B13 Reproduction

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B13 Reproduction

B13.1 Types of reproduction

Asexual reproduction involves only one parent, and cells divide by mitosis. There is no gamete fusion and clones (genetically identical) are made. Mitosis involves dividing into two genetically identical cells to grow or replace worn-out tissues.

Sexual reproduction is when a male and female gamete (sperm and ovum in animals, pollen and ovum in plants) fuse to form a zygote which develops into an individual. Gametes are formed by meiosis, and the chromosome number is halved. Genetic information is inherited from both parents and there is variation - not being genetically identical. Sexual reproduction is risky due to relying on two cells to meet and fuse, but introduces variation, important for a species' long-term survival.

B13.2 Cell division in sexual reproduction

Ova are made in the ovaries and sperm in the testes.

In meiosis, when a cell divides to form gametes, the DNA is copied, resulting in four sets of each chromosome forming a pair of chromatids, and then divides twice in quick succession to form four gametes, each with a single set of chromosomes and each genetically different, containing random mixtures of the original chromosomes.

When two gametes join, the zygote will have a full st of chromosomes and a unique genetic combination, then dividing by mitosis to form an individual, and cell number rising, and cells differentiating through development.

B13.3 The best of both worlds

Asexual reproduction is time and energy efficient since you don't have to find a mate, and is faster than sexual reproduction, although no variation is introduced, meaning that they could die by the same disease. Sexual reproduction, however, does introduce variation, which means that some can survive in environmental changes (natural selection).

Fungi use asexual reproduction. Fungal spores made by mitosis are clones. Two hyphae (a mass of thin threads) from different fungi join and the nuclei fuse for two sets of chromosomes, undergoing mitosis to make haploid spores with only one set of chromosomes. They reproduce sexually in adverse conditions.

In plants, flowers contain the sexual reproduction parts. Gametes are formed by meiosis. Pollen from one flower reaches the female parts of another plant - pollination. Flowers are adapted to attract animal pollinators - eg bees - or to catch pollen easily. Plants also reproduce asexually. New plants grow due to mitosis, even in adverse conditions.

Malarial parasites spend part of their life cycle in a female mosquito's body and thn another part in the blood and organs of a human - a combination of meiosis and mitosis, asexually only in adverse conditions. They produce asexually in the human body, and when the mosquito eats, the temperature drop between her and the human body triggers sexual reproduction and the zygotes undergo meiosis to produce new asexual parasites.

B13.4 DNA and the genome

Chromosomes are made up of long molecules of DNA - a polymer with a double helix structure. Genes are small sections of DNA, containing genetic information and coding for a specific sequence of

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