Lecture 18
Biology
DNA (DeoxyriboNucleic Acid)
DNA is a long double-stranded linear molecule composed of
multiple nucleotide sequences. It acts as a template for the synthesis of
RNA.
A. Nucleotides are composed of a base (purine or pyrimidine), a
deoxyribose sugar, and a phosphate group.
1. The purines are adenine (A) and guanine (G).
2. The pyrimidines are cytosine (C) and thymine (T).
B. The DNA double helix consists of two complementary DNA strands
held together by hydrogen bonds between the base pairs A-T and G-C.
C. Exons are regions of the DNA molecule that code for specific RNAs.
D. Introns are regions of the DNA molecule, between exons, that do not
code for RNAs.
E. A codon is a sequence of three bases in the DNA molecule that codes
for a single amino acid.
F. A gene is a segment of the DNA molecule that is responsible for the
formation of a single RNA molecule.
G. According to the Human Genome Study, there are approximately
25,000 genes in the human genome.
Lecture 18
Biology
RNA (Ribonucleic acid)
RNA is a linear molecule similar to DNA; however, it is single
stranded and contains ribose instead of deoxyribose sugar and uracil (U)
instead of thymine (T). RNA is synthesized by transcription of DNA.
Transcription is catalyzed by three RNA polymerases: I for rRNA, II for
mRNA, and III for tRNA.
A. mRNA
carries the genetic code to the cytoplasm to direct protein
synthesis .
1. This single-stranded molecule consists of hundreds to thousands of
nucleotides.
2. mRNA contains codons that are complementary to the DNA codons
from which it was transcribed, including one start codon (AUG) for
initiating protein synthesis and one of three stop codons (UAA, UAG, or
UGA) for terminating protein synthesis.
3. mRNA is synthesized in the following series of steps.
a. RNA polymerase II recognizes a promoter on a single strand of the
DNA molecule and binds tightly to it.
b. The DNA helix unwinds about two turns, separating the DNA strands
and exposing the codons that act as the template for synthesis of the
complementary RNA molecule.
c. RNA polymerase II moves along the DNA strand and promotes base
pairing between DNA and complementary RNA nucleotides.
Lecture 18
Biology
d. When RNA polymerase II recognizes a chain terminator (stop
codons—UAA, UAG, or UGA) on the DNA molecule, it terminates its
association with the DNA and is released to repeat transcription.
e. The primary transcript, pre-mRNA after the introns are removed,
associates with proteins to form hnRNP.
f. Exons are spliced through several steps, involving spliceosomes
producing an mRNP.
g. Proteins are removed as the mRNP enters the cytoplasm, resulting in
functional mRNA.
h. RNA segments remaining from the transcription process as introns
were once thought to be degraded and recycled because they were
believed to have no function. However, recent evidence shows that these
RNA segments perform regulatory functions that parallel regulatory
proteins related to development, gene expression, and evolution.
B. tRNA is folded into a cloverleaf shape and contains approximately
80 nucleotides, terminating in adenylic acid (where amino acids attach).
1. Each tRNA combines with a specific amino acid that has been
activated by an enzyme.
2. One end of the tRNA molecule possesses an anticodon, a triplet of
nucleotides that recognizes the complementary codon in mRNA. If
recognition occurs, the anticodon ensures that the tRNA transfers its
activated amino acid molecule in the proper sequence to the growing
polypeptide chain.
C. Ribosomal RNA associates with many different proteins
(including enzymes) to form ribosomes.
Lecture 18
Biology
1. rRNA associates with mRNA and tRNA during protein synthesis.
2. rRNA synthesis takes place in the nucleolus and is catalyzed by RNA
polymerase I. A single 45S precursor rRNA (pre-rRNA) is formed and
processed to form ribosomes as follows:
a. Pre-rRNA associates with ribosomal proteins and is cleaved into the
three sizes (28S, 18S, and 5.8S) of rRNAs present in ribosomes.
b. The RNP containing 28S and 5.8S rRNA then combines with 5S
rRNA, which is synthesized outside of the nucleolus, to form the large
subunit of the ribosome.
c. The RNP containing 18S rRNA forms the small subunit of the
ribosome.
Lecture 18
Biology
Chromosomes
A- Structure: Chromosomes consist of chromatin extensively folded into
loops; this conformation is maintained by DNA-binding proteins. Each
chromosome contains a single DNA molecule and associated proteins,
assembled into nucleosomes, the structural unit of chromatin packaging.
1. Extended chromatin forms the nucleosome core, around which the
DNA double helix is wrapped two full turns.
a. The nucleosome core consists of two copies each of histones H2A,
H2B, H3, and H4. Nucleosomes are spaced at intervals of 200 base
pairs.
b. When viewed with TEM, extended chromatin resembles beads on a
string; the beads represent nucleosomes and the string represents linker
DNA. DNA is supported by the nucleosomes that function.
Nucleosomes support DNA and regulate its accessibility for replication
and transcription as well as for its repair. Chromatin is packaged into
30-nm threads as helical coils of six nucleosomes per turn and bound
with histone H1.
2. Condensed chromatin contains an additional histone, H1, which
wraps around groups of nucleosomes, thus forming 30-nm-diameter
fibers of helical coils of six nucleosomes per turn, which is the structural
unit of the chromosome.
B. G-banding is observed in chromosomes during mitosis after staining
with Giemsa, which is specific for DNA sequences rich in adenine (A)
and thymine (T). Banding is thought to repre-sent highly folded DNA
Lecture 18
Biology
loops. G-banding is characteristic for each species and is used to
identify chromosomal anomalies.
C. Karyotype refers to the number and morphology of chromosomes
and is characteristic for each species.
1. Haploid number (n) is the number of chromosomes in germ cells (23
in humans).
2. Diploid number (2n) is the number of chromosomes in somatic cells
(46 in humans).
D. Genome, the total genetic complement of an individual, is stored in
its chromosomes. In humans, the genome consists of 22 pairs of
autosomes and 1 pair of sex chromosomes (either XX or XY), totaling
23 pairs, or 46 chromosomes.