HS120: DNA & Cell Cycle Flashcards
What is DNA?
- aka deoxyribonucleic acid
- it is the genetic material of the cell
- structure: double-stranded polynucleotide chain wound into a double helix
What is the structure of DNA?
- each nucleotide is made of: a nitrogeneous base, a deoxyribose sugar, & a phosphate molecule
- nucleotides are linked via phosphodiester bonds between the sugar molecules
- 2 types of bases: purines & pyrimidines
How is the double helix established?
via the formation of hydrogen bonds between the complementary bases of each strand of DNA molecule.
What are the types of bases?
- purines: adenine & guanine
- pyramidines: cytosine & thymine
Where are the hydrogen bonds in the double helix formed?
- Adenine & thymine (AT)
- guanine & cytosine (GC)
What does the phosphodiester bond create?
Creates the sugar-phosphate backbone of a polynucleotide chain
Define DNA replication
the process by which the genome’s DNA is copied in cells
Define gene
- a union of genomic sequences encoding a coherent set of potentially overlapping functional products
- it is a specific segment of the DNA molecule that codes for the synthesis of a particular protein/ set of closely-related proteins
What is the progress of gene expression?
- DNA undergoes transcription to become mRNA
- mRNA undergoes translation to become protein
What are some numbers regarding DNA?
- human cell DNA is ~2m long, 3.2 billion base pairs
- length of DNA molecule is 100 000 times longer than the nuclear diametre
- human genome encompasses the entire length of human DNA, containing genetic information that is packaged in to 46 chromosomes
How is DNA organized?
- Beads on a string
- Solenoid
- Loop domains of chromatin
- Condensed heterochromatin
- Chromosome at metaphase
What is beads on a string?
these are 10nm- diametre particles that represent the first level of chromatin folding & are formed by the coiling of DNA molecules around a protein core called a nucleosome.
What is a nucleosome?
a protein that produces the initial organization of free double-stranded DNA into chromatin.
What is the structure of nucleosomes?
- octameric core complex made up of 4 types of histones
- about 150 bp (base pairs) of DNA are wrapped around each of these core complexes.
What are the types of histones?
- H2A, H2B, H3, H4
- there are 2 copies for each of these histones in a given nucleosome.\
- outside of the nucleosome, there is a large histone (H1) that is associated with both the wrapped DNA & surface of the core.
What is the microsopy of these nucleosomes?
- under EM, a series of nucleosomes resembles “beads on a string”
- 50-80bp of linker DNA separating each bead
What is the structural nature of nucleosomes?
- they are structurally dynamic
- modification & rearrangement of these histones allows temporary unwrapping of the DNA
- usefule for: arrival of enzymes & other proteins required for replication & gene transcription
What is solenoid organization of DNA?
how often is the rate of folding?
- 10-11nm fibre of nucleosomes & DNA which undergoes helical folding to produce 30nm chromatin fibril.
- DNA helical folding occurs every 6 nucleosomes. (for ever 6 nucleosomes, there is a turn)
What is loop domains of chromatin organization?
- 300nm
- chromatin that is anchored into a chromosome scaffold OR nuclear matrix composed of NONhistone proteins.
What is condensed heterochromatin?
700nm in length
What is the length of chromosome at metaphase?
1400nm
What is chromatin?
when can it be seen?
- chromatin is a thread-like structure formed of DNA & histones
- represents the relaxed, uncoiled chromosomes of the interphase nucleus
What are the types of chromatin?
- euchromatin
- heterochromatin
both found in most nuclei of nondividing cells
What is euchromatin?
aka extended chromtin:
- active form of chromatin
- rich with genes
- occupies the more central regions of the nucleus
- euchromatin predominateds in active cells [ex. large neurons]
What does euchromatin look like in LM?
with H/E staining, it is:
lightly-stained basophilic areas
Euchromatin under EM?
under EM, it is:
finely-dispersed granular material; electron lucent
What is heterochromatin?
- condensed inactive form of chromatin
- little to no transcriptional activity
- more abundant in cells with little synthetic activity [ex. circulating small lymphocytes]
LM of heterochromatin:
intensely basophilic clumps
EM of heterochromain:
coarse, electron-dense material.
Where can chromatin be found?
distribution of chromatin (3 regions)
- marginal/ peripheral chromatin: found at the periphery of the nucleus
- nucleolus-associated chromatin: found in assocation with the nucleolus
- chromatin island (karyosomes): discrete bodies of chromatin irregular in size & shape. Found throughout the nucleus.
What are the 2 types of heterochromatin?
- constitutive heterochromatin
- facultative heterochromatin
What is constitutive heterochromatin?
- generally similar in all cell types & contains mainly repetitive, gene-poor DNA sequences
- centromeres & telomeres are made of constitutive heterochomatin.
