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B2.7 Cell Division and inheritance

B2.7 Cell Division and inheritance

Summary

Characteristics are passed on from one generation to the next in both plants and animals. Simple genetic diagrams can be used to show this. There are ethical considerations in treating genetic disorders.

Genetic material

  • In the nucleus of a typical human body cell there are 23 pairs of chromosomes.
  • We inherit one set of 23 chromosomes from each of our parents.
  • Chromosomes are made from a large molecule called DNA (deoxyribose nucleic acid).
  • DNA consists of two strands coiled into a double helix structure.
  • DNA has 2 main roles:
    1. It can replicate prior to cell division (mitosis or meiosis).
  1. Its code is used to synthesise proteins.
  • A gene is a small section of DNA.
  • Each gene codes for a particular combination of amino acids which make a specific protein.
  • These proteins determine our characteristics.
  • Some characteristics are controlled by a single gene.

Cell Division and Growth

  • New body cells are produced:
    • When the animal is growing.
    •  To repair damaged tissues.
    • To replace worn out tissues.
  • Mitosis occurs:
    • All of the chromosomes are replicated.
    • The nucleus is divided in 2.
  • This results in two genetically identical
  • The cell divides in 2 to form 2 genetically identical cells.
  • Some cells undergo cell division again and again.
  • Some cells carry out their function then die.

Asexual reproduction

•    The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells.
•    They contain the same genes as the parents.

Cell differentiation
•    Differentiation results when some genes are turned on, some are turned off.
•    Once the cells are specialised they carry out their role.
•    Most types of animal cells differentiate at an early stage.
•    Many plant cells retain the ability to differentiate throughout life.
•    In mature animals, cell division is mainly restricted to repair and replacement.

Stem cells

•    Stem cells can be made to differentiate into many different types of cells e.g. nerve cells.

•    There are very few stem cells in an adult.

•    There is currently a lot of research involving the use of stem cells to treat various diseases and injuries.

•    Cells can be taken from human embryos, adult bone marrow and umbilical cords.

•    Treatment with these cells may help cure conditions such as paralysis.

•    However, many people are concerned about the use of human embryos to treat diseases.

•    They feel that all embryos have got the potential to become a baby, and that they should not be used in this way.

Sexual reproduction

Gamete formation
•    Gametes are sex cells (sperm and egg cells).
•    Sperm formed in the testes.
•    Egg cells are formed in the ovaries.

•    A cell containing a full set of chromosomes (chromosomes in pairs) divides to form cells with half the number set of chromosomes (a single set):

o  Meiosis occurs:

    Copies of the chromosomes are made.

    The nucleus divides twice to form 4 nuclei.
o    Then the cell divides twice to form four gametes.

    Each has a single set of chromosomes.

 

Fertilisation

•    When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed.

•    A new individual then develops by this cell repeatedly dividing by mitosis.

Variation
Asexual reproduction leads to very little variation:
•    Organisms that reproduce asexually create new individuals by mitosis.
•    They are genetically identical to the parent.
•    They may develop differently due to differences in their environment.

Sexual reproduction leads to much more variation:

•    Meiosis ensures that all gametes contain the same genes, but have a different selection of alleles.

•    Also, it is random which sperm fertilises which egg.

•    Therefore all individuals (except for identical twins) produced sexually are genetically different.

Mendel’s discoveries
•    Mendel worked out the main principles of inheritance in the 19th century.

•    He studied inheritance in pea plants.

•    He noticed that certain characteristics that were shown by 2 pea plants were not always shown in their offspring.

•    However, when he crossed these offspring together, the characteristics sometimes reappeared in the next generation.

•    He carried out thousands of crosses with pea plants, and he found that many characteristics were inherited in predictable patterns.

•    He proposed the idea of separately inherited factors:
o    Each individual inherits a set of factors from each of their parents.

o    It is the combination of these characters that determines the characteristics of an individual.

