Reproduction

• Meiosis vs Mitosis


  Meiosis is the formation of four non-identical cells from one cell.

  Mitosis is the formation of two identical cells from one cell.
• Sexual Reproduction


  Involves the fusion of male and female gametes to form a zygote.

  Gametes are sperm and egg in animals, pollen and egg in plants.

  During fertilisation the nuclei of the male and female gamete fuse so there is genetic variation in the offspring.

  Formation of gametes involves meiosis.
• Asexual Reproduction


  Does not involve gametes or fertilisation.

  Only one parent required so there is no mixing of genetic information, therefore offspring are genetically identical to the parent and to each other.

  Only mitosis is involved.
• Meiosis


  Cells in the reproductive organs divide by meiosis to form gametes.

  The number of chromosomes must be halved when gametes are formed so the offspring has the correct number of chromosomes when fertilisation occurs.

  This halving occurs during meiosis in which the chromosome number is halved from 46 (diploid) to 23 (haploid).
• Meiosis: Process

  Each chromosome is duplicated, forming 46 pairs of X-shaped chromosomes.

  First Division: chromosome pairs line up along the centre of the cell and are pulled apart so that each new cell only has one copy of each chromosome; results in two daughter cells with 46 chromosomes.

  Second Division: chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart. A total of 4 haploid daughter cells with 23 chromosomes will be produced.
• Fertilisation


  Gametes join at fertilisation to restore the normal amount of chromosomes, forming a zygote containing the full number of chromosomes (46).

  The zygote divides by mitosis to form two new cells, which continue to divide, forming an embryo after a few days.

  Cell division and eventually the cells begin differentiating into specialised cells to form all the body tissues of the offspring.
• Advantages and Disadvantages of Sexual Reproduction

  Advantages: Increases genetic variation, so the species has a higher likelihood of adapting to a change in the environment which can provide a survival advantage to prevent the species from going extinct. Sexual reproduction can be controlled via selective breeding to produce livestock with desired characteristics, increasing food production.

  Disadvantages: Takes a lot of time and energy to find mates, produces less offspring and at a slower rate than asexual.
• Advantages and Disadvantages of Asexual Reproduction

  Advantages: Faster than sexual reproduction, more energy and time efficient as organisms do not need to find a mate, only one parent needed, many identical offspring can be produced under favourable conditions.

  Disadvantages: No genetic variation in population as offspring are genetically identical to parents and each other, so population is vulnerable to environmental change.
• Organisms that Reproduce Both Ways

  Malarial parasites produce asexually inside the human host, but sexually inside the mosquito.

  Many fungi reproduce asexually by releasing spores but also reproduce sexually to give variation.

  Many plants produce seeds sexually, but also reproduce asexually by runners e.g. strawberry plants, or by bulb division such as daffodils.
• Genome


  The entire genetic material of an organism.

  The whole human genome has now been studied and this will have great importance for medicine in the future.

  It has improved our understanding of the genes linked with different types of diseases and inherited genetic disorders.

  The human genome also allows us to study human migration patterns from the past as different populations of humans living in different areas have developed slight differences in their genomes.
• DNA


  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.

  DNA is contained within structures called chromosomes.
• Gene


  A small section of DNA on a chromosome.

  Each gene codes for a specific sequence of amino acids, and these sequences form different types of proteins.

  Genes control our characteristics as they code for proteins that have an important role in what our cells do.
• Structure of DNA: Nucleotides

  DNA is a polymer - a molecule made from repeating subunits called monomers.

  The monomers that make up DNA are called nucleotides.

  Each nucleotide consists of a sugar attached to a phosphate, and one of four different bases attached to the sugar.

  There are four different types of nucleotides: each with the same phosphate and sugar, but differ from each other with the base attached, which can be either A, T, C or G.

  
• Structure of DNA: Base Pairing

  The bases on each strand pair up with each other, holding the two strands of DNA in the double helix.

  The bases always pair up in the same way: A with T, and C with G.

  This is called complementary base pairing.

  
• Coding for Amino Acids

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

  The order of bases in a sequence controls the order and different types of amino acids that are joined together.

  These amino acid sequences then join together to form a protein.

  In this way, it is the order of bases in DNA that determines which proteins are made.
• The Double Helix


  The sugar-phosphate section of the nucleotide forms the backbone of the DNA strand, and the base pairs of each strand connect the two strands together.

  
• Protein Synthesis: mRNA


  Proteins are made in the cell cytoplasm on structures called ribosomes.

  Ribosomes use the base sequences in DNA to make proteins.

  DNA cannot travel out of the nucleus to the ribosomes as it is too large, so the base code of each gene is transcribed onto small RNA molecules called messenger RNA (mRNA).

  The DNA strands are un-winded and the sequence of bases is copied into the mRNA molecules, which are now templates.

  The templates exit the nucleus and enter the cytoplasm, where they attach to the ribosomes.
• Protein Synthesis: Ribosomes


  The ribosome reads the code on the templates in groups of three as each triplet/codon of bases codes for a single specific amino acid.

  Carrier molecules bring specific amino acids to add to the growing protein chain in correct order.

  Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein.
• Structure of Proteins


  When the protein chain is complete, it folds up to form a unique shape.

  This unique shape allows the protein to enable a specific function. For example, proteins can either be:

  Enzymes.

  Hormones.

  Structural proteins (e.g. collagen)
• Mutations


  Random changes in the sequence of DNA bases within a gene or chromosome.

  Mutations occur continuously, and can sometimes lead to a change in the protein the gene codes for.

  However most mutations do not alter the protein or alter it slightly so that is function or appearance is not changed.
• Types of Mutation: Insertion


  A new base is randomly inserted into the DNA sequence.

  This changes the amino acid that would have been coded for by the original base triplet.

  This also has a knock-on effect by changing the base triplets further on in the sequence.

  
• Types of Mutation: Deletion

  A base is randomly deleted from the DNA sequence.

  This changes the amino acid that would have been coded for by the original base triplet.

  This also has a knock-on effect by changing the base triplets further on in the sequence.
• Types of Mutation: Substitution


  A base in the DNA sequence is randomly swapped for a different base.

  This only changes the amino acid for the triplet the mutation occurs in and does not have a knock-on effect.
• Effects of Mutations


  Most mutations do not alter the protein or only alter it slightly so the appearance or function is not changed.

  However a small number of mutations code for a significantly altered protein with a different shape.

  E.g. if the shape of an enzyme's active site changes the substrate may no longer be able to bind to it.

  E.g. structural proteins like collagens may lose their strength if their shape is changed.
• Gene Switching


  Not all parts of DNA code for proteins; some non-coding parts can switch genes on and off.

  This means they can control whether or not a gene is expressed.

  Mutations in these areas of DNA can affect how genes are expressed.
• Gamete


  Sex cells.

  Pollen and egg cells in plants; sperm and egg cells in animals.
• Chromosome


  Thread-like structures of DNA, carrying genetic information in the form of genes.

  Located in the nucleus of cells.
• Gene


  Short lengths of DNA found on chromosomes that code for specific proteins.
• Alleles


  Different versions of a particular gene.
• Dominant allele


  Always expressed, even if only one copy is present.
• Recessive allele


  Only expressed if two copies are present; therefore no dominant allele present.
• Homozygous


  If the two alleles of a gene are the same, the individual is homozygous.
• Heterozygous


  If the two alleles of a gene are different, the individual is heterozygous.
• Genotype


  The combination of alleles that control each characteristic.
• Phenotype


  The observable characteristics of an organism e.g. eye colour.