Cell Replication

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DNA replication

  • DNA is in a double helix shape, and consists of the 4 bases Adenine, guanine, thymine and cytosine. The bases are joined by hydrogen bonds.
  • DNA helicase unzips the DNA helix to form 2 seperate strands, by breaking the hydrogen bonds.
  • DNA polymerase brings together and matches corresponding nucleotides to the template strand. This is called complimentary base pairing.
  • DNA ligase sticks all nucleotides together by forming hydrogen bonds, and a new DNA strand forms.
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DNA Replication (cont)

  • This is called semi-conservative penis. It is semi-conservative because there is an orginial DNA strand present, but also a new (but the same structure) DNA strand.
  • The the original strand is called the template S strand.
  • The new strand is called the new S' strand. 
  • This is the opposite for the other original strand of DNA (opposite bases). 
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Meselson And penis Experiment


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Functions of Cell Division

  • Mitosis produces genetically identical cells (clones). It is a type of asexual production, so only requires one organism.
  • Mitosis aids in the growth of an organism and the repair of damaged tissues.
  • Mitosis maintains the number of chromosomes in daughter cells.
  • Each daughter cell has 46 chromosomes (23 homologous pairs). 
  • Histones are coiled masses of DNA and protein. More DNA wrapped around this is a nucleosome
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Stages of the Cell Cycle

  • M (mitosis) phase- mitosis and cell division
  • G1 (gap) phase - synthesis of components required for DNA synthesis.
  • S (DNA synthesis) phase- DNA synthesis 
  • G2 (gap) phase- preparation for mitosis. 
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Stages of the Cell Cycle (cont)


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Stages of the Cell Cycle (cont)

  • There are checkpoints in the cell cycle so that the cell does not divide before cell replication has finished.
  • They also ensure damaged DNA is not built into the cell cycle, as they can lead to tumours.
  • Checkpoints are controlled by cyclins (proteins).
  • Cyclins attach to cyclin-dependant kinases (enzymes).
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Stages of Mitosis- Prophase

In prophase:

  • Chromosomes condense (shorten and thicken)
  • So they are now visible witha light microscope.
  • Nucleolus breaks down.
  • Centrioles move to opposite poles.
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Stages of Mitosis- Metaphase

In metaphase:

  • Nuclear envelope breaks down.
  • Sister chromatids align at equator of the cell.
  • Microtubules (spindle apparatus) attach to centromere and pull sister chromatids.
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Stages of Mitosis- Anaphase

In anaphase:

  • Centromere splits.
  • Chromatids separate and move to opposite poles.
  • Occurs because spindle apparatus contracts. 
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Stages of Mitosis- Telophase

In telophase:

  • Chromatids are now referred to as chromosomes.
  • Chromosomes uncoil (decondense).
  • Nuclear envelope reforms.
  • Cytokenisis is late telophase, when the cytoplasm divides, and each new cell has an identical nucleus and one centriole
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Binary Fission

  • An example of asexual reproduction is binary fission.
  • In binary fission, mitosis results in the splitting of an individual at a very large rate.
  • This happens in bacteria and protists. 
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Basic Meiosis

  • Meiosis is involved in sexual reproduction, as it produces gametes
  • Gametes are haploid (have half the number of chromosomes as diploid cells).
  • They have 23 chromosomes, one from each homologous pair in a body cell.
  • Meiosis allows for variation- cells produced are genetically different from eachother and other parent cells
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Complex Meiosis

  • There are two stages of meiosis: Meiosis 1 and meiosis 2. The stages are the same as the stages of mitosis. Just before meiosis 1 a diploid cell doubles its number of chromosomes (interphase). 
  • It then undergoes meiosis 1 where 2 bivalents (chromosomes pairs) split and when the cell divides you get 2 diploid cells with the normal chromosome number. 
  • These turn into 4 cells with half the number chromosomes as a normal diploid cell by the 2 cells splitting in meiosis 2
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Variation in meiosis means cells produced are different from eachother and parent cell. It happens in 3 ways:

