DNA Flashcards
carbohydrates
Chemical compounds that contain carbon (C),
hydrogen (H) and oxygen (O) usually with a H:O ratio
of 2:1, and the empirical formula Cm(H2O)n
- monosaccharides
(Glucose, Fructose,Galactose) - disacharides
(sucrose, lactose, maltose) - oligosacharides
(raffinose, stachyose) - polysacharides
(starch, glycogen, cellulose)
lipids
- Heterogeneous group of organic
compounds that are insoluble in
water and soluble in non-polar
organic solvents. - Part of cell membranes
- Help control what goes in and out
of cells. - Help with moving and storing
energy, absorbing vitamins and
making hormones
nucleic acids
- Principle information molecules of the cell
(genetic machinery of the cell) - Two main types:
DNA: Deoxyribonucleic acid
RNA: Ribonucleic acid
proteins
- The ‘doers’ of the cell
- Execute the tasks assigned by the genetic
information. - Most diverse macromolecules in the cell.
- Proteins direct virtually all activities of the cell.
- Amino acids
- Peptides
- Polypeptides
what gave rise to the first cell?
Theory: Enclosure of self-replicating
RNA in a phospholipid membrane gave rise to the first cell.
def: genes
Genes: segments of DNA that encode RNA (or proteins) – functional
units of inheritance
def: transcription
Transcription: Process by which nucleotide gene sequence is copied
into RNA
def: translation
Translation: Nucleotide sequence of RNA is used to specify the
order of amino acids in a protein
def: archaea
Archaea: Found in the ocean and also extreme environments
(E.g. Extreme thermophiles, halophiles, acidophiles etc.) Cell
walls lack peptidoglycan.
def: bacteria
Bacteria: Wide range of environments (soil, water, other
organisms) Cell walls usually present and contain peptidoglycan
def: cyanobacteria
Cyanobacteria: Largest/most complex prokaryotes; bacteria in
which photosynthesis evolved
Escherichia coli (E. coli)
Rod-shaped (bacillus)
Rigid cell wall: polysaccharide and peptides;
maintains shape and provides some protection
against osmotic stress; porous
Plasma membrane: phospholipid bilayer and
associated proteins
DNA: single circular chromosome in nucleoid (not
membrane-enclosed)
Ribosomes: sites of protein synthesis, ~30,000 found
in the cytoplasm
def: nucleus
Nucleus: Largest organelle; Contains linear DNA molecules; Site
of DNA replication, RNA synthesis
def: mitochondria
Mitochondria: Sites of oxidative phosphorylation; generate ATP
def: chloroplasts
Chloroplasts: Sites of photosynthesis, only found in plants/some
algae
def: lysosomes
Digestion of macromolecules
def: peroxisomes
various oxidative reactions
def: vacuoles
-Plant cells; digestion of macromolecules, storage (waste products and
nutrients)
def: endoplasmic reticulum (ER)
Membrane network extending from the nuclear envelope; Protein
processing and transport; Lipid synthesis
def: golgi apparatus
Receives proteins from the ER; Protein processing and sorting; Lipid
synthesis (plants), cell wall polysaccharide synthesis
def: cytoskeleton
- Network of protein filaments
extending throughout the cytoplasm - Provides structural framework
- Determines cell shape and
organization - Involved in movement of whole cells,
organelles, and chromosomes during
cell division
Endosymbiotic Theory
Eukaryotic cells may have arisen from a fusion of genomes from Bacteria and the Archaea
EVIDENCE:
Mitochondria and Chloroplasts:
Similar to bacteria in size;
Reproduce by dividing in two;
Contain their own genetic systems (DNA ,RNA and ribosomes);
Double membranes
what are the 3 main tissue systems in plants?
- Ground tissue:
Parenchyma cells – site of
metabolic reactions, including photosynthesis.
Collenchyma and sclerenchyma – have thick cell
walls and provide structural support - Dermal tissue:
Covers the plant surface;
Protective coat; Allows absorption of nutrients - Vascular tissue:
Xylem and Phloem – elongated
cells that transport water and nutrients throughout
the plant
what are the 5 main tissue types in animals?
