Chapter 10.1, Problem 4MI

Summary Introduction

To describe:

The three processes that occur during sexual reproduction.

Introduction:

Reproductive cells which pass on genetic traits from the parent to child are produced by the process of meiosis. In order to maintain the same number of chromosomes in each generation, an organism produces gametes. Gametes are sex cells and have half the number of chromosomes.

When gametes combine in fertilization, the number of chromosomes is restored.

Answer to Problem 4MI

	Meiosis	Fertilization	Crossing Over
What happens?	Cell division reduces the chromosome number by half.	Two haploid gametes, one from each parent fuse together.	Parts of homologous chromosomes get exchanged.
What is the product?	Four haploid cells are formed.	A diploid fertilized egg is formed.	New combinations of genetic material on chromosomes are formed.

Explanation of Solution

Meiosis involves two consecutive cell divisions called meiosis I and meiosis II. In meiosis I the stages are interphase I, prophase I, metaphase I, anaphase I and telophase I. During interphase I cells carry out cellular activities, replication of DNA and synthesis of proteins.

When one haploid gamete fuses with another haploid gamete, the process is called fertilization. A cell with n number of chromosomes is called a haploid cell. The result of fertilization is a diploid cell with 2n number of chromosomes; n chromosomes from male parent plus n chromosomes from female parent.

Crossing over- During prophase I, the replicated chromosomes appears as two sister chromatids. The homologous chromosomes condense and begin to form pairs in a process called synapsis. The homologous chromosomes are held tightly together along their lengths. Some segments of homologous chromosomes get exchanged during synapsis. This process is called crossing over. It is a process during which chromosomal segments are exchanged between a pair of homologous chromosomes. This results in exchange of genes between homologous chromosomes.