Biology Chapter 8 - Asexual and Sexual Reproduction

Reproduction

The creation of a new generation of organisms from existing cells. Can be asexual or sexual.

Asexual reproduction

Reproduction involving one parent and without the fusion of gametes, producing 2 daughter cells identical to the parent, either through mitosis (eukaryotes) or binary fission (prokaryotes)

6 types of asexual reproduction

Fission

Budding

Vegetative propagation

Spore formation

Fragmentation

Parthenogenesis

Fission

Rapid equal division of parent cell into 2 new cells through mitosis followed by cytokinesis, that occurs in unicellular protists like amoeba and paramecium.

Binary fission vs fission

Binary fission is simpler, occurs in bacteria rather than protozoan, and involves just a replication of 1 circular chromosome and plasmids, then cytokinesis.

Budding

Development of a new organism from the outgrowth of a parent organism that can detach and grow into an adult, that occurs in yeast, protozoan hydra, and some flatworms

Vegetative propagation

Separation of plant parts to form new, independent plants from leaves, stems and underground stem or roots. E.g. runners, cuttings and bulbs. Often used in horticulture to produce genetically identical plants in many flowering plants and trees. One type is grafting.

Grafting

Where a part of a plant (scion) is joined to another similar plant (stock). Carried out on most fruit trees and roses

Spore formation

Spores are hardy, self-contained, small and light capsules which are released from the parent into the environment and are able to germinate into new individuals. E.g. prokaryotes, protists, fungi, moss and ferns.

Fragmentation

Parent breaks into fragments, each capable of growing independently into a new organism. E.g. hydra, flatworms, starfish

Parthenogenesis

Offspring develops from egg or female gamete without the prior fertilisation from the male gamete. E.g. daphnia, bees, stick insects, some reptiles, sharks

Advantages of asexual reproduction

- Advantage in ideal, unchanging environment where individuals are well adapted to the environment

- Only one parent required so no energy wasted on finding a mate

- Allows for efficient and rapid reproduction under suitable conditions, where offspring will all be well-suited to the environment

Disadvantages of asexual reproduction

- Lack of genetic diversity in the population

- Unfavourable conditions can wipe out the population

- Large number of offspring may have to compete for limited resources

Sexual reproduction

Fusion of haploid gametes/sex cells produced through meiosis from somatic cells to make a unique diploid zygote with a full complement of DNA for the species

How does meiosis contribute to genetic variation?

- Independent assortment - the random distribution of chromosomes from a homologous pair into gametes.

- Crossing over - exchange of segments of homologous chromosomes during meiosis resulting in new combinations of alleles

Spermatogenesis

Each of the four haploid cells produced from a male germline cell will undergo differentiation and become 4 sperm cells, two carrying 'X', two carrying 'Y' chromosomes.

ADD IN slide 18

Oogenesis

Cytoplasm divides unevenly, with one cell receiving more (incl. all organelles) and the other 3 polar bodies disintegrating.

ADD IN slide 19

Advantages of sexual reproduction

- Infinite genetic variation

- in a changing environment, at least some individuals will survive to carry on the species

Disdvantages of sexual reproduction

- the need to find a mate

- requires more energy.

- usually limited to the times of year which gives the young more chance of survival

Plant gametes

Male - pollen

Female - ovules

Cloning

Allows for the production of genetically identical organisms

Plant cloning

Asexual reproduction e.g. cuttings, tissue culture

Animal cloning

Embryo splitting or Somatic Cell Nuclear Transfer

Embryo splitting

Egg cells are fertilised by sperm in vitro, then after the zygotes divides the two cells are split, each developing into an embryo that are treated and allowed to develop before being implanted into surrogate mothers.

Offspring are genetically identical to each other but not related to surrogate mothers

ADD IN slide 25

SCNT

Somatic Cell Nuclear Transfer. A nucleus from a mature somatic cell of a donor is inserted into an ovum which has had the nucleus removed. It is incubated and prepared, then surgically implanted into surrogate uterus to develop normally. Low success rate, with some offspring showing deformities.

ADD IN slide 26

Applications of animal cloning in agriculture

Increase stock with desirable traits.

Applications of plant cloning in agriculture

Vegetative propagation to produce identical plants for economic reasons e.g. Cavendish bananas, which are all sterile so each new plant must be manually cut from existing banana roots.

Micropropogation/tissue culture

A type of propagation that uses small pieces of plant to grow a callus, which is treated with plant growth factors to produce new plants in vast numbers economically