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9.3 Blueprint of life: 3. Chromosomal structure
Extract from Biology Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW
[Edit: 4 Jun 09]
Prior learning: Stage 5: Models, Theories and Laws 5.8.2 (a) (b) (c) (d) and (e).
Recall statements in Preliminary course: Preliminary module 8.3 (subsection 7)
Background
The discovery of the structure of chromosomes aided the understanding of the mechanisms of inheritance. Chromosomes were first identified as coloured components of a cell that became visible during cell division. It became clear that the behaviour of chromosomes during cell division and fertilisation matched the behaviour of factors as described by Mendel.
outline the roles of Sutton and Boveri in identifying the importance of chromosomes
- Two scientists are credited with the discovery of the role of chromosomes in 1902. They were the German scientist Theodor Boveri and the American microbiologist Walter Sutton.
- Boveri worked on sea urchins and showed that their chromosomes were not all the same and that a full complement was required for the normal development of an organism.
- Sutton worked on grasshoppers and showed that their chromosomes were distinct entities. He said even though they duplicate and divide they remain as a distinct structure. He associated the behaviour of chromosomes with Mendel's work on the inheritance of factors and concluded that chromosomes were the carriers of hereditary units.
- Together their work became known as the Sutton-Boveri chromosome hypothesis.
describe the chemical nature of chromosomes and genes
- Chromosomes consist of 40% DNA and 60% protein (histone). Short lengths of DNA make up genes so genes have the same chemical composition as DNA. DNA is described in more detail in Part 4.
identify that DNA is a double-stranded molecule twisted into a helix with each strand, comprised of a sugar-phosphate backbone and attached bases, adenine (A), thymine (T), cytosine (C) and guanine (G), connected to a complementary strand by pairing the bases, A-T and G-C
- In summary, DNA is a nucleic acid in the shape of a double helix. Each strand of the helix consists of four different nucleotides made up of deoxyribose sugar, a phosphate molecule and a nitrogen base. The helix is like a twisted ladder. The backbones of the structure, or the sides of the ladder, consist of the deoxyribose sugar and phosphate molecules. The bases form the rungs between the sides of deoxyribose sugar and phosphate molecules and are complementary (only pair with their matching base). Adenine pairs with thymine and guanine pairs with cytosine.
Structure
of DNA 
Access Excellence, The National Health
Museum, USA
Nucleotide
Access Excellence, The National Health
Museum, USA
process information from secondary sources to construct a model that demonstrates meiosis and the processes of crossing over, segregation of chromosomes and the production of haploid gametes
- Use plasticine or pipe cleaners to model the process of meiosis to demonstrate crossing over, segregation of chromosomes and the production of haploid gametes. Or use prepared slides showing meiosis.
- Or use secondary sources, such as the Internet sites listed below, to investigate the process of meiosis.
- Process the information by finding the pattern of chromosome behaviour that occurs in all cells.
Cell Division: Meiosis and Sexual Reproduction
M.J. Farabee, Estrella Mountain Community College, Avondale, Arizona, USA
Crossing
over and recombination
Access Excellence, The National Health Museum, USA
Meiosis animation
California State University, USA
explain the relationship between the structure and behaviour of chromosomes during meiosis and the inheritance of genes
- Chromosomes are made of DNA. Genes are coded within the DNA on the chromosomes. During division each chromosome (which therefore includes the genes) makes a complete copy of itself. The new chromosome is attached to the original chromosome by a centromere. In the initial division of meiosis the homologous chromosomes line up in matching pairs and one of each pair of homologous chromosomes moves into a new cell. Next the duplicated chromosomes separate to single strands resulting in four sex cells that are haploid, (ie contain half the chromosome number of the original cell).
- The genes are located on the chromosomes. They are duplicated during the first stage of meiosis and are then randomly assorted depending on which chromosomes from each pair enters which new haploid cell during the first and second division.
explain the role of gamete formation and sexual reproduction in variability of offspring
- Gamete formation results in the halving of the chromosome number (n) (diploid to haploid) and sexual reproduction results in combining gametes (haploid to diploid) to produce a new diploid organism (2n). The processes involved in forming this new organism result in variability of the offspring.
- Gametes are formed during the process of meiosis. In meiosis there are two
stages that lead to variability. These are:
- random segregation of individual chromosomes with treir associated genes ie, different new combinations of the original maternal and paternal chromosomes and
- the process of crossing over where the maternal and paternal chromosomes of each pairmay exchange segments of genes making new combinations of genes on the chromosomes.
- In sexual reproduction each female or male cell produces 4 sex cells (gametes) from the process of meiosis. Each of these sex cells is haploid (has half the normal chromosome number) and has a random assortment of genes from the parent. The genes (Mendel's alleles) are separated and the sex cells have a random assortment of dominant and recessive genes. More variability is introduced depending on which sex cells are successful in fertilisation. The resulting embryo has a completely different set of genes from either of the parents.
solve problems involving co-dominance and sex linkage
-
Solve problems by using identified
strategies such as Punnett squares to develop a range of
answers for a particular problem.
Use the web pages below as a secondary source of information. They contain worked solutions and tutorials to problems involving co-dominance and sex-linkage.
Co-dominance problem
Biology Project, University of Arizona
USA Bloodtype calculator
Biology Project, University of Arizona
USAPredicting human blood types
Biology Project, University of Arizona
USASex-linked inheritance problem set
Biology Project, University of Arizona
USASex-linked inheritance
Access Excellence, The National Health Museum,
USAHaemophilia
Access Excellence, The National Health Museum,
USA
describe the work of Morgan that led to the identification of sex linkage
- Thomas Morgan worked on the fruit fly Drosophila melanogaster. He looked at crosses between red- eyed and white-eyed flies and found that the results could not be accounted for by simple Mendelian crosses. He showed that some genes were sex-linked because they were located on the X chromosome.
Sex-linked
animations
Cold Springs Harbor Laboratory, Dolan DNA Learning Center, Columbia Unversity,
USA. (Some audio requires Quicktime plug in to be installed)
explain the relationship between homozygous and heterozygous genotypes and the resulting phenotypes in examples of co-dominance
- If an individual has two different alleles (heterozygous) for a characteristic,
then often one will be dominant while the other is not expressed and is said
to be recessive. In some cases however, both alleles are expressed in the
phenotype and the two alleles are said to be co-dominant. In this case both
alleles are labelled with upper case letters.
(Please note that this is not "incomplete dominance" where each allele partly suppresses the expression of the other as in pink flowers of snapdragons. The HSC Biology syllasbus does not refer to incomplete dominance. Some web sites will have these two concepts confused. If in doubt, check in written references.)
- An example of co-dominance is human blood groups. There are three alleles
for blood type A, B and O. O blood type is recessive to both A and B but A
and B are co-dominant and form a fourth blood type AB.
Alleles present Blood type AA or AO A BB or BO B OO O AB AB
- Another example of co-dominance is in coat colour of Shorthorn cattle.
These animals have an allele for both red and white hair. As neither is dominant,
cattle with both alleles have a mixture of red and white hairs scattered over
their bodies and are called roan. Red cattle have the alleles RR while white
cattle have WW. In the F1 (first) generation all of the offspring will be
roan, RW.
R R W RW RW W RW RW When the roan cattle are crossed:
R W R RR RW W RW WW Half the offspring will be roan while a quarter will be red and quarter white.
describe the inheritance of sex-linked genes, and genes that exhibit co-dominance and explain why these do not produce simple Mendelian ratios
- Mendel was fortunate in his choice of factors as they all showed dominant/recessive characteristics. However, sex-linked genes and genes that are co-dominant do not display the phenotype ratioos predicted by Mendel's laws.
- An example of sex-linked inheritance is red-green colour blindness in humans.
The gene is carried on the X chromosome and there is no corresponding gene
on the Y chromosome. Therefore males need only one allele for colour blindness
on the X chromosome while females require two. This results in many more males
being colour blind than females because the father would have to be colour
blind and the mother either colour blind or be a carrier for colour blindness.
As you would expect the sex of offspring to be 50% male and 50% female the
occurrence of colour blindness is higher in males than would be expected from
a simple pair of dominant and recessive genes. Take the cross between a normal
female XN XN and a colour-blind male X n
Y.
XN XN X n XN X n XN X n Y XN Y XN Y
All offspring have normal sight. But if the female is a carrier for colour blindness and crosses with a normal male then 50 % of the males will be colour blind and none of the females.
| XN | X n | |
|---|---|---|
| XN | XN XN | XN X n |
| Y | XN Y | X n Y |
- Human blood types are another example of co-dominance. Human blood types give different results from Mendelian ratios. When a homozygous male with AA alleles crosses with a homozygous female with BB alleles then all of the offspring will be a different phenotype from the parents (group AB).
identify data sources and perform a first-hand investigation to demonstrate the effect of the environment on phenotype
Background
Studies on identical twins separated at birth are useful to determine how much the phenotype is determined by the environment. Identical twins have the same genotype, so any differences in phenotype could be determined by the environment. Other studies that are useful are long-term studies on height of individuals. For example, Japanese people who grew up in America on average were taller than Japanese people who grew up in Japan. Better nutrition was responsible for the Japanese people to reach their genetic potential. This has been shown in an increase in the height of the average Japanese person over the last fifty years as nutrition has improved. |
-
Identify data sources to analyse the
complex problem of environmental effect on phenotype to
determine appropriate ways to research the problem.
Gene-environment interaction
Wikipedia
- Perform a first hand investigation and identify the variables that need to be kept constant and demonstrate the use of a control. For example, a first-hand experiment can be done on seedlings to find the effect of soil nutrients on plant height. One group of 100 plants (control) is given soil containing all nutrients while another group of 100 plants (replication) is given soil with the nutrient nitrogen missing. The average heights of each group can be compared to see if the average is affected by the different environmental conditions. Other differences might be observed. In this experiment it is necessary to ensure that only one variable is different from the control. Factors that need to be controlled are the amount of light, the amount of water and any pests or diseases. It is important to use safe working practices. When working with soil wear gloves to prevent exposure to soil bacteria. Be aware of any sharp gardening tools and wear covered shoes in case of an accident. If the skin is cut when using soil it is necessary to ensure that a tetanus immunisation is up to date.
outline ways in which the environment may affect the expression of a gene in an individual
- The appearance of an individual is not based solely on their genetic information. The environment of the organism also plays a part.
- Hydrangeas are plants that have different flower colour (pink or blue) depending
on the pH of the soil they are grown in. In acid soils (less than pH 5) Hydrangeas
are blue. In soils that have a pH greater than 7 Hydrangeas are pink. The
pH has an effect on the availability of other ions in the soil and it is these
ions that are responsible for the colour change.
Burke’s Backyard Don Burke, Australia
Hydrangeas! Hydrangeas!Judith King, USA
- Another example of the influence of the environment on the appearance is
the height of plants. Genetically identical plants will grow to different
heights if they are exposed to different growing conditions.
- The environmental factors that influence growth in a forest:
Forest Measuring and Monitoring Cris Brack, Department of Forestry, Australian
National University, ACT
How Trees Grow in the Urban EnvironmentUniversity of Florida, USA
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