B2.7 Cell Division and Inheritance

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Mitosis

The chromosomes in the nucleus of a body cell contain the genetic information (genes) of the cell. The chromosomes are in pairs. Body cells have 2 sets of chromosomes.

Mitosis occurs:

  • During growth - to produce more body cells
  • To produce replacement cells- when body cells are damaged
  • During asexual reproduction

In mitosis

  • Copies of the genetic material are made
  • The cell divides once
  • The two new cells are genetically identical

http://www.bbc.co.uk/staticarchive/35b00c8ee0b890b07e171021b11f65e9da836dfd.gif

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Meiosis

Gametes have only one set of chromosomes, so cells in reproductive organs (eg testes and ovaries) divide to form gametes.

Meiosis allows gametes to be formed

In meiosis:

  • Copies of the genetic information are made
  • The cell divides twice to form four gametes
  • Each gamete has a single set of chromosomes, and are not identical

Sexual fertilisation

When gametes join, a single body cell with new pairs of chromosomes is formed, and an individual is formed by this cell dividing by mitosis. 

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Stem Cells

  • Animals
    • Most cells differentiate at an early stage
    • In mature animals, cell division is mainly for repair and replacement
  • Plants
    • Many cells divide and differentiate into specialised cells throughout life
  • The cells that are able to divide to form different specialised cells (differentiate) are stem cells
    • Adult stem cells
      • From bone marrow
      • No risk of rejection if patient's own cell
      • Can only make the type of specialised cell in the tissue they are found in
      • Limited quantity
    • Embryonic stem cells
      • From early embryo
      • Can be differentiated into any kind of human cell
      • Can form tumours, could have been born
  • Treatment with stem cells can help conditions such as paralysis
    • Enable scientists to learn about cells
    • Drug testing
    • Help with transplants
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Genes and Alleles

  • Chromosomes
    • Found in the nucleus
    • One pair carries the genes that determine sex
      • Female = XX
      • Male = XY
    • In pairs in body cells
    • Large molecule of DNA
      • DNA is a long, coiled molecule formed from two strands in a double helix
      • A gene is a short section of DNA.
        • Each codes for a particular combination of amino acids that makes a specific protein
        • May have different forms (alleles)
        • Characteristics controlled by a single gene = monohybrid inheritance
  • Alleles
    • Each produces a different form of the protein the gene codes for
    • When gametes fuse, one of each pair of alleles comes from each parent (leads to variation)
    • Dominant = controls development of a characteristic when present on only one of the chromosomes (capital)
    • Recessive = only controls development of a characteristic if the dominant is not present (lower)
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Key Terms

Allele - alternate form of a gene found at the same position on a chromosome

Genotype - alleles/genes you have

Phenotype - physical characteristics you have

Heteroygous - An individual with alternative alleles on the two chromosomes

Homozygous - An individual with the same alleles on both chromosomes of the pair 

Genetic Diagrams: http://www.bbc.co.uk/staticarchive/c2a81343d5d11d90790b71fd5681c8d9776816e5.gif

Punnett Squares:  http://study.com/cimages/multimages/16/heterozygous_cross_punnett_square.png

DNA Fingerprinting:  http://www.bbc.co.uk/staticarchive/1fbfc092849ed1a787667d10d02786370520cceb.gif

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Genetic Disorders

Some disorders are caused by genes, so can be inherited

Polydactyly

  • Extra fingers or toes
  • Caused by a dominant allele of a gene
    • DD = Polydactyly
    • Dd = Polydactyly
    • dd = Usual
  • Can be passed on by only one parent with the disorder

Cystic Fibrosis

  • Affects cell membranes so makes mucus stickier than usual
  • Caused by a recessive allele of a gene
    • ff = has cystic fibrosis
    • FF = does not have cf and cannot pass it on
    • Ff = does not have cf but can pass it on (carrier)
  • Must be inherited by both parents
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Embryo Screening

Tests embryo for specific alleles, to see if the child will have an inherited disorder. 

  • Economic issues
    • May cost parents more to support a child with a disorder
    • A person with a disorder may need a lot of hospital treatment or drugs
  • Social issues
    • Unknown who will care for the child if the parents cannot
    • Unfair on other children if the child with the disorder needs extra care
  • Ethical issues
    • Wrong to abort an embryo that carries the disease
      • Embryos may have the same right to life as an existing person
    • Carries a risk of damaging the embryo
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Mendel

  • Studied inheritance of characteristics in pea plants
  • Produced pure-breeding plants 
    • Produced the same characteristic every time when bred with each other
    • Made crosses by hand so he was certain which parent plants produced which offspring
    • Repeated hundreds of times
  • Published work in 1865 but wasn't widely accepted
    • Later work linked his 'inherited factors' to genes and chromosomes
  • Characteristics are caused by separately inherited factors
    • Found that some characteristics of parent plants disappear in the first generation but reappear in the second, so the factors that cause the colours must be kept separate even in the first generation
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Comments

evie knapman

thanks these are really helpful and explain a hard topic! I'm aiming for an a/a* so don't know wether i need more detail?

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