Reproduction

- in asexual reproduction, there is no fusion of gametes and only one parent. There is no mixing of genetic information, leading to genetically identical offspring (clones)

- only mitosis is involved in asexual reproduction

- sexual reproduction reproduction involves the joining (fusion) of male and female gametes formed by meiosis

    ... meiosis leads to the formation if non-identical cells, sperm and egg cells in animals, and pollen and egg cells in flowering plants

- in sexual reproduction there is a mixing of genetic information that leads to variation in the offspring

- cells in the reproduction organs divide by meiosis to form the gametes (sex cells)

- body cells have two cells of chromosomes, gametes have only one set

- in meiosis, the genetic material is copied and then the cells 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 and as the embryo develops, the cell differentiate

- sexual reproduction produces variation that helps survival through natural selection if the environment changes. Natural selection is sped up by humans in selective breeding

- asexual reproduction needs only one parent, is time and energy efficient, often faster than sexual reproduction and many identical offspring are produced when conditions are favourable

- some organisms depend on both asexual and sexual reproduction depending on the circumstances

- malaria parasites reproduce sexually in mosquitoes and asexually in their human host

- many plants produce seeds asexually by spores but can also reproduce sexually to give variation

- many plants produce seeds sexually but also reproduce asexually, for example by runners or bulb division

- 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 been great importance for medicine in the future

- the genetic material in the nucleas of a cell is composed of DNA , DNA is a polymer made up of two strands forming a double helix 

- 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 long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of four bases- A, C, G or T. Each unit of a sugar, phosphate and base is known as a nucleotide 

- a sequence of three bases is the code for a particular amino acid

- the order of bases controls the order in which amino acids are assembled to produce a particular protein

- in the complementary strands of DNA, a C is always linked with a G on the opposite strand and a T to an A

- proteins are synthesised according to a template, carrier molecules bring specific amino acids to add to the growing protein chain in the correct order

- when the protein chain is complete it folds up to form a unique shape that enables the protein to carry out its funtion in the cell

- not all parts of the DNA code for proteins, non-coding parts switch genes on or off, so variations in these areas of DNA can affect how genes are expressed

- a change in the DNA structure may result in a change in the protein synthesised by the gene

- mutations occur continuously, most do not alter the protein, or they alter it so slightly that the function is not affected

- a few mutations code for an altered protein with a different shape, affecting the function. this may be an advantage or a disadvantage 

- some characteristics are controlled by a single gene, each gene may have different forms called alleles

- the alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as the phenotype

- if the two alleles are the same, the individual is homozygous for that trait, but if the alleles are different they are heterozygous

- a dominant allele is always expressed in the phenotype, even if only one copy is present, a recessive allele is only expressed if two copies are present 

-most characteristics are the result of multiple genes interacting, rather than a single gene

- direct proportion and ratios can be used to express the outcome of a genetic cross

- use punnett squares and family trees to understand genetic inheritance

- construct a punnett square diagram to predict the outcome of a monohybrid cross

- ordinary human body cells contain 23 pairs of chromosomes:

    22 control general body characteristics only,

    but the sex chromosomes carry the genes that determine sex

- in human females the sex chromosomes are the same (**) whilst in males the sex chromosomes are different (XY)

- some disorders are inherited 

- polydactyly is a dominant allele which can be inherited from either or both parents

- cystic fibrosis is a recessive phenotype and is caused by recessive alleles which must be inherited from both parents

- cells from embryos and fetuses can be screened for the alleles that cause many genetic disorders

- embryo and fetal cells are used to identify genetic disorders but screening raises economic, social and ethical issues