reproduction

• Only one parent is needed in asexual reproduction. There is no fusion of gametes so genetic material does not mix, which means that the offspring produced through this process are genetically identical clones to the parent.
• Examples of organisms that use asexual reproduction include:

  • bacteria
  • production of spores by fungi
  • some plants, such as strawberries, use runners
  • formation of tubers in potatoes and bulbs in daffodils
• Asexual reproduction uses the process of mitosis to create the identical copies (clones) of the parent cell.

  Mitosis
• Mitosis is part of the cell cycle, which involves:

  • cell growth, and the increase of the number of structures in the cell (mitochondria, ribosomes)
  • genetic material is copied
  • mitosis occurs leading to chromosomes separation and cell division
• Mitosis is also used for:

  • growth
  • repair to damaged tissue
  • replacement of worn-out cells
• Two parents are needed in sexual reproduction. During this process the nuclei of the male and female gametes are fused in order to create a zygote. This process is known as fertilisation. The gametes contain half the number of chromosomes in each (haploid). When the male and female gametes combine they create the full complement of chromosomes (diploid) in order to create a human embryo.
• The gametes in:

  • animals are sperm and eggs
  • flowering plants are pollen and eggs
• The offspring produced in sexual reproduction are genetically different to each other and the parents. This process results in variation as it involves the mixing of genetic information.
• Sexual reproduction uses the process of meiosis, which creates gametes. The process of meiosis happens in the male and female reproductive organs.
• As a cell divides to form gametes:

  • copies of the genetic information is made
  • the cell divides twice to form four gametes, each with a single set of chromosomes (haploid)
  • all gametes are genetically different from each other
• Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.
• In humans, each gamete has half the number of the total 46 chromosomes that the body requires. Twenty three chromosomes within a gamete are referred to as a haploid. When the two gametes combine, they merge the two sets of chromosome to have 46, which are referred to as diploid.
• This produces a new cell called a zygote, which will mature into an embryo. The number of cells increase by mitosis, and as the embryo develops, the cells begin to differentiate (or specialise).
• The advantages of sexual reproduction:

  • Produces variation in the offspring
  • The species can adapt to new environments due to variation, which gives them a survival advantage
  • A disease is less likely to affect all the individuals in a population
  • Humans can speed up natural selection through selective breeding, which can increase food production
• The advantages of asexual reproduction include:

  • the population can increase rapidly when the conditions are favourable
  • only one parent is needed
  • it is more time and energy efficient as you don't need a mate
  • it is faster than sexual reproduction
• The disadvantages of sexual reproduction:

  • time and energy are needed to find a mate
  • it is not possible for an isolated individual
• The disadvantages of asexual reproduction include:

  • it does not lead to variation in a population
  • the species may only be suited to one habitat
  • disease may affect all the individuals in a population
• Some organisms reproduce by sexual and asexual reproduction, but this depends on the particular circumstances.
• Fungi reproduce sexually to generate variation
• Fungi release spores by asexual reproduction
• Plants use sexual reproduction to produce seeds
• Plants such as strawberries reproduce asexually by sending out runners, or daffodils when their bulbs divide
• Malarial parasites reproduce sexually in the host mosquito
• Malarial parasites reproduce asexually in the human host
• The genetic material in the nucleus of a cell is composed of a chemical called DNA
• DNA is a polymer, a large and complex molecule. It is made up of two strands forming a twisted ladder structure called a double helix. It carries the genetic code, which determines the characteristics of a living organism.
• Except for identical twins, each person's DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, which can form a 'bar code' that is different from one person to the next.
• The cell's nucleus contains chromosomes. These are long threads of DNA, which are made up of many genes.
• A gene is a small section of DNA in a chromosome. Each gene codes for a particular sequence of amino acids in order to make a specific protein. It is the unit of heredity, and may be copied and passed on to the next generation.
• The genome of an organism is the entire genetic material of that organism. The whole human genome has been studied, and this has great importance for medicine.
• In order to exploit its secrets, it is vital that the human genome is fully understood.

  It enables us to:

  • search for genes linked to different types of disease
  • understand inherited disorders and their treatment
  • trace human migration patterns from the past
• Scientists are searching for disease associated genes. One example was those that can contribute to breast cancer, which are known as BRCA1 and BRCA2. Mutations in these genes account for approximately 10% of all inherited breast cancer cases detected.
• Scientists detected BRCA1 and BRCA2 genes by studying families where breast cancer was known to have been inherited between individuals. They were able to create a pedigree analysis, which is similar to a family tree diagram that showed the close relationship of those affected and unaffected within the family.
• The pedigree analysis illustrates the inheritance pattern of the disease to be determined. This enabled scientists to test DNA of the affected and unaffected individuals to identify differences. It is now possible to detect the presence of the genes by having a simple blood test.
• ames Watson and Francis Crick worked out the structure of DNA in 1953. By using data from other scientists (Rosalind Franklin and Maurice Wilkins) they were able to build a model of DNA. The X-ray crystallography data they used showed that DNA consists of two strands coiled into a double helix.
• DNA is a polymer made from four different nucleotides. These are arranged in a repeating fashion. Each nucleotide consists of alternating sugar and phosphate sections with one of the four different bases attached to the sugar.
• Each strand of DNA is made of chemicals called bases. Note that these are different to bases in relation to acids and alkalis in chemistry.
• There are four different bases in DNA:

  • thymine, T
  • adenine, A
  • guanine, G
  • cytosine, C
• There are chemical cross-links between the two strands in DNA, formed by pairs of bases. They always pair up in a particular way, called complementary base pairing:

  • thymine pairs with adenine (T-A)
  • guanine pairs with cytosine (G-C)
• A sequence of three bases is the code for a particular amino acid, which is known as a triplet or the triplet code