In sexual reproduction, reproductive cells are produced through a special type of cell division called“meiosis.” The mechanism by which the chromosome number is halved is the key to diversity among offspring.
Basic Knowledge
Meiosis: A special type of cell division that occurs when reproductive cells are produced. The chromosome number becomeshalf(1/2).
Human somatic cells: 46 chromosomes(23 pairs) → reproductive cells(sperm and eggs): 23 chromosomes.
Through fertilization, 23 chromosomes in an egg + 23 chromosomes in sperm = 46 chromosomes again, maintaining the original chromosome number.
As a result, offspring inherit half of their genetic information from each parent.
Genes are arranged on chromosomes, and the substance of genes is DNA(deoxyribonucleic acid).
Relationship among terms: cell ⊃ nucleus ⊃ chromosome ⊃ gene = DNA(a gene is a meaningful section of DNA).
In somatic cells, chromosomes exist in pairs(homologous chromosomes), with one chromosome in each pair inherited from the father and the other from the mother. During meiosis, each pair separates and enters a reproductive cell.
Meiosis(Cell division that produces reproductive cells. The chromosome number becomes half)
Homologous chromosomes(A pair of chromosomes with the same shape and size. One comes from the father and one from the mother)
Heredity(The transmission of traits and characteristics from parents to offspring)
Gene(A unit of genetic information. A meaningful region of DNA)
DNA(Deoxyribonucleic acid. The substance of genes. It has a double-helix structure)
Trait(A physical characteristic that appears through heredity, such as flower color or blood type)
Further Exploration (Background and Significance)
Diversity produced by meiosis:
① Random combinations of homologous chromosomes: Humans have 23 pairs of homologous chromosomes. Whether the chromosome inherited from the father or the mother enters a reproductive cell is independently and randomly determined for each pair. The number of combinations is 2²³ ≒ about 8 million.
② Combinations during fertilization: Father's sperm(8 million combinations) × mother's egg(8 million combinations) = about 70 trillion combinations. In addition, crossing over(advanced topic) occurs during meiosis, so even siblings are born as genetically very different individuals(except identical twins).
This is the fundamental reason why sexual reproduction creates “the ability to adapt to changing environments.”
The double-helix structure of DNA was elucidated by Watson and Crick in1953. The sequence of four types of bases, A, T, G, and C, represents genetic information. In the 21st century, the Human Genome Project was completed, and the entire human DNA sequence was decoded.
- Meiosis = produces reproductive cells and halves the chromosome number
- Humans: 46 chromosomes in somatic cells→23 in reproductive cells→46 after fertilization
- Relationship: cell⊃nucleus⊃chromosome⊃gene=DNA
- Homologous chromosomes = one from the father and one from the mother form a pair
- Half of the genetic information is inherited from each parent through fertilization
- Siblings are genetically different(diversity)
- DNA double helix = Watson and Crick(1953)
Points to Note (Often Confused)
① Distinguish between somatic cell division(the chromosome number remains unchanged) andmeiosis(the chromosome number becomes half). ② Genes and DNA are nearly synonymous, but strictly speaking, a gene is a meaningful section of DNA. ③ Humans have 46 chromosomes, or 23 pairs. Do not confuse the number of chromosomes with the number of pairs. ④ New genetic information is not created through fertilization; rather, the parents' genetic information is combined.
Practice
- How does the chromosome number change during meiosis?
- State the chromosome numbers of human somatic cells and reproductive cells, respectively.
- Show the containment relationship among cells, nuclei, chromosomes, genes, and DNA using arrows.