What is facultative heterochromatin?
- contains regions of DNA with genes where transcription is inactive
- these regions of inactivity are different from cell to cell.
- can undergo reversible transitions from: [compact, transcriptionally silent states] to: [open, transcriptionally active conformations].
What is a Barr Body?
- the inactive X chromosomes present in human females.
What does the Barr body represent?
it is a region of facultative heterochromatin that can be used to identify the sex of a fetus.
How does the Barr body behave?
- it remains tightly coiled
- OTHER X chromosome is: uncoiled, transcriptionally active & NOT visible. (i.e. euchromatin)
How does the X chromosome in males behave?
- euchromatic; both the X chromosome & Y chromosome in males remain euchromatic.
Where can the sex chromosome be found?
examples & in which cells
- at the edge of the nuclear envelope (peripheral chromatin) in smears of oral epithelial cells
- small drumstick-like evaginations of the nuclei of the neutrophils
What is an indicator of a cell’s metabolic & biosynthetic activity?
The ratio of heterochromatin to euchromatin, seen with nuclear staining.
Which cells does euchromatin predominate in?
large neurons
Where does heterochromatin predominate in?
small lymphocytes
Define chromosomes
chromatin fibres that become so condensed & tightly coiled during mitosis & meiosis that they are very visible with L/M.
What is the structure of chromosomes?
- sister chromatids held together at their centromere by complexes of cohesin proteins
- centromeres: located near middle of chromosomes
- telomeres: located at ends of chromosomes
What are sister chromatids?
A duplicated chromosome consists of 2 genetically identical chromatin units called sister chromatids.
How many DNA strands are there in a chromosome?
- before replication: each chromosome would have 1 (double-stranded) DNA molecule
- after replication: eache chromosome have 2 (double-stranded) DNA molecules.
each DNA molecule makes up 1 chromatid, and after doubling, it is still considered 1 chromosome, except now it has sister chromatids.
What is a karyotype?
a picture of all the chromosomes found in a cell
- describes the chromosomes count & shows their characteristics such as length of chromosome, position of centromere, banding pattern, etc.
How many chromosomes are there in a somatic cell?
- 46 chromosomes, arranged in 23 pairs
- in each pair, there is a maternal chromosome & a paternal chromosome
- there are 22 homologous pairs: autosomes
- 1 pair of sex chromosomes: XX for females & XY for males
- In females, chromosome number is: 44 + XX
- in males, chromosome number is: 44 + XY
What is the nature of somatic cells?
- diploid (2n), 23 pairs of chromosomes & 2 copies of each one.
What is the nature of gametes?
aka mature ova or spermatozoa:
- haploid (1n), with one copy of each chromosome
- sperm: (22+X) OR (22+Y)
- mature ova: (22+X)
Define cell proliferation
Process by which cells increase in number due to cellular growth & cell division.
Define cell differentiation
Process by which unspecialized cells undergo structural & functional changes to become specialized cells with distinct characteristics & functions
Define cell renewal
when cells divide & multiply to form new cells, tissues, & organs most notably during embryonic development.
How does cell renewal look like in adults?
not all cells in adults retain the ability to divide & reproduce. (not as common as in embryos)
Classification of somatic cells in adults based on their mitotic activity:
- static cell populations (permanent)
- stable cell populations (quiesecent)
- renewing cell populations (labile)
What are static cell populations?
cell pop. consists of cells that no longer divide (postmitotic cells).
examples:
1. CNS cells
2. skeletal & cardiac ms. cells
What are stable cell populations?
aka quiescent:
consists of cells that divide episodically & slowly to maintain normal tissue or organ structure.
- these cells may be stimulated by injury to become more mitotically active.
examples:
- liver cells
- periosteal & perichondrial cells
- smooth ms cells
- endothelial cells of blood vessels
What are renewing cell populations?
- regular mitotic activity
- under normal physiologic conditions, cells continue to multiply throughout life.
What are types of renewing cell pop.?
- slowly renewing
- rapidly renewing
What are examples of slowly-renewing populations?
- smooth muscle cells of hollow organs
- fibroblasts of the uterine wall
- epithelial cells of the lens of the eye
What are examples of rapidly-renewing cell pop.?
- blood cells
- epithelial cells + dermal fibroblasts of the skin
- epithelial cells + subepithelial fibroblasts of the mucosal lining of the GIT.
What are stem cells?
a small population of undifferentiated cells whose cycling serves to renew the differentiated cells of tissues as needed.
How do stem cells divide?
infrequently & asymetrically:
- one daughter cell remains a stem cell
- the other (progenitor cell/ transit-amplifying cell) commits to a path of differentiation.
