Genetics Questions

Question 1

Define the term “Gene”

Answer 1

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

Question 2

Outline the relationship between a gene and a chromosome

Answer 2

A gene occupies a specific position on a chromosome; this specific position is called locus.

Genes can be linked into groups, and each group = one type of chromosome.

Question 3

Define “alleles” and outline how they are formed

Answer 3

• Alleles are the various specific forms of a gene.
• New alleles are formed by mutation, and they differ from each other by one or only a few bases.
• Most animal have 2 copies of each type of chromosome, and each copy may have same or different alleles; but only one allele can occupy the locus of a gene on a chromosome.

Question 4

Outline the definition of a genome

Answer 4

• The genome is the whole of the genetic information of an organism.
• The size of a genome is therefore the total amount of DNA in one set of chromosomes in that species.
• It can be measured in millions of base pairs of DNA.

Question 5

List the number of genes of one plant, one bacterium, one species with more genes and one with fewer genes than a human.

Answer 5

**The number of genes in a species should not be referred to as genome size as this term is used for the total amount of DNA.

Estimated number of protein-coding genes in humans is 21 000.

Escherichia Coli (Bacteria): less genes than humans
~4 200

Oryza Sativa (Rice): plant, more genes than humans
~38 000

Gallus gallus (Chicken): animal, less genes than humans
~1 700

Daphina pulex (water flea): animal, more genes than humans
~31 000

Question 6

Explain the causes of sickle cell anemia

Answer 6

• The cause of sickle cell anaemia is due to the base substitution mutation in the DNA.
• In DNA sense strand gene that codes for hemoglobin protein, GAG is being mutated to GTG (thymine substituted adenine)
• Which then codes for valine instead of glutamic acid on the SIXTH amino acid.
• This causes a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.

Question 7

Outline the human genome project and its outcome

Answer 7

Outcomes of the HGP:
- knowledge of location of human genes / position of human genes on chromosomes;knowledge of number of genes/interaction of genes / understanding the mechanism of mutations;
evolutionary relationships between humans and other animals;
discovery of proteins / understanding protein function / detection of genetic disease;
leads to the development of medical treatment/enhanced research techniques;
knowledge of the base sequence of genes/study of variation within genome;

Question 8

Distinguish between prokaryotic and eukaryotic chromosomes

Answer 8

• Prokaryotes have one chromosome consisting of a circular DNA molecule, they reproduce asexually through binary fission.
• Some prokaryotes also have plasmids but eukaryotes do not.
• Plasmids are used to transfer genetic information from one bacteria to another.
• They are also used in laboratories to genetically modify a prokaryote.
• Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
• In a eukaryote species there are different chromosomes that carry different genes, with both coding and non-coding DNA.
• Eukaryotes have different types of chromosome with 2 alleles of each type.

Question 9

Describe what homologous pairs are in relationship to diploid and haploid nuclei.

Answer 9

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. A same type of chromosome can be identified by its length and shape (have same length and same position of centromere).

Diploid nuclei have pairs of homologous chromosomes; they have 2 types of chromosomes, meaning they have 2 genes copies (alleles) for each trait. A somatic cells are diploid and divide by mitosis.

Haploid nuclei have one chromosome of each pair, as they only possess a single copy (one allele) for each trait. Sex cells are haploid and they divide by meiosis.

Question 10

State why chromosome number and type is a distinguishing characteristic of a species.

Answer 10

The number of chromosomes is a characteristic feature of members of a species.
In order to reproduce, the species have to have the same number of chromosomes in order to form homologous pairs in zygotes.

Question 11

Describe the process of creating a karyogram, and its uses

Answer 11

-a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
-a cell is “frozen” in metaphase by the application of chemicals that disrupt the mitotic spindle.
-a hypotonic solution is added;
-water enters the cell causing it to swell and burst, separating the chromosomes from each other.
-the chromosomes are stained and viewed with a microscope.
-the images of the chromosomes are then organized in a standard pattern, from longest chromosomes to the smallest;
-with heterosomes at the end

Karyograms can be used to deduce sex and diagnose Down syndrome in humans. The 23rd pair of karyogram reveals the gender.
Down syndrome can be identified as such patients have 3 copies of chromosome 21 (trimosy 21).

Question 12

Distinguish between heterosome and autosomes.

Answer 12

Heterosomes are sex chromosomes, they are the 23rd pair of chromosomes. X is big and long, Y is small and short and contains SRY gene for development of male characteristics.

Heterosomes are homologous in females (XX) but not in males (XY)

Autosomes are chromosomes that do not determine sex (the rest of the somatic cells)

Question 13

Describe Cairns’ technique for measuring the length of DNA molecules, his conclusion.

Answer 13

Autoradiography is used through the use of electron microscopes.

1. Allows bacterium to absorb 3H-Thymidine (Tritiated thymidine)
   -contains tritium, a radioactive isotope of hydrogen, so radioactively labelled DNA was produced by replication in the E. coli cells.
2. Cells were then placed onto a dialysis membrane and their cell walls were digested using the enzyme lysozyme.
   -cells were gently burst to release their DNA onto the surface of the dialysis membrane.
3. A thin film o photographic emulsion was applied to the surface o the membrane
   -being left in darkness for weeks
   -some o the atoms o tritium in the DNA decayed and emitted high energy electrons, which react with the film.
   -each point where a tritium atom decayed there is a dark grain.

The film showed that prokaryotic chromosomes are circular, and the length and width of the chromosomes can be determined.

Conclusions:
-chromosome in E. coli is a single circular DNA molecule with a length o 1,100 microm. (the E coli cells is only 2 microm!)
-prokaryotic chromosomes are circular
-measured the lengths of chromosomes.
-he also observed the DNA replication fork.

Question 14

Outline the process of meiosis.

Answer 14

a. meiosis reduces a diploid cell into (four) haploid cell(s);
b. (during prophase I) homologous chromosomes pair up/synapsis;
nuclear membrane degenerates
centrioles move to opposite poles
c. chromatids (break and) recombine / crossing over followed by condensation.
d. (metaphase I) (homologous chromosomes) at the equator of the spindle / middle of cell;
e. (anaphase I) (homologous) chromosomes separate and move to opposite poles;
f. (telophase I) chromosomes reach poles and unwind WTTE;

Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

g. (prophase II) chromosomes (condense and) become visible, new spindles form;
h. (metaphase II) chromosomes line up at the centre of the cells/equator;
i. (anaphase II) sister chromatids separate;
j. (telophase II) chromatids reach the poles and unwind;