Science unit 4.1

What is sexual reproduction? 

• sexual reproduction: a type of reproduction in which genetic material from two different cells combine to produce an offspring.   

• The cells that combine during sexual reproduction are called sex cells or gametes. 

  • sex cells are formed in reproductive organs

  • egg: female sex cell; formed in an ovary

  • sperm: male sex cell; formed in a testis

• fertilization: a reproductive process during which an egg cell and a sperm cell join to form a zygote. 

  • zygote: the new cell that forms when a sperm cell fertilizes an egg cell

  • a zygote develops into a new organism by mitosis

Fig. 1: A litter of kittens. Not all of the offspring are identical because of sexual reproduction. 

Diploid Cells

• Organisms that reproduce sexually produce two types of cells: body cells and sex cells. 

• diploid cells (2n): cells with pairs of chromosomes

  • one comes from your mother and the other comes from your father

• homologous chromosomes: pairs of chromosomes that have genes for the same trait arranged in the same order; one from each parent

• All body cells have a diploid number of chromosomes. 

• Different species have different numbers of chromosomes. 

Fig. 2: An organism’s chromosomes can be matched as pairs of chromosomes that have genes for the same trait. Humans have 46 total chromosomes and 23 homologous pairs. 

Haploid Cells

• Organisms that reproduce sexually also produce sex cells: egg or sperm cells. 

  • Sex cells are always haploid. 

• haploid cells (n): cells that contain only one chromosomes from each pair. 

  • haploid sex cells are produced by meiosis 

• meiosis: a type of cell division that produces haploid sex cells

  • only occurs in reproductive organs

  • one diploid cell divides to form four haploid cells

Remember, haploid sex cells come together to create a diploid zygote. (n)+(n)=(2n)

Fig. 3: All of the cells in your body came from one cell: a fertilized egg. 

Advantages of Sexual Reproduction

• The biggest advantage of sexual reproduction is the genetic variation it produces. 

• Offspring inherit DNA from each parent. Different DNA means each offspring will have different traits. 

• Genetic variation occurs in all organisms that reproduce sexually. 

  • Different traits means different individuals may have an advantage in some situations.

  • Genetic variation leads to evolution. 

• Ex. Gros Michel bananas went extinct in the 1950s b/c of a fungus called Panama disease

  • All Cavendish bananas are clones of each other (no genetic diversity) and susceptible 

Fig. 5: A Cavendish banana that is infected with Panama fungal disease. 

Phases of Meiosis

Meiosis is the division of a diploid cell to produce four haploid cells. 

• Meiosis I: tetrads (aka duplicated homologous pairs) are separated

  • Prophase I

  • Metaphase I

  • Anaphase I

  • Telophase I

• Meiosis II: sister chromatids are separated

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Fig. 4: Meiosis of a cell with 4 total chromosomes.  

Meiosis I: homologous pairs are separated

Cells go through interphase, including chromosome duplication, prior to meiosis I. Remember, each duplicated chromosomes consists of a two sister chromatids joined by a centromere. 

Meiosis I: 

• prophase I: duplicated chromosomes condense; homologous chromosomes come together to form a pair called a tetrad; nuclear envelope and nucleolus disintegrate; spindle apparatus forms; crossing over occurs 

• metaphase I: pairs of homologous chromosomes are brought to the center of the cell by the spindle apparatus

• anaphase I: homologous pairs are pulled apart and the spindle apparatus pulls sister chromatids to opposite poles of the cell

• telophase I: cytoplasm divides during cytokinesis; the nuclear envelopes and nucleoli form

At the end of meiosis I, you are left with 2 daughter cells with a duplicated set of haploid chromosomes. 

Phases of Meiosis: Meiosis I

Fig. 6

Meiosis II: sister chromatids are separated

The two cells produced by meiosis I do not go through interphase before meiosis II. Cells do not duplicate their chromosomes before meiosis II. 

Meiosis II: 

• prophase II: nuclear envelope and nucleolus disintegrate; spindle apparatus forms

• metaphase II: sister chromatids are brought to the center of the cell by the spindle apparatus

• anaphase II: sister chromatids are pulled apart and the spindle apparatus pulls individual chromatid to opposite poles of the cell

• telophase II: cytoplasm divides during cytokinesis; the nuclear envelope and nucleoli reform

At the end of meiosis II, you are left with 4 genetically unique haploid cells. 

Phases of Meiosis: Meiosis II

Fig. 7

Why is meiosis important? 

• Meiosis forms sex cells with the correct number of haploid chromosomes. 

  • When fertilization occurs and two sex cells combine, the resulting zygote has a diploid number of chromosomes. 

  • (n) + (n) = (2n)

• Meiosis increases genetic diversity. 

  • Crossing over: the exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.

  • Random assortment of chromosomes being pulled apart. 

How do mitosis and meiosis differ? 

• Mitosis and meiosis have very similar steps, but there are a few key differences. 

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• Meiosis creates four genetically different haploid sex cells by duplicating the chromosomes and dividing the nucleus twice. 

• Mitosis creates two identical diploid body cells by duplicating the chromosomes and dividing the nucleus once. 

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