What is the cell cycle?
a series of events within the cell that prepare the cell for dividing into 2 daughter cells
What are the 2 major events in a cell cycle?
- M phase
- Interphase
What is interphase?
a longer period of time during which the cell increases its size & content, also replicating its genetic material.
What is the M phase?
aka mitosis,
a short period of time during which the cell divides its nucleus & cytoplasm, giving rise to 2 daughter cells.
Which phase is the quiescent phase?
the G0 phase is the stable phase or resting state of the cell.
What is interphase and what are its 3 phases?
it is the time between 2 successive mitotic events.
1. G1 phase (gap 1)
2. S phase (synthesis)
3. G2 phase (gap 2)
outside of the interphase is the G0 phase, just after mitosis and before G1.
What is G1 phase?
- the longest and most variable part of the cycle
- period of cell growth
- nucleoli are reestablished
- synthesis of RNA, regulatory proteins that are essential to DNA replication, & enzymes necessary to carry out synthetic activities in prep for the next mitosis.
- duplication of cell organelles
- cell volume is restored to previous size.
What is the S phase?
- DNA replication
- beginning of centrosome distribution
- nucleoproteins, like histones, are imported & incorporated into DNA molecule, forming chromatin material.
What is G2 phase?
- short period between the end of DNA replication and beginning of mitosis
- RNA & proteins essential to cell division are synthesized (just like in G1)
- energy is stored for mitosis
- tubulin is synthesized for assembly into microtubules required for mitosis
- DNA replication is analyzed for possible errors, & any of these errors are corrected.
What can happen to cells after becoming highly differentiated?
- they may cease to undergo mitosis permanently (neurons & skeletal ms cells)
- or temporarily cease to undergo mitosis and then return to the cell cycle much later (eg. hepatocytes/ liver cells)
regulation of the cell cycle
What is the cell cycle controlled by?
- Signals from outside the cell (signal transduction
- Check points
What is regulation of cell cycle using signal transduction?
- stimulatory signals: mitogens or growth factors bind to cell surface receptors & trigger a cascade of kinase signalling in the cells
- inhibitory signals: ex. cytokines
What is cell cycle regulation via check points?
- cyclin & cyclin-dependent kinases.
- genes: P53 & RB1 (retino blastoma)
What do internal quality control mechanisms do?
they control transition between cell cycle stages.
What are the 3 checkpoints?
- restriction checkopoint: checkpoint of no return
- G2/M checkpoint
- metaphase spindle checkpoint
What is the restriction checkpoint?
G1/S checkpoint prerequisites:
- cell size
- nutrients
- molecular signals
- DNA integrity
Once these aspect of the cell are found to be in perfect condition for mitosis, synthesis phase begins.
What is the G2/M checkpoint?
checks for:
1. DNA integrity: is any of the DNA damaged?
2. DNA replication: was the DNA completely copied during S phase?
What is the M/spindle checkpoint?
- it is the checkpoint that ensures all chromosomes will be segregated.
- checks whether all chromosomes are properly attached to spindle or not.
- also governs the alignment of the chromosomes & integrity of the spindles.
“makes sure the rollercoaster is ready to go” everyone with seatbelts on, no holding hands, seats aren’t lose, etc.
What are the cell cycle regulators?
- proto-oncogenes
- tumor suppressor genes: RB1 & P53
- cyclins & CDKs
What are proto-oncogenes?
- genes that control the proliferative pathways of the cell.
WARNING
structural mutuations or changes in expression of proto-oncogenes can convert them into oncogenes which cause for uncontrolled cell growth, potentially becoming cancer.
Mechanism of RB1 gene/ RB protein:
- binds to E2F (phosphorylates RB)
- E2F is released
- E2F migrates to nucleus to induce transcription
- production of protein factors for cell cycle progression
- cell cycle progresses!
Mechanism of P53 gene:
- P53 will etiher cuase for apoptosis or cell cycle arrest.
- apoptosis: death & elimination of damaged cells
- cell cycle arrest: halting of cell cycle, DNA repair, & then cell cycle restarts
works to restore cellular & genetic stability after DNA damage/ hypoxia/ cell cycle abnormalities occur.
What are cyclins & CDKs?
cyclins are a famiy of cytoplasmic proteins that activate CDKs (cyclin-dependent kinase) to essentially modify protein that is involved in regulating the cell cycle.
- cyclin is binded to CDKs to modify these proteins.
- cyclin is destroyed once protein modification is complete.
How can CDKs be inhibited? When does this happen?
When a problem at the checkpoint cannot be quickly corrected while cell cycle is halted:
- proteins coded for by the tumor suppresor genes (esp. P53) redirect the cell toward apoptosis/ cell death.
- Progression is halted when adverse conditions (G1/S restriction check point) are present.