More Diverse than plants with 5 main tissue types:
1.Epithelial cells: form sheets that
cover the surface of the body and
line internal organs
- Connective tissues: Include
bone, cartilage, and adipose
tissue. Loose connective tissue is
formed by fibroblasts - Blood: Several cell types
* Red blood cells (erythrocytes) for oxygen transport
* White blood cells (granulocytes, monocytes, macrophages, and lymphocytes) for
inflammatory reactions/immune response - Nervous tissue: Composed of supporting cells and nerve cells (neurons), and
various types of sensory cells - Muscle cells: Production of force and movement
yeasts
- Simple Eukaryotes
- Model for fundamental studies of
eukaryote biology
S. cerevisiae (most commonly studied yeast):
12 million bp DNA (6,000 genes)
Distinct eukaryotic features:
- membrane-enclosed nucleus,
- genomic DNA arranged in 16 linear
chromosomes,
- subcellular organelles in cytoplasm
Fast doubling time
Mutants have expanded understanding of:
- DNA replication
- Transcription
- RNA processing
- Protein sorting
- Regulation of cell division
C. elegans (nematode)
- Relatively simple, multicellular eukaryote
- 100 million bp DNA (19,000 genes)
Drosophila melanogaster
The fruit fly D. melanogaster has been a crucial model organism
in developmental biology
180 million bp DNA (14,000 genes)
Easily maintained and bred in the laboratory
Short reproductive cycle (2 weeks) – genetic experiments
Embryos develop outside
Short lifespan (monitor development)
Advances in understanding the
molecular mechanisms in animal
development – body plan of complex
organisms
Parallels between genes in Drosophilia
and vertebrates
Wound healing, tissue regeneration,
drug discovery, genetic disease,
Parkinson’s disease research,
neurodevelopment
Arabidopsis thaliana
- Relatively simple organism: suitable
model organism for study of plant
molecular biology and development - 125 million bp DNA (26,000 genes,
unique 15,000) - Easily grown and genetically
manipulated in the laboratory - Used to elucidate mechanisms such
as flower development in higher
plants
Induced Pluripotent
Stem Cells (iPSCs)
■ Take adult skin biopsy
■ Grow cells in tissue culture
■ Treat with special cocktail of
chemicals to reprogram
■ Cells forget they were skin
cells and revert to basic stem
cells
■ Can be programmed into any
cell type
HeLa cells
■ First immortalised human cell
line.
■ The oldest and most commonly
used human cell line in scientific
research.
■ Derived from cervical cancer
cells taken on February 8, 1951,
from named Henrietta Lacks, a
31-year-old African-American
mother of five, who died of
cancer on October 4, 1951
■ HeLa cells have an active version of
telomerase during cell division, which copies telomeres over and over again
■ Prevents the incremental shortening of telomeres that is implicated in aging and eventual cell death
■ Results in unlimited cell division and immortality
how many elements are essential for life?
- About 26 of the 92 elements are
considered essential for life - Carbon, hydrogen, oxygen and
nitrogen make up 96% of living
matter - Most of the remaining 4% consists
of calcium, phosphorus, potassium
and sulfur - Implications for amino acids and
thus protein composition
hydrogen bonds
Weak intermolecular interactions between partial
charges on polar molecules
Collectively forms a strong bond; individually can
form and break easily as they are individually
relatively weak bonds
Often represented as dashed or dotted lines
e.g. H-bonds between certain amino acid side chains and between C=O and N-H of amino acids in alpha helices and beta sheets
van der waals’ forces
- Interactions between non-polar amino acid side
chains. - Allow even non-polar groups to form favourable
electrostatic interactions with one another. - Non-polar bonds can have a transient polarity and
induce a transient polarity on a neighboring non-polar
bond. - VdW forces are especially important in the
hydrophobic interior of a protein.