  • Any sperm can meet any egg.
  • Independent assortment of each homologous pair means each gamete gets a different combination of maternal/paternal chromosomes.
  • Recombination (crossing over) is when enzymes cut parts of the chromatids in a chromosome pair at points called chiasmata. The chromosomes then recombine, genetic information is shared between.  
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Variation (penis)


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  • Gametogenesis is the process in which the gametes sperm and egg cells are formed.
  • Sperm cells are produced by spermatogenesis, which occurs in the testes continuously from puberty.
  • Egg cells are produced by oogenesis, which occurs in the ovaries from birth
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  • A primordial germ cell (diploid) enters the gonad and becomes a spermatogonium
  • The spermatogonia replicate by mitosis in the testes.
  • A spermatogonium enters into meiosis. In the meiosis interphase, the number of chromosomes in the diploid cell doubles to 92. This is a primary spermatoctye
  • After meiosis 1, there are 2 diploid cells with the normal 46 chromosomes. These are called secondary spermatocytes.
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Spermatogenesis (cont)

  • After meiosis 2, each secondary spermatocyte has split, forming a total of 4 haploid cells called spermatids
  • The spermatids then undergo differentiation and become mature sperm
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  • Oogenesis is very similar in process to spermatogenesis. 
  • The terms oogonium, primary/seconday oocyte and mature egg are used instead of spermatogonium, primary/seconday spermatocyte and mature sperm respectively.
  • During meiosis 1, the primary oocyte developes coat.
  • At the end of meiosis 1, the double-diploid cell does not divide into 2 diploid cells , but into one secondary oocyte and one polar body.
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Oogenesis (cont)

  • Only the secondary oocyte enters meiosis 2, and at the end it divides into a mature egg and a second polar body, so only one egg is formed.  
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Adaptions of Sperm

  • The acrosome is a membrane-bound storage site for enzymes that digest the layers surrounding the ovum and allows the sperm's nucleus to penetrate the egg.
  • The nucleus contains haploid genetic information. The genetic information is highly condensed to reduce the space and energy needed to transport it. 
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Adaptions of Sperm (cont)

  • The middle peice contains tightly packed mitocondria to provide sperm with energy to lash its tail.
  • The tail contains microtubes which produce whip-like movements to keep sperm in suspension and move.
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Adaptions of Ova

  • The cytoplasm of an egg cell is covered by the corona radiata (top), and then the zona pellucida. These protect the egg cell's genetic information.
  • Before sperm are invlolved, the egg is still just a secondary oocyte. 
  • Sperm start to digest the zona pellucida (using enzymes). 
  • This is called the acrosome reaction. 
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Adaptions of Ova (cont)

  • When one sperm touches the surface membrane of the secondary oocyte (ovum),the second meiotic division occurs and it becomes a mature egg.
  • At the same time ion channels open in the ovum, hardening the surface to prevent other sperm from entering the ovum. 
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Gametogenesis in Plants

  • Diploid microspore mother cell divides by meiosis.
  • Just before meiosis 1 (interphase), mother cell is double-diploid.
  • Just after meiosis 1, 2 diploid cells are formed.
  • Just after meiosis 2, 4 haploid cells are formed.
  • Taking 1 haploid cell as an example, it will develop into a pollen grain, containing 2 haploid nuclei.
  • One is the tube nucleus, which produces the pollen tube that penetrates the ovary. The other is the generative nucleus, which fuses with the nucleus in the ovule to form a new individual. 
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Gametogenesis in Plants (cont)

  • The motherspore mother cell divides by meiosis to form 4 haploid cells.
  • 3 of these cells are degenerate, so only one continues developing.
  • The cell divides its haploid nucleus by mitosis to form 2 haploid nuclei. This happens twice more so 1 becomes 2 becomes 4 becomes 8 nuclei.
  • These form 2 polar bodies each with 3 nuclei, and 1 egg cell with 2 nuclei. 
  • The nuclei form together so for example an egg cell has 1 nucleus with 2n genetic information. 
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Fertilization in Plants

