Chapter 3.2

Question 1

Prokaryotes

Answer 1

Microscopic, single-celled organisms without specialised compartments inside the cell. Prokaryotes include two of the three domains of life: eubacteria and archaea.

Question 2

How many chromosomes do prokaryotes have?

Answer 2

In prokaryotes, there is usually a single chromosome that contains the entire genome. The chromosome is a closed-loop circular DNA double helix molecule. Prokaryotic DNA is called ‘naked DNA’ because, in contrast to eukaryotic DNA, it is not organised around associated histone proteins.

Question 3

Plasmids

Answer 3

prokaryotes often contain much smaller loops of DNA known as plasmids. A plasmid may contain one or several genes, often related to a specific function. Bacteria can easily exchange plasmids, sometimes even with a different species of prokaryote. Although plasmids are very common in prokaryotes, they are generally not found in eukaryotes, However, there are exceptions. For example, Saccharomyces cerevisiae, a unicellular yeast used in baking, contains a small plasmid.

Question 4

Eukaryotic species

Answer 4

They can be unicellular or multicellular. They separate their genetic material from the cytoplasm by containing it in the membrane-bound nucleus. Eukaryotic chromosomes consist of one or two long linear DNA molecules, depending on the phase of the cell cycle .The DNA molecule is wrapped around basic (alkaline) proteins called histones.

Question 5

How are chromosomes visible?

Answer 5

During interphase, when the information on the DNA is in active use, the chromosomes are visible only with an electron microscope in a less organised form called chromatin. This allows the DNA-transcribing enzymes to access the genes with the information they need to keep the cell functioning.

Question 6

Sister chromatids

Answer 6

The two lines that form the ‘X’ are identical copies of the chromosome

Question 7

Centromere

Answer 7

The place where the two strands of chromatids connect. It holds the sister chromatids of a chromosome together until they are separated in anaphase (at which point each chromatid is considered a chromosome).

Question 8

Homologous chromosomes

Answer 8

Homologous chromosomes carry the same set of genes in the same locations. However, there can be small differences between the nucleotide sequences of the same gene. Alleles usually differ from each other by only a few DNA base pairs , but may lead to different characteristics and phenotypes.
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. Diploid nuclei have pairs of homologous chromosomes.

Question 9

Eukaryotes and different chromosomes

Answer 9

In any eukaryote species, there are different chromosomes that carry different genes.

Question 10

Autosomes

Answer 10

chromosomes that do not code for sex

Question 11

Different organism genome sizes

Answer 11

In millions
T2 phage: 0.18
E. coli: 5
Drosophila melanogaster: 140
Homo sapiens: 3 200
Paris japonica: 150 000

Question 12

Genome size and number of genes

Answer 12

It is important to note that the genome size does not always correlate with number of genes. The main reason is that some parts of a chromosome, especially in eukaryotes, may not code for polypeptides.

Question 13

Haploid cells

Answer 13

Haploid cells are produced from diploid cells by meiosis. Haploid nuclei, which contain only one chromosome from each pair, are found only in your gametes, or sex cells (sperm and ova). Haploid nuclei have one chromosome of each pair of homologous chromosomes. Each chromosome in a homologous pair is referred to as a homologue .

Question 14

Tetrad

Answer 14

A structure consisting of two homologues

Question 15

Number of chromosomes to species

Answer 15

The number of chromosomes is a characteristic feature of members of a species. Usually, all individuals within a species have the same number of chromosomes (except in cases of chromosomal mutations). In some species, the different sexes have different numbers of chromosomes. However, even in these cases, individuals of each sex have a known characteristic chromosome number.

Question 16

Comparison of diploid chromosome numbers

Answer 16

Homo sapiens: 46
Pan troglodytes: 48
Canis familiaris: 78
Oryza sativa: 24
Parascaris equorum: 4

Question 17

What does a karyogram show?

Answer 17

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. A karyogram can be used to deduce the sex of an individual, to find missing or extra chromosomes, and to detect other chromosomal mutations. It is not able to detect differences in alleles or mutations that affect a single gene. It can determine sex as well as genetic disease’s like down syndrom

Question 18

What does a karyogram show?

Answer 18

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length. A karyogram can be used to deduce the sex of an individual, to find missing or extra chromosomes, and to detect other chromosomal mutations. It is not able to detect differences in alleles or mutations that affect a single gene. It can determine sex as well as genetic disease’s like Down syndrome

Question 19

Trisomy

Answer 19

During meiosis, when the gametes are formed, the resulting ova or sperm cells can sometimes contain an extra or missing copy of a chromosome. This is caused by the failure of a pair of homologous chromosomes or sister chromatids to separate and is known as non-disjunction . Zygotes formed when one gamete has an extra chromosome give rise to individuals with three copies of one particular chromosome, a condition called trisomy.

Question 20

Down syndrome

Answer 20

With most autosomes, trisomy is fatal. However, where there are three copies of chromosome 21 present (trisomy 21), Down syndrome occurs. This syndrome causes:
- hearing loss
- heart and vision problems
- intellectual disability
- slower growth > smaller stature.
The severity of these symptoms varies greatly between individuals. There is no cure for Down syndrome, but with early intervention and therapy many individuals with Down syndrome lead independent and fulfilling lives

Question 21

autoradiography

Answer 21

a technique that uses X-ray film to visualise the two-dimensional distribution of a radioactively labelled substance. Autoradiography allows radioactive molecules to, in effect, take pictures of themselves which is used for measuring the length of DNA molecules.

Question 22

Images of DNA

Answer 22

Cairns first produced images of DNA molecules from E. coli through:
- producing thymidine (the nucleotide containing thymine) labelled with radioactive hydrogen ( 3 H).
- growing E. coli bacteria in a medium containing the radioactive thymidine. When the E. coli cells replicated their DNA, they used the radioactive thymine, creating radioactive DNA.
- lysing the cells (ruptured their cell walls and membranes) to release the cell contents, including the intact bacterial DNA, onto slides.
- covering the slides with photographic emulsion and store them in the dark for two months.
- High energy electrons emitted by the radioactive decay of 3 H within the DNA caused the appearance of dark spots on the photographic emulsion.
The pattern of dark spots indicated the presence of labelled DNA.
- The length of the E. coli chromosome could then be measured and was worked out to be about 1 mm, nearly 1 000× longer than the typical E. coli cell.