Hydrophobic effect
- Not a force per se, however considered the major
driving force for the folding of globular proteins. - Hydrophobic interactions cluster hydrophobic groups
away from water (think of oil and water not mixing). - Non-polar amino acids have a strong tendency to
cluster together, away from water, allowing very
favourable H-bonds to form between water and
hydrophilic amino acid residues
how are units of macromolecules grouped together?
Single units = Monomers
A few units joined together = Oligomers
A large number of monomers joined = Polymers
nucleic acids
- Principle information molecules of the cell
- Two main types: DNA and RNA
DNA: Deoxyribonucleic acid
- The genetic material of all life (except some viruses)
RNA: Ribonucleic acids
- Messenger RNA (mRNA) carries information from DNA to the ribosomes
- Ribosomal RNA (rRNA) and transfer RNA (tRNA) are involved in protein
synthesis
- Other RNAs involved in regulating gene expression and processing transport of
RNAs and proteins (miRNA, lncRNAs, etc)
- RNA can also catalyse some chemical reactions
The chemical structure of nucleic acids
Nucleotides:
■ 5 - carbon sugar
- DNA = deoxyribose
- RNA = ribose
■ Nitrogenous base
■ 1 or more phosphate groups linked to the 5’ carbon of the sugar
DNA and RNA are polymers of nucleotides
Nucleic Acids: Nitrogenous bases
Purines: Adenine and Guanine
Pyrimidines: Cytosine, Thymine (DNA only) and Uracil (RNA only)
Base pairing:
Occurs between pyrimidines and purines
DNA
A = T
G ≡ C
RNA
A = U
G ≡ C
Nucleic Acids: Phosphate group
- Attached to the 5’ carbon on the 5-C-sugar
- 1 or more phosphates attached
Example:
ATP (Adenosine triphosphate) the principal form of
chemical energy within cells.
Other Nucleotides:
■ Nucleotides are not exclusively
found in DNA or RNA
■ Can be involved in other
biochemical processes and
pathways within the cell.
■ e.g. ATP, AMP, dAMP, cAMP
Nucleic Acids : Polymerization & Directionality
- Polymerisation of nucleotides involves phosphodiester bonds between the 5’
phosphate of one nucleotide and the 3’ hydroxyl of another - Gives polynucleotide chains a sense direction: polynucleotides are always
synthesized in the 5’ to 3’ direction - Dehydration synthesis: production of a H20 molecule during the reaction
- Oligonucleotides are polymers of only few nucleotides
- RNA and DNA are polynucleotides and may contain thousands or millions of
nucleotides - RNA mainly single stranded
- DNA double stranded
3D structure of DNA
- Double helix
- Polynucleotides formed by phosphodiester
bonds - Strands run anti-parallel * H-bonds pair the bases on opposite strands,
which run antiparallel - Bases are on the inside * Inert sugar-phosphate backbone on outside * Structure stabilized by Hydrogen bonds
Nucleic Acids: Complementarity / Double Helix
- Information in DNA and RNA is always conveyed by the order of the bases
- DNA is made up of 2 polynucleotide chains that run anti-parallel to each other
- Complementary base pairing allows one strand of DNA (or RNA) to act as a
template for synthesis of a complementary strand - Nucleic acids are thus capable of self-replication
- The information carried by DNA and RNA directs synthesis of specific proteins,
which control most cellular activities - While DNA was first discovered and isolated by Friedrich Miescher in 1869, it
would take another 75 years before it was shown to hold the genetic blueprint
of life…
DNA is the genetic material and the
‘transforming principle’
Griffith’s experiment (1928):
There is a substance (transforming principle) in cell extracts that is
capable of transforming a non-pathogenic bacterial strain into a
pathogenic form
Avery, MacLeod and McCarthy Experiment (1944):
Digestion of cell extracts with proteases, carbohydrates and RNAse
did not destroy the transforming principle but treatment with DNAse
did -> DNA is the transforming principle
Hersey-Chase Experiment (1952):
Radio-labelled DNA (but not protein) from bacteriophages entered
bacterial cells upon infection -> DNA is the genetic material
The role of messenger
RNA
While DNA had been shown to determine the
order of amino acids in a protein sequence, it
did not infer/prove that DNA itself directs
protein synthesis
Experiment by Brenner, Jacob and Meselson
determined that RNA and not ribosomes are
the intermediate messenger molecule
between DNA and proteins
– identified messenger RNA (mRNA)
In addition to mRNA, two other types of RNA molecules are involved in
protein synthesis: transfer RNA (tRNA) and ribosomal RNA (rRNA)
the genetic code
64 different codons:
61 specify amino acids
3 are used as stop signals
Universal: used by all
organisms
and
Redundant: multiple
codons for most amino
acids
Expression of Genetic Information
The order of the bases (A, T, G and C) must specify the genetic
information but how exactly does this determine protein
structure?
