MCB4304 Exam Questions & Answers 2024/2025
Why does the estimated number of protein coding genes in prokaryotes correlate well with genome size? - ANSWERSBecause most genes are coding regions and prokaryotic DNA do not have introns
Why does the estimated number of protein coding genes in eu...
Why does the estimated number of protein coding genes in prokaryotes correlate well with genome
size? - ANSWERSBecause most genes are coding regions and prokaryotic DNA do not have introns
Why does the estimated number of protein coding genes in eukaryotes correlate poorly with genome
size? What is the estimated number of genes in humans? - ANSWERSBecause Euks have introns and
some of our genes are regulatory sequences and non-coding DNA. many genes can give rise to several
alternative variant proteins, so that the total number of proteins specified by the genome is substantially
greater than the number of genes. We have about 24,000 genes
Gene density is defined as the number of genes per mega base pair; genes/MB. What is the approximate
gene density in E. coli? In humans? - ANSWERSIn bacteria, there are about a 1000 genes per million base
pairs. In mammals, there are about 10 genes per million base pairs. Gene density for E. coli is about 0.92
genes/MB. In humans, the genome size is 3,200,000 MB and the number of genes coding for proteins is
24,000. So, the gene density for humans is about 0.0075
Approximately how many nucleotide pairs are present in the human genome? What percentage of these
are unique sequences? - ANSWERS3.2E9 nucleotide pairs. 50% are unique seq
What are the three DNA sequences required to produce a stable eukaryotic chromosome? -
ANSWERSTelomere, replication origin, centromere
What is meant by the "histone code"? Describe how the recruitment of a code-reader-writer complex
can spread chromatin changes along a chromosome - ANSWERSThe histone code is a hypothesis that the
transcription of genetic information encoded in DNA is in part regulated by chemical modifications to
histone proteins, primarily on their unstructured ends. First a gene regulatory protein can bind to a
section of the DNA between histones. Then a histone modifying enzyme (writer) can bind to the
regulatory protein. Then a code reader protein can come in and bind to the histone and the "writer"
forming a code-reader-writer complex and this complex can keep recruiting "readers" and "writers" to
the histone bead and spread along it which could cause a number of things. It could condense the
chromatin or it could help initiate or block transcription.
, Describe the two unusual chromosome structures (one in amphibia and the other in Drosophila) that
have been influential in proposing the presence of looped domains. - ANSWERSLampbrush
chromosomes. Each chromosome replicates to begin meiosis and the homologous replicated
chromosomes pair to form a highly extended structure containing four replicated DNA molecules, or
chromatids. These chromatids are organized into a series of large loops emanating from a linear
chromosomal axis, and many of these loops are active in RNA synthesis.
Polytene chromosomes. In the larval salivary glands of Drosophila, the interphase homologous
chromosomes undergo multiple mitoses without cytokinesis (cell division), resulting in about 1000
copies of each homologous chromosome instead of two. Moreover each of these 1000 identical
sequences remain side by side in register, forming a giant polytene chromosome, containing visible
bands and interbands with recognizable thickness and spacing. The chromatin in the bands is more
condensed than that in the interbands and 95% of the DNA is in bands.
What is meant by a conditional mutation, a loss of function mutation, and a null mutation? -
ANSWERSConditional Mutation: produces its phenotypic effect only under certain conditions, called the
restrictive conditions. Under other conditions - the permissive conditions - the effect is not seen. For a
temperature-sensitive mutation, the restrictive condition typically is high temperature, while the
permissive condition is low temperature.
Loss of Function Mutation: either reduces or abolishes the activity of the gene. These are the most
common class of mutations. Loss-of-function mutations are usually recessive- the organism can usually
function normally as long as it retains at least one normal copy of the affected gene.
Null Mutation: a loss-of-function mutation that completely abolishes the activity of the gene
What is the difference between an intragenic suppressor mutation and an extragenic suppressor
mutation? - ANSWERSSuppressor Mutation: suppresses the phenotypic effect of another mutation, so
that the double mutant seems normal. An intragenic suppressor mutation lies within the gene affected
by the first mutation; an extragenic suppressor mutation lies in a second gene - often one whose product
interacts directly with the product of the first.
How is a complementation test used to determine whether two mutations are in different or in the same
genes. - ANSWERSan individual who is homozygous for one mutation is mated with an individual who is
homozygous for the other. The phenotype of the offspring gives the answer to the question. If the hybrid
offspring shows normal phenotype: one normal copy of each gene is present. If hybrid offspring shows
mutant phenotype: no normal copies of the mutated gene are present.
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