Sexual reproduction is how most animals and plants reproduce. 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 fertilising an ovum from the female. 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. In 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]
The consequence of assortment and crossing over makes it certain that no two offspring of the same mother and father are identical, except for identical twins. They are 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:
- 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.
- Disadvantages: Requires two parents. So, supposing the total number of eggs to be the same, a population reproducing sexually would produce only half as many offspring as a population reproducing asexually.
Various terms[change | change source]
Gametes are specialized sex cells formed in gonads by gametogenesis. The male gamete is the sperm, and the female is ova.
Spermatogensis process which produces haploid sperm.
Oogenesis process which produces haploid egg cells.
Differences Between sperm and ovum: Sperm: male sex cell, microscopic, many mature created at one time. Ovum: female sex cell, largest cell in body, created from birth, fixed number able to mature.
Fertilizations: sperm penetrates membrane of ovum. The structure that now exists (single cell) is called a zygote.
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 1 side begin to move in and form a 2 layered embryo. Developed into 3 layered embryo which is endoderm, mesoderm, and ectoderm.
Growth and Differentiation differentiation: changing of unspecialized embryonic cells into specializd cells to tissue and organs.
Why do cells differentiate? Parts of DNA turned on/off determines the role of a cell.