1957 Sickle-cell anaemia studies:
First link between genetic variation and an alteration to the
amino acid sequence (Glutamic acid (charged)->Valine
(hydrophobic))
Patients with this inherited disease had altered haemoglobin
molecules resulting from a single amino acid substitution.
Indicates that possibly some variation in the DNA sequence
correlated to a change in the amino acid sequence (GAG-
>GTG)
i.e. order of DNA nucleotides determines the order of amino acids
■ With few exceptions (some viruses contain RNA instead of DNA), all
organisms utilize the same genetic code—strong support for the
conclusion that all present-day cells evolved from a common ancestor.
■ The mode of replication of viral RNA was determined by studies of RNA
bacteriophages of E. coli.
■ These viruses encode an enzyme that catalyzes synthesis of RNA from
an RNA template (RNA-directed RNA synthesis).
■ Most animal viruses replicate in this way, but one group (RNA tumor
viruses) requires DNA synthesis in infected cells.
■ These viruses (now called retroviruses) replicate via synthesis of a DNA
intermediate, a DNA provirus.
■ Reverse transcriptase can be used experimentally to
generate DNA copies of any RNA molecule.
■ This has allowed mRNAs of eukaryotic cells to be
studied using the molecular approaches currently
applied to the manipulation of DNA.
Reverse transcription
■ This hypothesis was initially met with disbelief
because it reverses the central dogma.
■ Later, an enzyme that catalyzes synthesis of DNA from
an RNA template (reverse transcription) was
discovered (reverse transcriptase enzyme).
■ Reverse transcription has other broad implications.
■ It also occurs in cells and is frequently responsible for
transposition of DNA from one chromosomal location
to another
Unicellular Genomes
- In bacteria, most of the DNA encodes proteins (little non-coding DNA), often arranged in operons (groups of genes with related function expressed together)
- The E.coli genome is twice the size of H. influenzae: contains 4.6 Mb
and 4,000 genes (90% of DNA is protein-coding) - Simplest eukaryotic genome is Saccharomyces cerevisiae (yeast):
contains 12 Mb and 6,000 genes (70% of DNA is protein-coding; only
4% of its genes have introns)
the human genome
Number of protein-coding genes: 21,000 (1.2 % of genome)
Very unexpected result (Estimated 100,000)
40% of human proteins are related to proteins in simpler
eukaryotes; most involved in basic cellular processes
Most unique proteins have similar domains to other species
but arranged in different combinations
Mice, rats, and humans have 90% of their genes in common
Humans and chimpanzees share ~ 98.8% DNA similarity
Human Accelerated regions (HARs)
■ HARs represent conserved genomic loci with elevated divergence in humans.
■ ~ 3,000 of human accelerated regions: enriched in genes related to DNA
interaction, transcriptional regulation and neuronal development.
■ If some HARs regulate human-specific social and behavioral traits, then
mutations would likely impact cognitive and social disorders.
■ NPAS3 (neuronal PAS domain-containing protein) gene contains 14 HARs.
