Reproduction

13 REPRODUCTION
Reproduction: one of the fundamental characteristics of all living things, in order to maintain the specie.
Each organism has a particular method of reproduction, but all will fit into 2 categories:
* Sexual reproduction.
* Asexual reproduction.
In both cases, in reproduction, each new organism obtains a set of chromosomes from its parent or parents.
Chromosomes: formby long threads of DNA, and it contains the GENES.

Asexual reproduction
Definition: process resulting in the production of genetically identical offspring from one parent.
Asexual reproduction involves just one parent. Some of the parent organism´s cells divide by mitosis (kind of cell division). This produces new cells containing exactly the same genes as the parent cell (geneticallyidentical). E.g.: one parent cell is going to divide. Before doing so, the DNA is going to replicate (make a copy of itself), and then give 2 daughter cells which are going to have the same genetic information as the parent.
Examples of asexual reproduction:
* Bacteria: Bacteria reproduce by binary fission.

The cells of bacteria, unlike animal or plant cells, do not have nucleus containingchromosomes and therefore they do not divide by mitosis, but by binary fission (splitting into two). In this process, the DNA (single, circular molecule) is replicated (copied) just when the cell is going to divide. Then, after division, the 2 new cells are going to have the same genetic information as their parent. Bacteria are able to divide every 20 minutes under the right conditions.

*Potatoes reproduce asexually by producing stem tubers
Many plants can reproduce asexually. Gardeners and farmers make a good use of this, because asexual reproduction can quickly and efficiently produce many new plants, all of them genetically identical to one another. If the grower has found a plant with the characteristics wanted, asexual reproduction is going to be more advantageous.


Forexample, potatoes reproduce using stem tubers, which are used for energy reserve(starch)
The tubers are harvested for human consumption, and some are saved to produce next year´s crop. Potatoes are then cut into pieces, making sure of leaving the eyebuds on them, and planted. From each piece a new potato plant will grow, and it will be genetically identical to the parent plant.
Sexual reproduction:Definition: the process involving the fusion of haploid nuclei to form a diploid zygote and the production of genetically dissimilar offspring.
In sexual reproduction the parent organism produce sex cells called GAMETES (e.g. eggs and sperm in humans) produced by a special kind of cell division called MEIOSIS.
Sexual reproduction involves FERTILISATION: Gametes join and their nuclei fusetogether forming a ZYGOTE: (new cell form by fertilization). The zygote divides continuously to form a new organism.
(see figure 13.6 from the book)
FERTILISATION:

Gametes have half the normal number of chromosomes: Human cell: 46, (23 pairs of different kinds, two sets of chromosomes in each cell), meanwhile human gametes (egg and sperm):23 chromosomes.
In sexual reproduction, each new organismobtains a set of chromosomes from each parent, 1 set from the mother and 1 from the father.
Cell with 2 complete sets: diploid (2n), and cell with only 1 set: haploid (gametes)(n)
As mentioned before, chromosomes are long threads of DNA found in the nucleus of a cell, contain set of instructions known as GENES. Genes present slightly variation from one another in different individuals.Comparison between sexual and asexual reproduction:
SEXUAL | ASEXUAL |
Offspring genetically dissimilar to parents | Offspring genetically identical |
Needs sex cells (gametes) | Reproduction occurs as a result of cell division (mitosis) |
Involves fertilisitation (fusion of gametes) | DO not need sex cells |
2 parents involved | 1 parent involved |

SEXUAL REPRODUCTION IN FLOWERING...