Medical and Molecular Genetics
Lecture 3 Genome Oganization
1) List the constituents of chromatin and state their functions; describe how
chromatin forms a chromosome; define the terms histone, nucleosome,
chromatosome, scaffolding, heterochromatin, and euchromatin. Chromatin
consists of histone proteins, which condense the DNA so that it can fit within the
nucleus, and non-histone proteins, which are involved in transcription, DNA
replication, and maintenance and remodeling of chromatin structure. Chromatin must
be densely packed to form a chromosome. First, approximately 146bp of DNA are
wrapped around an octomer of histones (two of each: H2A, H2B, H3, and H4)
creating a nucleosome. Each nucleosme is separated by 20-100bp bound to H1, which
serves to further bind DNA forming the chromatosome. The chromatosomes form a
solenoidal coil with about 6 chromatosomes per turn. The fibrils form loops of
variable lengths and are anchored to a scaffolding comprised of nonhistone proteins.
These loops coil again to form minibands which are arranged along a central axis to
from the arms of a mitotic chromosome. When viewed under a microscope, dark
bands can be seen on each chromosome called heterochromatin consisting of highly
condensed DNA. Light bands are called euchromatin and consist of DNA that is less
condensed and transcriptionally active.
2) Describe how and why chromatin is thought to restructure itself in order to
regulate transcription. Chromatin must decondense in order for the replication
machinery and transcriptional machinery to gain access to the DNA. The dynamic
restructuring of chromatin is carried out by multiprotein complexes that are called
chromatin remodeling factors.
3) List and state the functions of the elements needed for chromosomal replication
and segregation. At least three types of cis-acting elements are required including:
origins (autonomously replicating sequences (ARS)), telomeres, and centromeres.
Origins are the sites at which DNA replication is initiated on the chromosome and
contain two functional sites: one, a specific segment of DNA that is recognized by a
large protein complex known as the origin recognition complex, and two, an adjacent
AT-rich region so that DNA replication can initiate. Telomeres are required for
replicating the tips of chromosomal DNA and consist of multiple repeats of a specific
DNA sequence found at the ends of linear chromosomes. Centromeres are required
for the segregation of replicated chromosomes to daughter cells and are the sites of
attachment for sister chromatids as well as the mitotic spindle during mitosis (forming
the kinetochore with other structural proteins).
4) Distinguish between unique and repetitive DNA; describe how these types of
DNA are distributed in the genome and state their functions. Unique DNA
contains sequences that don’t have a repetitive base pair organization. This accounts
for over 50% of the human genome but only 14% are contained in exons. Repetitive
DNA consists of two classes: highly repetitive and moderately repetitive. About 30%
of the genome consists of repetitive DNA that functions in maintaining the structure
of DNA in the nucleus. Highly repetitive DNA has over a million copies, comprises
15% of the genome, and is transcriptionally inactive in regions like the centromere or
telomere. It may function in regulation (trinucleotide repeats function in disease
states). Moderately repetitive DNA is diverse and well distributed with coding
regions such as rRNA and mostly noncoding regions that may be transcribed (why?),
the Alu family (5%) of genome may be related to transposons.
5) List the characteristic features of the mitochondrial genome. It is 16,569bp in
length, circular, has no repetitive DNA, and doesn’t associate with histones but rather
bacteria-like DNA- binding proteins. It encodes 2rRNAs, 22tRNAs, DNA
polymerase, and 13 polypeptides in the electron transport chain. Mutations in these
genes can be related to disease states and the mitochondrial gene sequence is used to
track evolutionary lineage.