Genes and Genomes Flashcards
Define what is meant by the term ‘genome’.
Full set of genes or chromosomes contained in a haploid organism or the gamete of a diploid organism
What is the difference between euchromatin and heterochromatin?
Heterochromatin is a tightly bound structure of the chromosome, which localizes to the ends and centromere
regions of the chromosome. It is composed of highly repetitive DNA and is not often actively transcribed. Euchromatin is a
more loosely bound form of chromatin that contains a number of genes and other sequences that are more actively
transcribed.
What is a transposon?
Transposons are DNA segments that are capable of moving (by the mechanism of transposition) to different
locations within the genome of a particular cell.
What is the link between repetitive DNA and biological complexity of an organism?
Biological complexity is characterized by a number of features, including multicellularity, compartmentalization
and cellular specialization. The proportion of repetitive DNA in an organism’s genome is inversely proportional to the
amount of repetitive DNA. Therefore, primitive prokaryotes will lack repetitive DNA and the entire genome will be devoted
to coding sequences, giving these organisms a high gene density. In contrast, eukaryotes have repetitive sequences in
addition to coding sequences. The more advanced multicellular eukaryotes will have a greater proportion of repetitive DNA
and a lower gene density, compared to single-celled eukaryotes like yeast.
Consider the figure below that displays the structure of the globin genes from plants (plant globin and
leghemoglobin) and human (myoglobin, α-globin and β-globin) origin. Using specific information from the figure,
discuss the following:
a. The link between intron/exon number and position in related genes
Similar genes show similar intron-exon gene structure and therefore should have the same number of introns at
similar positions in the gene. Genes for related proteins from the same organism should be more similar to a similar protein
in a different organism. In the 5 genes of the globin family, the two plant globins have the same number of introns, namely 3
introns, which occur at similar positions within the two genes. In comparison the human globin genes all possess 2 introns,
which also occur at similar positions in the gene. In addition, two of the three introns in the plant globins are conserved with
respect to position to the two human globin introns.
The correlation between gene size and the organism from which the gene arises
There is no correlation between the total size of the gene and the organism from which the gene arises. The size of
the different globin genes in humans varies from 677 to 8659bp. Apart from the myoglobin gene, the total sizes of the globin
genes from plants and humans is similar. The size of the gene product (protein) is similar for all proteins.
c. The link between intron size in related genes.
There is no correlation between intron size and the different genes. Intron size at a particular position can vary
considerably, from 4800 in the myoglobin at position 31 to 130 at the equivalent position in β-globin and 122 in plant
globin.
Name one experiment providing direct evidence for DNA being the genetic material of the cell
Avery, Macleod, and McCarty’s transformation experiments; or Hershey and Chase’s blender experiments
in bacteriophages
Name two processes that do not conform to the “central dogma of molecular biology”
reverse transcription, viral RNA replication (+ sense to – sense RNA), the fact that not all RNA is
translated into protein (some becomes rRNA, tRNA), the ability of prions to produce different proteins by
conformational changes to existing proteins
Define the term “gene” as accurately as possible
A gene is a region of DNA that controls a discrete hereditary characteristic and is transcribed as a
single unit. It encodes a set of closely related polypeptides or an RNA molecule. And encompasses
coding DNA sequences, non-coding regulatory DNA sequences and, in the case of eukaryotes, introns
What is an intron?
Introns are stretches of intervening DNA between the exons of a gene that do not code for protein and are spliced
out during RNA processing.
Why is it an oversimplification to say that exons are short protein-coding sequences?
Exons are those segments of sequence that are spliced together after the introns have been removed from the pre-mRNA. Yes, the coding sequence is contained in exons, but it is possible for some exons to contain no coding
sequence. Portions of exons or even entire exons may contain a sequence that is not translated into amino acids.
These are the untranslated regions or UTRs. UTRs are found upstream and downstream of the protein-coding
sequence.
What is the C-value paradox? Elaborate on your answer encompassing the concept of complexity, gene
density, repetitive DNA, the “minimal genome”
The size of an organism’s genome does not necessarily correlate with biological complexity. (For example, the
cells of some salamanders may contain 40 times more DNA than those of humans.) In the lower-level life
forms, there is a correlation between complexity and genome size, but as we climb the evolutionary tree, the
relationship between the complexity of the organism and the content of DNA becomes obscure. This is called the Cvalue paradox. C-value is the total amount of DNA in a haploid cell and the C stands for constant or
characteristic. The reason for the paradox is that more complex organisms have a large amount of repetitive
and non-coding DNA (1 % exons in humans) and thus a lower gene density. Unlike total genome size,
minimum genome size (or “minimal genome”) is correlated with complexity. These are the genes essential for
survival
Name one advantage of having a large genome
Mutational load – buffering of the coding sequences due to the low probability of a mutation happening to occur in
such a small proportion of the genome.
Describe in detail how DNA is packaged as chromatin
The process starts when DNA is wrapped around histone proteins to form nucleosomes. Nucleosomes are made
up of double-stranded DNA that has complexed with histones. The core particle of each nucleosome consists of
eight histone molecules, two each of four different histone types: H2A, H2B, H3 and H4. Next the nucleosomes
are packaged into a thread, which is sometimes described as “beads on a string”. The end result is a fiber
known as chromatin, which is made up of 60% protein, 40% DNA. The chromatin fiber is coiled into a
structure called a “solenoid”. This fiber is then looped and coiled yet again, leading finally to the familiar
structures known as chromosomes.
