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CYTOGENETICS
Lesson
[TRANS] LESSON 09: DNA & CHROMOSOMES
WHAT IS DNA MADE OF?
COMPOSED OF:
● Sugar (deoxyribose) + phosphate
● Nitrogen base – attached to sugar
○ Adenine (A)
○ Cytosine ©
○ Guanine (G)
○ Thymine (T)
- DNA is made of four nucleotide building blocks.
(A) Each nucleotide is composed of a sugar
phosphate covalently linked to a base—guanine (G) in
this figure.
(B) The nucleotides are covalently linked
together into polynucleotide chains, with a
sugar–phosphate backbone from which the
bases—adenine, cytosine, guanine, and thymine (A, C,
G, and T)—extend.
(C) A DNA molecule is composed of two
polynucleotide chains (DNA strands) held together by
hydrogen bonds between
the paired bases. The arrows on the DNA strands
indicate the polarities of the two strands, which run
antiparallel to each other (with opposite chemical
polarities) in the DNA molecule.
(D) Although the DNA is shown straightened out
in (C), in reality, it is wound into a double helix, as shown
here.
POLARITY OF DNA
● Sugar phosphate backbone held together by
phosphodiester bonds
● 5’ end – fifth C in deoxyribose
○ Phosphate end
● 3’ end – third C in deoxyribose
● “The nucleotide subunits within a DNA
strand are held together by phosphodiester
bonds. These bonds connect one sugar to the
next. The chemical differences in the ester
linkages—between the 5ʹ carbon of one sugar
and the 3ʹ carbon of the other—give rise to the
polarity of the resulting DNA strand. For
simplicity, only two nucleotides are shown here.”
DOUBLE HELIX AND COMPLEMENTARY
● Two strands held together by hydrogen bonds
between bases ○ Purine - G and A ○ Pyrimidine - C and T
● Complementary base pairs
○ C and G
○ T and A
● Strands are antiparallel to each other
● 10 base pairs per helical turn
● “The two strands of the DNA double helix are held
together by hydrogen bonds between complementary base pairs.” ● 10 base pairs per helical turn ○ Major groove ■ Where most proteins can attach ○ Minor groove
■ Provides a unique chemical
environment
DNA IS THE MECHANISMS FOR HEREDITY
GENETIC CODE
● Combinations of DNA correspond to amino acid
● Dna is read and converted into amino acids ● “The nucleotide sequence of an mRNA is translated into the amino acid sequence of a protein via the genetic code. All of the three-nucleotide codons in mRNAs that specify a given amino acid are listed below that amino acid, which is given in both its three-letter and one-letter abbreviations.”
GENES
● Regions in DNA that contain information to make a
protein or other molecules
● “ Most genes contain information to make
proteins. Protein-coding genes each produce a set of RNA molecules, which then direct the production of a specific protein molecule. Note that for a minority of genes, the final product is the RNA molecule itself, as shown here for gene C. In these cases, gene expression is complete once the nucleotide sequence of the DNA has been transcribed into the nucleotide sequence of its RNA. ”
EUKARYOTIC CHROMOSOMES
● Long DNA strands that are packaged using proteins
● In humans – 46 chromosomes divided into 23 pairs
○ 22 pairs – homologous (homologs)
○ 2 sex chromosomes – nonhomologous
in males (XY)
● Genome
○ Full set of chromosomes
● “ Each human chromosome can be “painted” a
different color to allow its unambiguous identification. The chromosomes shown here were isolated from a cell undergoing nuclear division (mitosis) and are therefore in a highly compact (condensed) state. Chromosome painting is carried out by exposing the chromosomes to a collection of single- stranded DNA molecules that have been coupled to a combination of fluorescent dyes. For example, single-stranded DNA molecules that match sequences in chromosome 1 are labeled with one specific dye combination, those that match sequences in chromosome 2 with another, and so on. Because the labeled DNA can form base pairs (hybridize) only with its specific chromosome (discussed in Chapter 10), each chromosome is differently colored. For such experiments, the chromosomes are treated so that the individual strands of its double-helical DNA partly separate to enable base-pairing with the labeled, single-stranded DNA.” ● “(A) Micrograph showing the array of chromosomes as they originally spilled from the lysed cell. (B) The same chromosomes are artificially lined up in their numerical order. This arrangement of the full chromosome set is called a karyotype.”
