Chapter 3.2 Chromosomes

Question 1

Outline the characteristics of prokaryotic chromosomes

Answer 1

Single naked chromosome + plasmids

Naked chromosome:
Not associated with histone protein
Contains all the genetic information for basic life processes
One circular DNA molecule: one copy per gene (when not under cell division)

Plasmids:
Circular ring
Survival characteristics (e.g. antibiotic resistance)
Can be present in multiple copies even not under cell division
Can be incorporated into the nucleoid chromosome
Can be transferred between species (used for exchanging genetic information between bacteria) –> useful for GM

Question 2

Which technique is used to measure the length DNA molecules

Answer 2

Autoradiography (auto – automatic, radio – radioactive isotope, graphy – picture)

It was used to:

1) measure the length of DNA molecules,
2) reveal that prokaryotic chromosomes only have one DNA molecule

Question 3

Procedures of autoradiography

Answer 3

1. Labeling DNA molecules of E.coli: E.coli cells are grown in a medium with tritiated thymidine (which contains thymine (base used for making nucleotides for replication) with tritrium, a radioactive isotope of H)
2. Getting the DNA molecules: E.coli cells are put on a dialysis membrane where their cell walls are digested by lysosomes. The cells then gentally burst to release the DNA molecules onto the membrane
3. Applying a thin film of photographic emulsion to the membrane surface. The membrane is then left in the dark for 2 month.
4. During the 2-month period, tritrium found in E.coli DNA decays and release high-energy electrons, which then reacts with the emulsion and creates a dark grain on the film –> indicates the position of DNA
5. After 2 months, the developed film can then be analyzed with a microscope.

Question 4

Nucleosomes

Answer 4

Single strand of eukaryotic DNA rapped around histone proteins.

Question 5

Why karyotyping requires chromosomes from metaphase?

Answer 5

Eukaryotic chromosomes can only be seen by a LIGHT MICROSCOPE after supercoiling during mitosis/meiosis.

Question 6

Chromosomes can differ by…

Answer 6

1. Length (number of genes)
2. Loci (location of genes)
3. Position of centromeres (where chromatids are linked during cell division)

Question 7

What is the significance of organisms of the same species having the same genes at the same location?

Answer 7

Allows crossing over to take place during meiosis –> sexual reproduction

Question 8

Chromosome number: why important?

Answer 8

The number of chromosomes is a characteristic feature of members of the same species.

Question 9

Chromosome number of Parascaris equorum (horse threadworm)

Answer 9

4

Question 10

Chromosome number of Oryza sativa (rice)

Answer 10

24

Question 11

Chromosome number of Homo sapiens

Answer 11

46

Question 12

Chromosome number of Pan troglodytes (chimpanzees)

Answer 12

48

Question 13

Chromosome number of Canis familaris (dog)

Answer 13

78

Question 14

Describe the characteristics of homologous chromosomes

Answer 14

Homologous chromosomes have the same size and structure. They carry the same sequence of genes but not necessarily the same alleles of those genes.

Question 15

Somatic chromosomes

Answer 15

Body chromosomes (non-sex)
All somatic chromosomes are homologous
22 pairs in human

Question 16

Genome size of Escherichia coli (gut bacteria)

Answer 16

5 million base pairs

Question 17

Genome size of Homo sapien

Answer 17

3,000 million base pairs

Question 18

Genome size of T2 phage (virus that attaks E.coli)

Answer 18

0.18 million base pairs

Question 19

Genome size of Drosophila Melanogaster (fruit flies)

Answer 19

140 million base pairs

Question 20

Genome size of Paris japonica (woodland plant)

Answer 20

15,000 million base pairs

Question 21

Why chromosome number and genome size is not an accurate indicator of the complexity of a species

Answer 21

1. The number of functional genes differ from species to species
2. The amount of gene duplication varies (different genes –> 1 AA)

Question 22

Haploid

Answer 22

Half: gametes

One chromosome per pair (one allele per gene)

Question 23

Diploid

Answer 23

Double
Two chromosomes per pair (two alleles per gene)
Diploid cells are less susceptible to genetic diseases (two chromosomes)

Question 24

Which gene on the Y chromosome determines male feature development?

Answer 24

SRY gene

Testes, testosterone

Question 25

Karyotype

Answer 25

The genetic characteristic of an organism: number + type of chromosomes possessed

Question 26

Karyograms

Answer 26

Shows the chromosomes of an organism in homologous pairs of decreasing length. Patterns of banding allow chromosomes of similar size to be distinguished.

Uses chromosomes under metaphase: cells placed on a miscroscope slide –> cover slip gentally pressed against the slide –> cell burst to release chromosomes –> stained using dyes

Question 27

Uses of karyograms

Answer 27

1. Determine sex
2. Diagnosing Down syndrome and other illnesses casuec by disjunction (by extracting fetal cells from uterus during pregnancy)

Question 28

Karyotype and symptoms of Down syndrome

Answer 28

3 chromosomes at 21st pair

Symptoms: hearing loss, heart and vision disorders, later mental and physical growth