■ NPAS3: Brain-enriched transcription factor
(Maria V. Suntsova & Anton A. Buzdin, BMC Genomics, 2020)
Introns
- Found in most, but not all eukaryotic genes (not found in histone genes)
- Rarely found in prokaryotic genes (90% of DNA is protein-coding)
- Found in both plants and animals
- Present in low numbers in yeast (70% of DNA is protein-coding)
- Intermediate number in C. elegans, Drosophila and Arabidopsis
Introns and Exons
Similar observations soon made on cloned eukaryotic genes
Sequencing of cloned DNAs and cDNAs (complementary DNAs) indicated that the coding region of the mouse β-globin gene is interrupted by two
introns that are removed from the mRNA by splicing
The amount of DNA in Introns can be greater than Exons
Average human gene has 56,000 base pairs (56 kilobases, kb);
contains about 10 exons with 4.3 kilobases-kb
Introns make up >90% of the average human gene
Additional 3’ or 5’ untranslated regions (UTRs) within the exons
Roles of Introns
Many introns encode functional (non-coding) RNAs and a very small
amount encode proteins (nested genes)
Others contain regulatory sequences that control transcription and
mRNA processing
- regulatory sequences very important for expression of
different genes in different cell types in a complex organism.
Allow exons of a gene to be joined in different combinations –
alternative splicing; this gives rise to different proteins from the
same gene
Alternative splicing is common in complex organisms
Human genes: Alternative splicing can yield between 2 and several
thousand different mRNAs per gene; Average 6 alternatively spliced
mRNAs; Average 4 different proteins per gene
Non-coding Sequences
ENCODE Project (2003 - present)
- Investigation of function in the
human genome - Non-coding regions don’t encode
protein; but they are not ‘junk DNA’ - Roles in: - gene regulation
- structure and replication of eukaryotic chromosomes
- evolution of eukaryotic genome
- Undiscovered roles? Hot bed of research – are there keys to disease
mechanisms and novel therapies in this DNA (eg. MiRavirsen for Hep C)
What is in non-coding sequences?
ENCODE project (2003-present)
Discovered that 75% of the human genome is transcribed
This cannot all be accounted for by protein-coding genes
Includes:
* Some non-coding RNA molecules produced (tRNAs, rRNAs, long noncoding RNAs and others)
* Pseudogenes
!!! Many of the genetic alterations responsible for inherited
diseases may be due to mutations in non-coding regions,
rather than in the protein-coding region itself !!!
Repetitive Sequences
Highly repeated DNA sequences account for a large proportion of
complex eukaryotic genomes (100s-1000s of copies per cell)
Include: Simple-sequence repeats and LINEs or SINEs
Repetitive Sequences: Simple-sequence Repeats
Up to thousands of copies (repeats) of short sequences (1-500
nucleotides) arranged in tandem along a section of the chromosome
Can be separated on a density gradient based on their GC:AT ratio
Repeat sequence DNAs band as “satellites”, separate from main DNA
band (satellite DNAs)
Simple-sequence repeats are not transcribed (40% of Drosophila
genome, 10% of human genome)
Role in chromosome structure
Repetitive Sequences: SINEs and LINES
Other repetitive DNA sequences are scattered (interspersed) throughout the genome as opposed to being arranged in tandem repeats on a chromosome
Their transpositions into random sites, introducing mutations; expected to be harmful to the cell (eg. Cystic fibrosis, muscular dystrophy, inherited cancers)
Short-interspersed elements (SINEs): 100-300 bp; 13% of genome; transcribed; unknown function
Long-interspersed elements (LINEs): 4-6 kb; 21% of the genome; transcribed; some encode proteins (e.g. reverse transcriptase) but unknown function
Both SINEs and LINEs are examples of transposable elements
(capable of moving to different sites in genomic DNA)
SINEs and LINEs are retrotransposons as their movement to other parts of the genome involves reverse transcription (RNA ® DNA)
Repetitive Sequences:
Movement of Retrotransposons
Retrotransposons (e.g. LINEs and SINEs) account for at least 34% of
human genome
Function/Effect on the Genome:
Depends on their new integration location:
- May disrupt the expression of a gene and lead to some inheritable
disorders (cystic fibrosis, hemophilia etc.) or cause some cancers
- Contributes to genetic diversity as can regulate the expression of
nearby genes following transposition
Other Repetitive Sequences
Retrovirus-like elements also move within the genome by reverse transcription; 2-10kb; 8% of human genome
DNA transposons move through the
genome by being copied and reinserted as
DNA sequences; no reverse transcription;
80-3,000 bp; 3% of human DNA
Gene Duplications: Gene Families and Pseudogenes
Genomes can contain multiple copies of many genes (some are
non-functional). Why?
