Saturday 19 January 2019

Errors in meiosis essay

Errors in meiosis                               Louise Usher                                                            
                                                             

Meiosis is a crucial part of creating early life.  As humans, we have 46 chromosomes for all body cells.  Prior to reaching this stage of development,  gametes join to go through various steps of fertilization in order to form a zygote.  The zygote contains 23 pairs of chromosomes.  23 single chromosomes from the sperm and 23 from the oocyte.  Therefore creating the correct number of 46 chromosomes needed.  Thus,  a complete set of chromosomes from each parent is now stored as the cell goes into the next stage of development.

Errors in meiosis can be the result of several outcomes.  Genetic changes can cause diseases in these cases. 

As females have produced all their egg cells prior to birth, the age of the mother will also indicate the age of the eggs.  Womens eggs may take up to 45 years to reach complete meiosis whereas sperm is being produced all the time.  A baby will have a higher risk of genetic chromosomal abnormalities if the mother is older. 
Should an error occur in oocyte or sperm,  the resulting baby will have this error in every cell of their body.  Ref: geneticseducation.nhs.uk [accessed March 11th 2016]

New alterations in the DNA sequence is likely to relate more to the fathers age. Ref: Voet, Voet and Pratt, Fundementals of Biochemistry, Wiley.



A genetic disorder is a problem caused by abnormalities in the genome.  Some genetic disorders can be inheritated from the parents.  Non heritable disorders of the genes can create defects in eukariotic cells caused by new mutations or changes to DNA while meiosis takes place. 

Genetics is central to biology. Underlying all life processes is gene activity. DNA is made up of two chains.  Each chain consists of building blocks nucleotides.  Within the nucleotides is deoxyribase, a phosphate group and a base.  The four bases are adenine, guanine, cytosine and thyamine.  In RNA, uracil occurs in place of thiamine.
The sequence of these bases within a strand determines the genetic information stored within that strand. Ref: Peter J Russell.  iGenetics 2nd edition.  Pearson.

While a change in the sequence of the gene may have no effect (known as polymorphism) there is also a chance of a severe disruption of the function of certain genes. Ref: geneticseducation.nhs.uk [accessed March 11th 2016]
Should gene function be disrupted in this way, diseases can result  These are known as mutations.
You can see below in figure 1 the differences in functional and non functional protein.





 



Fig 1 – functional and non functional protein

Variations in a DNA sequence depends on a number of factors
·      The size of the variant
·      The pathogenicity of the variant

Single nucleotide polymorphisms (SNP’s) are common.  This is just one type of a variant.  Rare genetic conditions can be created but mutations are specific to only an individual family. Ref: Peter J Russell.  iGenetics 2nd edition.  Pearson.



Depending on how the error occurs determines if it is
·      Non – disjunction
·      Segregation errors
·      Translocation errors
·      Recombination errors

Segregation errors have eggs or sperm which create too many or too few chromosomes.  Fertilised eggs may have an extra chromosome of a particular pair (trisomy). Monosomy is one chromosome fewer in each cell.  Turner syndrome and Klinefelter syndrome can result from this.

Translocation errors have no crossover occurring.  Often resulting in cancers forming.  Burkitt lymphoma affects chromosomes 8 and 14.  Chronic myelogenous leukemia affects chromosomes 9 and 22. 

Recombination errors involves the swapping of genetic materials between both chromosomes of the same pair (homologous).  Side by side, they pair up, break, swap DNA and rejoin.  If the chromosomes realign and are misaligned, duplications can occur.  Extra genetic material or deletions are likely to occur which involves missing genetic material. 
In non-homologous pairs, the exchange gives chromosome translocations.



Reciprical translocation involves the swapping of material.
Chromosomes can also stick together end to end which is known as Robertsonian translocation.
Translocations can lead to extra copies of genes causing over expression resulting in disrupted cell function.  Therefore, the loss of generic material may lead to the cell missing copies of genes essential to this activity.

Congenital diseases occur when there are errors in Meiosis.  While the list is endless and growing all the time (some illnesses still are awaiting scientific evidence to confirm if they are due to meiosis errors or not) many diseases are born from such errors.

Muscular dystrophy is characterized by insufficient protein ‘dystrophin’ as you can see below in figure 2.
 

