Sexual reproduction

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

Sexual reproduction is how most animals and plants reproduce.[1] Some protists and fungi also reproduce this way. Organisms that reproduce sexually have two different sexes: male and female. Offspring is made by a sperm fertilizing an ovum from the female involves meiosis is very slow method of multiplication. Different steps are involved in the process.

Cell biology[change | change source]

The cells of an animal or higher plant have two sets of chromosomes: they are diploid. When gametes (sex cells) are produced, they have only one set of chromosomes: they are haploid. They have undergone a process of cell division called meiosis. Two things happen during meiosis, each of which makes the offspring more variable. That means they are different from the parents and from each other.

Assortment[change | change source]

Assortment is when the double set of chromosomes becomes a single set in each gamete. Of each pair of chromosomes, which one goes into a single gamete is random. Because the gene alleles on each chromosome are not always the same, this means that there is genetic variation between gametes. This process was Mendel's 'first law', the law of segregation.

Crossing over[change | change source]

Because crossing over occurs during meiosis, this increases the variety of the chromosomes. This makes recombination possible.

The consequence of assortment and crossing over makes it certain that no two offspring of the same mother and father are identical. Identical twins are the exception, being identical genetically because they developed from the same fertilised egg.

Advantages and disadvantages[change | change source]

There are advantages and disadvantages of sexual reproduction, compared to asexual reproduction. The main issues are:

  1. Advantages: More variation assists with survival. It increases the chance that at least some offspring of a parent survive. To give an example, suppose a deadly infection occurs in the population. Greater variety increases the chance that some of the population will survive.
  2. Disadvantages: Requires two parents. So, supposing the number of eggs per female to be the same, a population of animals reproducing sexually would produce only half as many offspring as a population reproducing asexually, such as starfish do.

Various terms[change | change source]

  • Gonads are specialized sex organs where gametes are formed. In the male, the gonad is the testes; in the female, the gonad is the ovaries.
  • Fertilization: sperm penetrates the cell membrane of ovum. What now exists is a single cell called a zygote.
    • Internal Fertilization: sperm are deposited in or near the female reproductive tract. This requires physical contact between parents but requires fewer gametes to be released and higher chance of survival. It is associated with parental care and protection of embryos
    • External Fertilization: eggs are shed by the female which are fertilised by sperm in the external environment. This doesn’t require physical contact between parents but requires a large number of gametes to be released and has a lower chance of survival.
  • Cleavage: early stage of embryo development. Cell number increases by cell division.
    • 1. Morula: Solid ball of cells
    • 2. Blastula: hollow ball of cells filled with fluid (blastocoel)
    • 3. Gastrulation: Blastula continues to grow, cells continue reproducing using mitosis. Several hundred cells on one side begin to move in and form a two-layered embryo. It develops into a three-layered embryo with endoderm, mesoderm, and ectoderm.

Related pages[change | change source]

References[change | change source]

  1. Pang K. 2004. Certificate Biology: new mastering basic concepts. Hong Kong.