•    Mendel’s discoveries were not recognised until after his death – Why?
•    He published his work in an obscure journal
•    DNA, chromosomes and genes were not yet discovered
•    People could not accept the link between plants and humans.

Inheritance
•    Each gene may have different forms called alleles.

•    Eg. There is a gene for eye colour. Everyone has 2 copies of this gene. Alleles for eye colour may be blue, brown, green etc.

•    Therefore each person may have 2 different alleles for eye colour.
•    When we are conceived, we receive one copy of each gene from each parent.
•    Therefore we have two copies of every gene, but they may be 2 different alleles.
•    Different combinations of alleles may lead to differences in the characteristic.

•    An allele, which controls the development of a characteristic when it is present on only one of the chromosomes, is a dominant allele.

•    An allele, which controls the development of characteristics only if the dominant allele is not present, is a recessive allele.

•    Phenotype is a description of how a characteristic is expressed. This can be influenced by genetic or environmental factors.

•    Genotype is a description of the alleles an individual possesses for a characteristic.
•    A homozygous genotype has 2 identical alleles.
•    A heterozygous genotype has two different alleles for a gene.

Eg tongue rolling
•    There is a gene that control our ability to roll our tongues.
•    There are 2 alleles for this gene:
•    The allele that allows us to roll our tongues is dominant (R)
•    The allele that prevents tongue rolling is recessive (r)

 

  • These are approximate ratios
  • There is a 75% chance of a tongue roller being born in this family.
  • There is a 25% chance of a non-tongue roller being born in this family.

  • There is a 50% chance of a tongue roller being born in this family.
  • There is a 50% chance of a non-tongue roller being born in this family.

 

Family Trees

  • The inheritance of traits in families can be illustrated using a family tree
  • In this example, the allele for attached earlobes is recessive.
  • F = allele for detached earlobes
  • f = allele for attached earlobes
  • FF and Ff = genotypes for detached earlobes
  • ff = genotype for attached earlobes

Inherited Conditions in Humans

 

Polydactyly:

  • People that inherit this condition have extra fingers or toes.
  • It is caused by a dominant allele of a gene.
  • If someone inherits one copy of this allele, they will develop the disorder.
  • It can therefore be passed on by only one parent who has the disorder.

Cystic fibrosis

  • A disorder of cell membranes.
  • It causes thick, sticky mucus to accumulate in the lungs and the digestive system.
  • This causes:

o    Lung infections

o    Problems with breathing

  • Problems with digestion and absorption.
  • It is caused by a recessive allele of a gene.
  • The parents may be carriers of the disorder without actually having the disorder themselves.
  • It can therefore be passed on by parents, neither of whom has the disorder.
  • To develop the disease, the allele must be inherited from both parents.

 

Embryo screening

 

  • People in families that have had certain genetic disorders can have a genetic test to see if they carry the allele for the disease.
  • If they do carry the allele, their embryos can be screened to see if it is affected.
  • They can then decide whether to have an abortion.
  • This is very controversial.
  • Also, many people are concerned about this because in the future it may enable people to choose other characteristics in their children.

 

Sex determination

  • In human body cells, one of the 23 pairs of chromosomes carries the genes which determine sex.
  • In females the sex chromosomes are the same (XX).
  • In males the sex chromosomes are different (XY).

  • There is a 50% possibility of being a girl or a boy.
  • As a foetus, we all start off with female characteristics.
  • Presence of Y chromosome causes male development.
  • Absence of Y chromosome continues female development.
  • The sex of the baby is determined by the sex chromosome inherited from the father.

 

DNA Fingerprinting

  • Each person (apart from identical twins) has unique DNA.
  • Samples of DNA can be found in blood, semen and saliva.
  • Special techniques are used to cut the DNA and then separate them according to length across a gel.
  • Sequences within the DNA can be identified.
  • Each individual’s DNA produces a specific pattern.
  • This can be used:

o    To identify criminals

o    To decide whether someone is the biological father of a child.