritance: Process by which genetic information is passed from parents to offspring.
• Genetic information is carried in the form of DNA located in chromosomes within a cell's nucleus.

DNA, Genes, and Alleles

• Chromosomes: Structures made of DNA containing genetic information.
• Gene: Length of DNA that codes for a protein, determining specific traits/characteristics (e.g., eye color, height).
• Alleles: Different versions of the same gene leading to variations in traits (e.g., blood types A, B, AB, O).

Sex Inheritance in Humans

• Humans have 23 pairs of chromosomes (46 total), known as diploid cells.
• Sex Chromosomes:
  • Male: XY (sperm carries X or Y).
  • Female: XX (egg carries X).
  • Gender of offspring depends on sperm cell's chromosome.
  • Demonstrated via a Punnett Square.

Role of DNA in Protein Synthesis

• DNA provides instructions for protein production.
• Proteins function as enzymes, cell membrane carriers, and neurotransmitter receptors.
• Gene Expression:
  • Only needed genes are expressed ("switched on") based on cell function.

Protein Synthesis Process

• Transcription:
  • DNA sequence of a gene is copied into mRNA in the nucleus.
  • mRNA (messenger RNA) is a single-strand copy of the gene.
• Translation:
  • mRNA moves to cytoplasm and ribosomes assemble amino acids into proteins.
  • Amino acid sequence determined by mRNA base sequence.

Haploid vs. Diploid Nuclei

• Haploid:
  • Nucleus with a single set of chromosomes.
  • Gametes (egg and sperm) have 23 chromosomes.
• Diploid:
  • Nucleus with two sets of chromosomes (23 pairs = 46 chromosomes in humans).

Mitosis

• Mitosis:
  • Nuclear division producing genetically identical cells.
  • Used for growth, repair, replacement of cells, and asexual reproduction.
• Role in organismal growth and tissue repair.
• Stem Cells:
  • Unspecialized cells that divide by mitosis.
  • Zygote is an example, differentiates into various cell types.

Meiosis

• Meiosis:
  • Nuclear division reducing chromosome number from diploid to haploid.
  • Involved in production of gametes (sex cells).
• Comparison:
  • Mitosis: 2 genetically identical diploid daughter cells, 1 division.
  • Meiosis: 4 genetically different haploid daughter cells, 2 divisions.

Conclusion

• Summary of mitosis and meiosis differences.
• End of Part 1 of Chapter 17 on inheritance.
• Encouragement to subscribe and engage with content for more biology revision videos.