6. inheritance and variation

• Mitosis & Meiosis

  • Mitosis is a type of nuclear division that gives rise to cells that are genetically identical
  • It is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction
  • Meiosis is a type of nuclear division that gives rise to cells that are genetically different
  • It is used to produce gametes (sex cells)
• Sexual Reproduction

  • Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
  • The gametes of animals are the sperm cells and egg cells
  • The gametes of flowering plants are the pollen cells and egg cells
  • Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
  • The formation of gametes involves meiosis
• Asexual Reproduction

  • Asexual reproduction does not involve sex cells or fertilisation
  • Only one parent is required so there is no fusion of gametes and no mixing of genetic information
  • As a result, the offspring are genetically identical to the parent and to each other (clones)
  • Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent
  • Only mitosis is involved in asexual reproduction
• Meiosis


  The process by which cells in reproductive organs divide to form gametes (sex cells)
• Meiosis

  1. Chromosomes double themselves as in mitosis

  2. Chromosomes line up in the centre of the cell

  3. Cells divide twice

  4. One copy of each chromosome passes to each gamete
• Meiosis

  • The number of chromosomes must be halved when the gametes are formed
  • This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
  • Gametes are haploid - having half the normal number of chromosomes
  • Meiosis produces four haploid cells
• If the chromosome number was not halved during meiosis

  There would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
• Process of meiosis

  • Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes
  • First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
  • Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
  • A total of four haploid daughter cells will be produced
• Importance of meiosis

  • Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants
  • Increases genetic variation of offspring
  • Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others
• Fertilisation

  1. Gametes join

  2. Restore normal number of chromosomes
• Zygote

  Fertilised egg cell
• The zygote contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete
• Cell division after fertilisation

  1. Zygote divides by mitosis to form two new cells

  2. Cells continue to divide

  3. After a few days form an embryo
• Cell differentiation

  1. Cell division continues

  2. Many new cells become specialised to perform particular functions

  3. Form all the body tissues of the offspring
• AdvantagesDisadvantages of meiosis

  advantageIncreases genetic variation

  disadvantage-Takes time and energy to find mates

  -The species can adapt to new environments due to variation, giving them a survival advantage

  -Difficult for isolated members of the species to reproduce

  -Disease is less likely to affect the population (due to variation)
• • An additional advantage of sexual reproduction is our ability to use it and control it for our own needs:
  • Natural selection can be speeded up by humans in selective breeding to increase food production
  • We have controlled sexual reproduction in cows and selectively bred them to produce offspring that produce more milk and more meat than they would have under natural conditions
• Advantages & Disadvantages of Asexual Reproduction 

  • The population can be increased rapidly when conditions are right
  • Limited genetic variation in population - offspring are genetically identical to their parents
  • Can exploit suitable environments quickly
  • The population is vulnerable to changes in conditions and may only be suited for one habitat
  • More time and energy-efficient
  • Disease is likely to affect the whole population as there is no genetic variation
  • Reproduction is completed much faster than sexual reproduction
  • As only one parent is needed,
• Malarial parasites

  • Malaria is caused by parasites that are carried by mosquitoes
  • The parasites are transferred to a human when the mosquito feeds on the human’s blood
  • These malarial parasites reproduce asexually in the human host, but sexually in the mosquito
• Fungi

  • Many fungi reproduce both asexually and sexually
  • These species of fungi release spores, which develop into new fungi
  • These spores can be produced via asexual or sexual reproduction
  • Spores that are produced via sexual reproduction show variation (they are genetically different from each other)
• Plants

  • Many plants produce seeds via sexual reproduction but are also able to reproduce asexually
  • They reproduce asexually in different ways:
  • Some plants (eg. strawberry plants) produce ‘runners’ (stems that grow horizontally away from the parent plant, at the end of which new identical offspring plants form)
  • Some plants (eg. daffodils) reproduce via bulb division (new bulbs form from the main bulb underground and then grow into new identical offspring plants
• The Genome

  • The entire set of the genetic material of an organism is known as its genome
  • Biologists now know the entire human genome (they have worked out all the genes that are found in humans)
• The Structure of DNA

  • The genetic material in the nucleus of a cell is composed of a chemical called DNA
  • DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms
  • DNA is a polymer made up of two strands forming a double helix
  • DNA is contained in structures called chromosomes
  • Chromosomes are located in the nucleus of cells
• Genes

  • A gene is a short length of DNA found on a chromosome
  • Each gene codes for a particular sequence of amino acids
  • These sequences of amino acids form different types of proteins
  • There are many different types of proteins but some example of these could be:
  • structural proteins such as collagen found in skin cells
  • enzymes
  • hormones
  • Genes control our characteristics as they code for proteins that play important roles in what our cells do
• The Human Genome Project


  The name of the international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings
• The Human Genome Project was completed

  2003
• The Human Genome Project

  • It was an international, collaborative research effort
  • Its goal was to determine the DNA sequence of the entire human genome and record every gene in human beings
• Human genome

  The complete set of genetic information in human beings
• The Human Genome Project was a very important breakthrough
• Importance of the Human Genome Project

  • Improved our understanding of the genes linked with different types of disease and inherited genetic disorders
  • Helped us in finding treatments
• The human genome has made it possible to study human migration patterns from the past
• Different populations of humans living in different parts of the world have developed very small differences in their genomes
• Nucleotides

  • DNA is a polymer (a molecule made from many repeating subunits)
  • These individual subunits of DNA are called nucleotides
  • Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar

  
• Base Pairing: Basics

  • There are four different nucleotides
  • These four nucleotides contain the same phosphate and deoxyribose sugar, but differ from each other in the base attached
  • There are four different bases: Adenine (A), Cytosine (C), Thymine (T) and Guanine (G)
• Base PairingHigher tier only

  • 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:
  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)
  • This is known as ‘complementary base pairing’
• Coding for Amino Acids

  • A sequence of three bases is the code for a particular amino acid
  • The order of bases controls the order and different types of amino acids that are joined together
  • These amino acid sequences then form a particular type of protein
  • In this way, it is the order of bases in the DNA which eventually determines which proteins are produced
• Double Helix

  • The phosphate and sugar section of the nucleotides form the ‘backbone’ of the DNA strand (like the sides of a ladder) and the base pairs of each strand connect to form the rungs of the ladder
  • It is this sequence of bases that holds the code for the formation of proteins
• Protein Synthesis

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

  2. Ribosomes use the sequence of bases contained within DNA to make proteins

  3. DNA cannot travel out of the nucleus to the ribosomes (it is far too big to pass through a nuclear pore) so the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA)

  4. mRNA can move out of the nucleus and attaches to a ribosome (the mRNA acts as a messenger between DNA and the ribosome)

  5. The correct sequence of amino acids are then brought to the ribosome and joined together

  6. This amino acid sequence then forms into a protein
• Protein Synthesis

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

  2. Ribosomes use the sequence of bases contained within DNA to make proteins

  3. DNA cannot travel out of the nucleus to the ribosomes, so the base code of each gene is transcribed onto an RNA molecule called messenger RNA (mRNA)

  4. mRNA can move out of the nucleus and attaches to a ribosome (the mRNA acts as a messenger between DNA and the ribosome)

  5. The correct sequence of amino acids are then brought to the ribosome and joined together

  6. This amino acid sequence then forms into a protein
• Function of RibosomesHigher tier only

  • The ribosome ‘reads’ the code on the mRNA in groups of three
  • Each triplet of bases codes for a specific amino acid
  • Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order
  • In this way, the ribosome translates the sequence of bases into a sequence of amino acids that make up a protein
  • 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
• The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein