Chromosonal Structure Flashcards
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.
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.
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).
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.
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.
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 (external website) Don Burke, Australia
Hydrangeas! Hydrangeas! (external website)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 sites (external website) University of Vermont
How Trees Grow in the Urban Environment (external website)University of Florida, USA
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.