Figure 2 – chromosomes for muscular dystrophy ref: www.geneticsformedics.com

Severe combined immunodeficiency (SCID) is a heritable disorder where individuals have no functional immune system.  Minor infections can cause death in such individuals at a very young age.  The FDA approved the first human gene therapy trial in 1990 on a girl called Ashanti DeSliva.   She has an autosomal form of SCID originating from mutation of the gene encoding adenosine deaminase (ADA) which is an enzyme.  Some white blood cells (T cells)  were isolated and mixed with a



retroviral vector carrying an inserted copy of ADA.  As the virus infected many of the cells, a copy of the ADA gene was inserted into the genome of some of the T cells.
Many treatments of injection of these genetically altered T cells into Ashantis bloodstream and she also periodically accepted injections of purified ADA protein. 
More recently SCID treatment has involved bone marrow stem cells being used (see fig 3 below) with in vitro repopulation of the number of ADA producing cells.  This gene therapy has been somewhat successful in restoring health of a small number of children to date yet this is still considered the most successful example of gene therapy. Ref: King, Cummings, Spencer, Palladino Concepts of Genetics Eleventh edition Pearson global [776-777]

Fig 3. Bone marrow stem cell treatment in SCID  ref: thriving.childrenshospital.org

There are many more similar autoimmune illnesses such as crohns disease being researched to confirm or deny if a chromosomal mutation is responsible for the illness.
Crohns disease is related to chromosomes 5 and 10.  Should an individual have variations to the ATG16LI, IRGM and NOD2 increase the risk of developing crohns disease. Also, the IL23R gene is associated with Crohns disease. However, there is also an element that genetic and environmental factors play a role in this disorder developing yet many of the causes still remain unknown. Ref: Peter J Russell.  iGenetics 2nd edition.  Pearson.



Many diseases result from chromosomal errors during meiosis. Huntingdons, muscular dystrophy, down syndrome, Turners syndrome, Cri-du-chat syndrome, Burkitt lymphoma,  Klinefelter syndrome and Cystic fibrosis are examples.

Cystic fibrosis has just 3 missing letters on chromosome 7.  This change effects the body’s epithelial cells that compromise the linings of the lungs, pancreas, liver, sweat glands, digestive tract and reproductive system.  Usually, the epithelial cells release

slippery mucus to act as a lubricant, trapping dust and bacteria.  However,  cystic fibrosis makes epithelial cells produce a protein.  This leads to thick sticky mucus which can block the bronchial tubes.  Symptoms caused are coughing, tiredness, fatigue and worse,  often leading to the need for organ transplants.  The digestive tract is also affected causing lack of nutrient absorbtion and bulky stools. Ref: Peter J Russell.  iGenetics 2nd edition.  Pearson.




Tay-sachs disease has just one abnormal chromosomal letter. Fatty materials in the brain should be dissolved under normal conditions.  However, with this error, the proteins do not work.  Fat builds up, crushing critical brain cells.  Infants with this disease appear to develop normally for the first few months of life.  As nerve cells become deposited with fatty particles the child becomes blind, deaf and unable to swallow.  Muscles also become atrophic. Ref: Steve Parker, The concise human body book, Dorling Kindersley




It is usual for the miracle of meiosis to happen without errors.  However,  so many individuals suffer with diseases caused by such errors.  While this causes sadness to all concerned and suffering, at what stage should medical intervention stop?
In vitro fertilization (IVF) has shown us that there is now technology to diagnose chromosomal abnormalities and therefore decide which humans are allowed to go on and develop as babies. 

Ethics dictate that this type of diagnosis should perhaps not prevent scientists from allowing these illnesses to be present in humans but rather develop ways to manage the illnesses. 












 Copyright - Louise Usher 2016

Bibliography


Peter.J.Russell iGenetics A molecular approach. Second Edition. Benjamin Cummings

Cystic Fibrosis, Sams Story
Bozeman Science “Mutations”
Arman Azad “Causes – Crohns Disease”
Renan Mauch “Cystic Fibrosis Pulmonary disease”
Shomus Biology “Chromosomal Disorders”
AK lectures “Chromosomal Deletion, inversion, duplication and translocation” “Aneuploidy and non disjunction”
Kristen Kopronski “Trisomy 21”

Steve Parker The Concise Human Body Book. Dorling Kindersley

Voet, Voet and Pratt Fundamentals of Biochemistry upgrade edition – Wiley

Klug, Cummings, Spencer, Palladino Concepts of genetics eleventh ed

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