  • Pollen grain lands on stigma. If the pollen grain is recognised as being of the same species as the plant, a pollen tube grows down through the style to the ovary.
  • The tube nucleus creates this tube, leaving just the generative nucleus.
  • The generative nucleus divides by mitosis to form 2 nuclei.
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Fertilization in Plants (cont) occ

  • The nuclei enter the ovary and double fertilisation occurs.
  • 1 nucleus fuses with the 2 polar bodies to form the endosperm.
  • The other nucleus fuses with the egg nucleus to form a zygote.
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Methods to Ensure Plant Fertilization

  • To be atractive to insects, plants will be brightly coloured, have a strong scent, have sweet nectar, and have sticky pollen.
  • To catch wind easiliy, plants will have light pollen, have feather stamen (outside plant), and a pollen shape to catch a high level of wind
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Methods to Ensure Animal Fertilization

  • Fertilisation occurs mainly in aqeuous environments to stop gametes drying out.
  • Some mating rituals increase frequency and fertilisation.
  • Male gametes are normally transferred directly to female gametes.
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  • Growth is a perminant increase in the number of cells, or in the mass/size of an organism.
  • There are 3 aspects of growth: cell penis, assimilation, and cell expansion.
  • The uptake of food/resources needed for new cell material is called assimilation
  • When the resources are used in/by cells cell expansion occurs. nb 
  • Measuring growth is difficult, because temporary growth (eg water gain) can affect your results.
  • 3 types:continuous, discontinuous and rapid.
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  • After fertilisation, a zygote is formed.
  • The zygote divides by mitosis to form a hollow bundle of cells called a blastocyst
  • The blastocyst implants in the uterus and eventually becomes an embryo, a foetus, and then a baby.
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Stem Cells

  • Stem cells are cells that are capable of becoming other types of cells.
  • They are unspecialised and divide repeatedly (no hayflick limit).
  • Totipotent stem cells are found in an early embryo, and can become any type of body cell or placenta.
  • Pluripotent stem cells are found in a blastocyst, and can become any body cell except for placenta.
  • Multipotent stem cells are found in a foetus or adult, and can become certain body cells only
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Determination and Differentiation

  • Cell determination occurs when a cell becomes 'destined' or committed to be a certain type of cell.
  • No physical change has occured though, so factors can still affect determination.
  • Following this, a cell differentiates (complete physical change) and becomes a specialised cell.
  • This is irreversable
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A clone is a genetically identical individual. There are 2 types of cloning:

  • Embryo cloning- Collect egg and sperm with desired genetic traits, and allow in vitro fertilisation to occur. This creates an embryo. Split the embryo into many identical embryos. Each embryo is implanted into a surrogate mother, and all the babies born are clones.
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Cloning (cont)

  • Somatic cell cloning- Collect somatic cell of desired individual for cloning. Collect egg cell from another animal and remove its nucleus. Fuse the two together with an electric current, and grow this in vitro to form an embryo. Implant the embryo into a surrogate mother. Baby born is a clone of the somatic cell parent. 
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Cloning (cont)

  • Somatic cell cloning- Collect somatic cell of desired individual for cloning. Collect egg cell from another animal and remove its nucleus. Fuse the two together with an electric current, and grow this in vitro to form an embryo. Implant the embryo into a surrogate mother. Baby born is a clone of the somatic cell parent. 
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Therapeutic Cloning

  • Therapeutic cloning is a way of growing stem cells for a sick individual.
  • The same steps as in somatic cell cloning are used to obtain an early embryo.
  • Stem cells are then taken from that early embryo and are cultured to grow a required tissue or organ.
  • Organ/tissue is transplanted into patient. 
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