inheritance, variation and evolution

• meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed.
• Sexual reproduction involves the joining (fusion) of male and female gametes: -sperm and egg cells in animals -pollen and egg cells in flowering plants.
• In sexual reproduction there is mixing of genetic information which leads to variety in the offspring. The formation of gametes involves meiosis.
• Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.
• Mitosis is the division of the parent cell into two genetically identical daughter cells. The cell now has double the chromosomes, more organelles and is bigger. It is ready to divide into two daughter cells.
• Meiosis is a type of cell division that results in the production of four genetically different daughter cells from a single parent cell. It is the process that produces reproductive cells, such as sperm and eggs, for sexual reproduction.
• mitosis = genetically identical
• meiosis = genetically different
• meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.
• Cells in reproductive organs divide by meiosis to form gametes.
• When a cell divides to form gametes (via meiosis): -copies of the genetic information are made - the cell divides twice to form four gametes, each with a single set of chromosomes - all gametes are genetically different from each other
• Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
• dna structure = double helix held together by complementary base pairs
• The basic units of DNA are nucleotides. These nucleotides consist of a deoxyribose sugar, phosphate and base.
• The genome of an organism = the entire genetic material of that organism.
• The genetic material in the nucleus of a cell is composed of a chemical called DNA.
• DNA is a polymer made up of two strands forming a double helix
• The DNA is contained in structures called chromosomes
• A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.
• The genome of an organism is the entire genetic material of that organism. The whole human genome has now been studied and this will have great importance for medicine in the future.
• gamete = a sex cell. they contain half the normal number of chromosomes (23). they are haploid cells
• chromosome = a thin strand of DNA. they are subdivided into genes.
• gene = a section of DNA which controls part of a cells chemistry
• allele = different versions for the same gene. (a person has an allele for blue eye colour and an allele for brown)
• alleles may be either dominant or recessive
• a dominant allele is always expressed
• the allele for brown eyes is dominant
• a recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene.
• the allele for blue eyes is recessive
• homozygous = 2 identical alleles AA or aa
• heterozygous = both alleles are different for the same characteristic Aa
• genotype = the collection of genes an organism has. this is inherited from the parents
• phenotype = the physical characteristics of an organism. this is determined by environmental AND genetic factors
• The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
• most phenotype features are the result of multiple genes rather than single gene inheritance.
• Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.
• Polydactyly (having extra fingers or toes) is caused by a dominant allele.
• Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
• Embryo screening involves the biopsy of embryos during IVF to assess for chromosome abnormalities before selection for transfer
• Ordinary human body cells contain 23 pairs of chromosomes