WHAT ARE CHROMOSOMES MADE OF?
● Genes – code for proteins
○ Coding regions ● Non-coding regions ○ Other biological functions ○ Unknown functions
SPECIALIZED DNA SEQUENCES FOR CELL
DIVISION
● Replication origin
○ Starting points for DNA replication
CHROMATIN
● A long string of nucleosomes and other
DNA-associated proteins ● Chromatin is tightly compacted by a “linker” histone ○ Histone H ● Chromatin isolated directly from an interphase nucleus can appear in the electron microscope as a chromatin fiber, composed of packed nucleosomes.
LOOPED DOMAINS
● Uses special nonhistone chromosomal proteins
● “ The chromatin in human chromosomes is folded
into looped domains. These loops are established by special nonhistone chromosomal proteins that bind to specific DNA sequences, creating a clamp at the base of each loop.”
ORGANIZATION OF DNA
● DNA packing occurs on several levels in chromosomes ○ Mitotic chromosomes – up to 10,000 times shorter ● “ DNA packing occurs on several levels in chromosomes. This schematic drawing shows some of the levels thought to give rise to the highly condensed mitotic chromosome. Both histone H1 and a set of specialized nonhistone chromosomal proteins are known to help drive these condensations, including the chromosome loop-forming clamp proteins and the abundant non-histone protein condensin”
WAYS TO ALTER NUCLEOSOME
● Used to make DNA more accessible
● Main mechanisms: ○ Chromatin-remodeling complexes ■ Locally alter nucleosome arrangement ○ Histone-modifying enzymes ■ Allow for attachment of other proteins ● Chromatin-remodeling complexes locally reposition the DNA wrapped around nucleosomes.
CHROMATIN IN THE INTERPHASE
EUCHROMATIN
● More extended DNA Regions
● Contain actively expressed genes ● Regions that are actively being used by the cell ● Controlled by histone modifications
HETEROCHROMATIN
● More condensed chromatin
● Causes genes to be silenced ● Controlled by histone modifications
TAKEAWAYS
● Life depends on the stable storage,
maintenance, and inheritance of genetic
information.
● Genetic information is carried by very long DNA
molecules and is encoded in the linear
sequence of four nucleotides: A, T, G, and C.
● Each molecule of DNA is a double helix
composed of a pair of antiparallel,
complementary DNA strands, which are held
together by hydrogen bonds between G-C and
A-T base pairs.
● The genetic material of a eukaryotic cell—its
genome—is contained in a set of chromosomes,
each formed from a single, enormously long
DNA molecule that contains many genes.
● When a gene is expressed, part of its nucleotide
sequence is transcribed into RNA molecules,
most of which are translated to produce a
protein.
● The DNA that forms each eukaryotic
chromosome contains, in addi- tion to genes,
many replication origins, one centromere, and
two telomeres. These special DNA sequences
ensure that, before cell division, each
chromosome can be duplicated efficiently, and
that the resulting daughter chromosomes can be
parceled out equally to the two daughter cells.
● In eukaryotic chromosomes, the DNA is tightly
folded by binding to a set of histone and
nonhistone chromosomal proteins. This complex
of DNA and protein is called chromatin.
● Histones pack the DNA into a repeating array of
DNA–protein particles called nucleosomes,
which further fold up into even more compact
chromatin structures.
● A cell can regulate its chromatin
structure—temporarily decondens- ing or
condensing particular regions of its
chromosomes—using chromatin-remodeling
complexes and enzymes that covalently modify
histone tails in various ways.
● The loosening of chromatin to a more
decondensed state allows pro- teins involved in
gene expression, DNA replication, and DNA
repair to gain access to the necessary DNA
sequences.
● Some forms of chromatin have a pattern of
histone tail modification that causes the DNA to
become so highly condensed that its genes
cannot be expressed to produce RNA; a high
degree of condensation occurs on all
chromosomes during mitosis and in the
heterochromatin of interphase chromosomes.