May be required to produce large amounts of certain proteins or
RNAs (e.g. histones, rRNAs)
Members of a group of related genes (a gene family) with altered
functionality and may be transcribed in different tissues or at
different stages of development
Other members will have undergone mutations that result in loss
of function; non-functional gene copies called pseudogenes
-> evolutionary relics; increase genome size (~11,000 in humans)
Example: Hemoglobin – α and β subunits are both encoded by gene families in the genome, with different members being expressed in embryonic, fetal
and adult tissues
Gene duplication can arise by:
1. Duplication of a segment of DNA resulting in the transfer of a
block of DNA to a new location in the genome, or
2. Duplication by reverse transcription of an mRNA, followed by integration of the cDNA copy into a new site on a chromosome
(retrotransposition) – similar mechanism to that used in
transfer of repetitive elements; this can lead to an inactive gene
copy (processed pseudogene) with no introns or chromosomal
sequences that direct transcription
Eukaryotic genomes
■ Autosomes: Any chromosome that is not a sex chromosome (X or Y chromosome). Humans have 22 pairs of autosomes.
■ Most eukaryotic cells (somatic cells) are diploidand have two copies of each autosomal chromosome and two sex chromosomes.
■ Sex cells (gametes) are haploid (one copy of each chromosome and one sex chromosome.
Diploid vs Haploid cells
Chromosomes were proposed as carriers of genes
Most cells of higher plants and animals are diploid (2n)
Meiosis gives rise to haploid cells (n)
Fertilisation gives a diploid organism (2n)
Experiments carried out with Drosophila melanogaster demonstrated that while some
traits are independent of each, other traits are inherited together (genetic linkage).
def: karyotype
A photograph of the complete diploid set of chromosomes groups in homologouspairs and arranged in order of decreasing size
Chromosome painting
Different staining techniques can be
used to distinguish chromosomes and have been used to map chromosome regions and genes.
G-banding: By staining with Giemsa
heterochromatin (AT-rich) stains darkly forming G-bands, while less densely packed euchromatin (GCrich) stains more lightly forming R bands.
Q-banding: Quinacrine banding.
C-banding: constitutive heterochromatin.
Chromosomes (Mitosis)
- As cells enter mitosis, chromosomes become highly condensed for distribution to the daughter cells
- Loops within interphase chromatin fold upon themselves and condense
10,000 fold to highly compact
metaphase chromosomes - No longer transcriptionally active
- Exact folding has not been defined but it appears to be highly reproducible
Centromeres (Mitosis)
Centromere:
Specialised region of the chromosome that plays an integral role in ensuring the correct distribution of duplicated chromosomes to daughter cells during mitosis
Interphase: DNA is replicated – 2 copies of each chromosome
(humans: 46 chromosomes become 46 pairs of sister
chromatids)
Centromeres
Centromere is a constricted chromosomal region:
- Site where sister chromatids are joined
- Attachment site for microtubules of the mitotic spindle fibers
Kinetochore:
Specialized structure formed by proteins at the centromere; Proteins form attachment between chromatid and microtubule, and also act as
motors to move chromatid along spindle fibers to poles of cell
Aside from yeast, specific eukaryotic DNA sequences that mediate centromere function have not been identified.
