Chapter 10 Cell Biology

corellation of genes, chromosomes, and DNA

➢Genes are discrete factors which are maintained throughout the
life of an organism and then passed on to each of its progeny.
➢Chromosomes are the physical carrier of Genes.
➢Genes are consist of DNA, a macromolecule.
➢Chromosomes are DNA molecule with the associated proteins

What is DNA

is the genetic material for all organism

DNA structure

Nucleotide is the building block of DNA
o It consists of a phosphate, a
deoxyribose, and a nitrogenous base
• pyrimidine or purine
‒ Two different pyrimidines:
thymine (T) and cytosine (C)
‒ Two different purines: adenine
(A) and guanine (G)

DNA is composed of a linear nucleotide
polymer
✓ DNA molecule is polarized
✓ Backbone: Deoxyribose and phosphates
linked by 3’,5’-phosphodiester bonds
✓Nitrogenous bases project out like
stacked shelves

Chargaff established rules after doing
base composition analysis:
✓ Number of adenine = number of
thymine
✓ Number of cytosine = number of
guanine
✓ [A] + [T] ≠ [G] + [C]

nucleotide

Nucleotides have a polarized structure where one end is 5’
and the other is 3’
✓ 5’: Phosphate group is attached to the carbon 5
✓ 3’: Hydroxy group is attached to the carbon 3

Watson- Crick Proposal of 1953

DNA is composed of two
chains of nucleotides.
2. The two chains spiral around
each other forming a pair of
right-handed helices.
3. The two chains are
antiparallel, running in
opposite directions.
4. The sugar-phosphate
backbone is located on the
outside of the molecule and
the bases are inside the helix.

The Watson-Crick
base pairs
The Watson-Crick Proposal
5. The two DNA chains are held
together by hydrogen bonds
between each base.
6. The double helix is 2 nm wide.
7. Pyrimidines are always paired
with purines.
8. Only A-T and C-G pairs fit
within double helix.

The Watson-Crick Proposal
9. DNA molecule has a major
groove (wider) and a minor
groove (more narrow).
10. The double helix makes a
complete turn every 10
residues (3.4 nm).
11. The two chains are
complementary to each other
➢ E.g.: 5’-ATGC-3’
3’-TACG-5’

structure of genome

The genome is the unique collective body of the
genetic material of an organism.
✓ E.g.: Human genome is essentially equal to all of
the genetic information stored in a single (haploid)
set of human chromosomes: 22 autosomes plus X
and Y sex chromosomes

complexity of genome

Important properties of DNA:
✓ Denaturation: The DNA double helices can be
separated into two strands when DNA solution is
heated to a certain temperature
o DNA melting
✓ Renaturation or reannealing: Single-stranded DNA
molecules are capable of reassociating with
correct base pairing
o Basis for the investigation of genome
complexity

complexity of viral and bacterial genome

The rate of renaturation of DNA from
bacteria and viruses depends on the
size of their genome
✓ The smaller the genome size, the
faster the renaturation
o With the same initial concentration,
the smaller the genome size, the
greater the number of genomes
present, the greater the chance of
collision between complementary
fragments

Same weight of DNA: 1 genome of bacterial equal to 6 genome of viral

complexity of the eukaryotic genome

Reannealing of eukaryotic
genomes shows three classes of
DNA:
✓ Highly repeated
✓ Moderately repeated
✓ Nonrepeated

Highly Repeated DNA Sequences
➢ Represent about 1-10% of total DNA
➢ Short and present in clusters in which the given
sequence repeats over and over without
interruption
➢ Classification:
✓ Satellite DNAs: Five to a few hundreds base pair
long for each, forming large clusters up to several
million bp
✓ Minisatellite DNAs: 10 to 100 base pair long for
each, forming up to 3000 repeats
✓ Microsatellite DNAs: 1 to 9 base pair long for
each, forming 10 to 40 bp long cluster

Moderately Repeated DNA Sequences
➢ Accounts for about 20 to 80% of the genome
depending on the organism
➢ Two classifications:
✓ Repeated DNA Sequences with Coding Functions
– include genes that code for ribosomal RNA
and histones.
✓ Repeated DNA Sequences that Lack Coding
Functions
o Scattered throughout the genome as
individual elements
o Two classes
• SINEs: short interspersed elements
• LINEs: long interspersed elements

Non-repeated DNA Sequences
➢ Single-copy DNA sequence
➢ Code for the majority of
proteins
➢ Less than 2% of human
genome encode for protein

stability of genome

whole genome duplication polyploidization

DNA sequence organization of the genome can be
changed rapidly
➢ In polyploidization, or whole‐genome duplication,
offspring have four chromosome homologues rather
than two
✓ Two related species can mate to form a hybrid
organism with the combined chromosomes from
both parents, or a 1-cell embryo can undergo
chromosome duplication and retain the DNA.
➢ Occurs often in plants, and in some amphibian animals.

the dynamic nature the genome: jumping genes

Repeated sequences are sometimes present in tandem
arrays, sometimes present on two or a few
chromosomes, and sometimes dispersed throughout the
genome.
➢Barbara McClintock suggested that genetic elements
were capable of moving around the genome.
➢She called this genetic rearrangement transposition,
and the mobile genetic elements transposable
elements.

Transposons:
✓ Transposable elements in the
bacteria
✓ Encodes a protein called
transposase
o Catalyze excision of
transposon from donor
DNA and insertion of the
transposon to the recipient
DNA
o Cut-and paste pattern

n the eukaryotes:
✓ Transposons: cut-and-paste
✓ Retrotransposons: copy-and-paste