3.2 Chromosomes

3.2 Chromosomes

Objectives: Compare prokaryotic and eukaryotic chromosomes. Understand the relationship of homolous chromosomes to each other. Analyse a karyotype (for gender and chromosome abnormalities). Familiarise yourself with Cairn’s technique for measuring DNA.

Keywords:

  • Prokaryotes: a domain which contains ‘true-bacteria’, cells have distinctive features including circular DNA without histones.
  • Eukaryotes: a domain which contains fungi, animalia, plantae, fungi, protists. Cells have distinctive features including linear DNA associated with histones.
  • Homologous chromosomes: Pairs of related chromosomes which may carry the same sequence of genes, but not necessarily the same alleles.
  • Diploid. Nuclei that contain two homolous chromosomes for each pair (e.g.. 23 pairs in humans = 46)
  • Haploid. Nuclei that contain one homologous chromosome for each pair (eg. 23 total in a human sperm or egg cell).
  • Cairn’s technique: A method for measuring the length of chromosomes which relies on autoradiography (image on X-ray film produced from radioactive decay).
  •  Karyogram: image which shows all the homologous pairs of chromosomes in order of decreasing size)
  • Karyotype: the number and type of chromosomes an individual of a species typically has

Comparing eukaryotic and prokaryotic chromosomes

Eukaryotic chromosomes are tightly-packed, three dimensional structures made from linear DNA molecules and histone proteins.

Task Q) When do chromosomes form?

00 Eukaryotic Chromosomes

 

 

image credit: studyblue.com

image credit: studyblue.com

Structure of a eukaryotic chromosome:

A single chromosome consists of two identical sister chromatids, joined by a region known as the centromere.

Task Q) Why are there two copies of the same information (sister chromatids?)

 

 

 

Bacterial chromosomes

Prokaryotes (the domain which includes  ‘true’ bacteria or “eubacteria” ), have different kinds of  chromosomes.

These take the form of a large ring of DNA called the bacterial chromosome and smaller extra rings of DNA called plasmids, which may or may not be present. The region where the bacterial chromosome is present is called the nucleoid, and differs from a eukaryotic nucleus in that there is no nuclear membrane present.

image credit: University of Waikato

image credit: University of Waikato

 

Comparing eukaryotic and prokaryotic chromosomes

Chromosome type Histone proteins Shape Plasmids
Eukaryote yes linear no
Prokaryote no circular sometimes (not always)

 

Task: Archean bacteria such as methanogenic bacteria (see topic 4), contain circular chromosomes, that are associated with histone proteins.

Create a Venn diagram showing the similarities and differences between chromosomes in eubacteria ‘true bacteria’ e.g. E. Coli, archaea bacteria eg. methanogenic bacteria, and eukaryotes eg. human cells.

 

What are Homologous chromosomes?

As Mendel discovered, alleles for a specific trait are often inherited in pairs (one from each parent). The same is true of chromosomes.

Homologous chromosomes carry the same sequence of genes, but not necessarily the same alleles, as each chromosome of the pair comes from a different parent.

There are 23 homologous chromosome pairs in the human genome, and 22 of the pairs can be identified because the chromosomes in the pair are of equal size.

Fig. 3 Homologous chromosomes

image credit: harvard.edu

image credit: harvard.edu

In the image, 2 pairs of homologous chromosomes are shown. They may carry different alleles for:

Eye colour (B – brown, b-blue)

And E (E-prominent eye brow ridge, e-less defined eyebrow ridge)

The alleles that you inherit for a specific characteristic depends on which of the two available chromosomes in a homologous pair, that you inherit.

Sperm of egg cells are haploid and will only contain one of each homologous chromosome pair.

 

 

 

Measuring the length of chromosomes

Cairns technique is a method for measuring the length of chromosomes. The three important stages are:

  1. Incorporation of radioactive hydrogen into the DNA (the radioactive isotope tritium is used H3)
  2. Using audioradiography (x ray film) to get an image of the chromosome. The cells need to be lysed (burst) and the DNA released onto a dialysis film.
  3. Measuring the size of the chromosome in the image

Cairn’s technique allows for measuring the length of uncoiled chromosomes (ie. captured during interphase).

Figure: A bacterial chromosome from E. Coli replicating. The two ‘bunny ears’ are comprised of newly formed DNA from the circular chromosome. The small image on the right is a drawing to clarify the main image.

image credit: Shaechter and Kolter

image credit: Shaechter and Kolter

Karyotype analysis

Karyotype: The number of and types of chromosomes an organism has is called it’s Karyotype.  A Karyogram is a diagram of a      Karyotype. By analysing a karyogram, sex and chromosome abnormalities may be determined.

A karyogram is made by manipulating a photograph of chromosomes, where the homologous chromosome pairs are organised from largest to smallest, the 23rd pair being the sex chromosomes. The other homologous chromosomes are called autosomes.

Fig. A Karyogram of a normal human karyotype

image credit: genegeek.ca

image credit: genegeek.ca

 

Key features of a karyogram:

  • the chromosome are arranged in homolous pairs
  • the homologous pairs are arranged in order of decreasing size
  • the sex chromosomes are the last chromosomes in the karyotype (pair 23)

Variations in Karyotypes:

  • The 23rd pair of homologous chromosomes has 2 variations. XX – which is female, and XY – which is male. This is the genetic basis for sex-determination.
  • A trisomy, is when an extra chromosome is inherited because of something called non-disjunction (failure of chromosomes to separate in meiosis 1). Down’s syndrome is caused by a trisomy of the 21st pair of homologous chromosomes.

 

document version:

 

Activity: Finding the loci of human genes using an online database

Plot the loci of these 3 human genes on the Karyotype diagram you downloaded (mark where they are)

  1. TP 53 – tumor suppresent gene
  2. CFTR – the gene coding for chloride channel protein. A defective allele of this gene causes cystic fibrosis
  3. F8 – the gene that codes for factor VIII one of the clotting factors needed to clot blood. Haemophilia is causes by a defective allele of this gene

Link: http://www.omim.org

Choose search gene map, enter name of gene, find gene locus, remember how to read gene locus and plot gene locus on your karyogram (downloaded above).

 

Activity 2: Identifying a karyotype.

Try to assemble a karyogram from the photographs given. Determine: gender and whether the person has Down’s syndrome or one of the other chromosome abnormalities:

  • Klinefelters syndrome: An trisomy of the 23rd pair, so XXY (associated with infertility)
  • Edward syndrome: Trisomy at chromosome 18 (heart and kidney defects)
  • Patau syndrome: Trisomy at chromosome 13 (nervous system disorder including vision)

 

Sex and gender; transgender issues.

Karyogram analysis can be used as a basis for sex determination. However the occurrence of XXY trisomy at position 23 results in an ambiguous sex determination.

The paradigm shift regarding gender determination has been a change from gender being a scientific construct, to gender being a social or personal defined concept.

The consciousness of protecting the rights of gender-ambiguous or transgender individuals presents important ethical questions for todays societies.

https://www.wnyc.org/story/sex-testing-gone-scrutiny-continues/

http://www.itv.com/news/2018-02-22/transgender-kids-what-is-the-right-way-to-deal-with-the-growing-number-of-children-exploring-their-gender-identity/

 

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