Centromeres do have a unique chromatin structure
In centromeres:
- Histone H3 is replaced by a H3-like variant (CENPA/Centromere Protein A)
- CENP-A is found in all studied eukaryotic centromeres
- CENP-A containing nucleosomes are required to assemble other kinetochore proteins
Assay of a centromere in yeast
(Saccharomyces cerevisiae)
DNA sequences of centromere
regions were first defined in yeast
Followed the segregation of plasmids
during mitosis
* Functional centromeres: equal
plasmid distribution among
daughter cells
* Non-functional centromeres:
many daughter cells fail to inherit
plasmid DNA
Identified sequences important for
centromere function
Epigenetic inheritance of CENP-A
Unique chromatin structure allows
centromeres to be stably maintained at cell division
Example of epigenetic inheritance
Epigenetic inheritance is the transfer of information to progeny that is not based on DNA sequences. The information is carried on the histones
When chromosomal DNA replicates, the parental nucleosomes are distributed to the 2
progeny strands
These CENP-A-containing nucleosomes direct
the assembly of new CENP-A-containing nucleosomes into chromatin
Telomeres
Telomeres are the sequences at the ends of eukaryotic chromosomes
Critical roles in chromosome replication and maintenance
Identified after it was noted that broken chromosomes were
highly unstable in eukaryotic cells – must be some specific sequences needed at the ends (termini) of chromosomes to confer stability
Tetrahymena (protozoan)
-Telomeres from Tetrahymena were added to the ends of linearised yeast plasmid DNA
-Plasmids gained the ability to replicate in a linear form
-Conclusion: Telomeric DNA sequences are required
for the replication of linear DNA molecules
-DNA sequences of telomeres from
various eukaryotes are similar
-Repeats of simple-sequence DNA;
clusters of G residues on one strand
-Repeated 100s-1000s of times
Telomere maintenance may be
important in determining the
lifespan and reproductive capacity
of the cell
Telomerase
The ends of linear chromosomes cannot be replicated by DNA
polymerase
DNA polymerase extends a growing DNA chain (Lecture 5) but it cannot start replicating from the end (terminus) of a linear DNA
molecule
Telomerase plays a critical role in replicating the ends of linear DNA
molecules
Telomerase: Reverse transcriptase enzyme that replicates telomeric
DNA sequences
Because of their importance in reproductive capacity and cellular
lifespan, telomeres and telomerase are the subject of many aging
and cancer studies
Chromatin
Chromatin: complexes between eukaryotic DNA and proteins
Experiments using nuclease (an enzyme that breaks down
DNA (DNAse)) to digest chromatin yielded nucleosome core
particles (beads on electron microscopy):
Nucleosome:
- 147 base pairs; 1.67 turns around a histone core
- Histone core: 2 molecules of H2A, H2B, H3 and H4 (core histones)
-Basic structural unit of chromatin
-DNA wrapped around a histone core
-Described by Roger Kornberg (1974)
Chromatosome:
- 166 base pairs; 2 full turns around the histone core
- held in place by one molecule of histone H1 (linker histone)
- Chromatin condensation has an important role in regulating gene
expression - Two forms of chromatin: Euchromatin and Heterochromatin
- Euchromatin: chromatin is less condensed and distributed throughout the nucleus; most interphase chromatin exists in this form (cells are not dividing); transcription is active; DNA can be replicated ahead of cell division
- Heterochromatin: chromatin is in a highly condensed state; 10 % of
interphase chromatin is heterochromatin; transcriptionally inactive; can contain highly repeated DNA sequences or genes that are not expressed in those specific cells
Histones
Major proteins of chromatin
- Small, basic proteins (high arginine/lysine content)
- Facilitates binding to negatively charged DNA
- Five major types (H1, H2A, H2B, H3, H4)
- Highly conserved among eukaryotes
Fluorescence in situ hybridization
(FISH):
a kind of cytogenetic technique which uses fluorescent probes to bind parts of the chromosome that show a high degree of sequence complementarity. Fluorescence microscopy can be used to find out where the fluorescent probe bound to the chromosome
Non-coding DNA and gene regulation
■ Looping of DNA to bring long-range
regulatory elements close together.
■ Local (cis) gene regulation
■ Cross-chromosome (trans) gene
regulation
