Cell and Molecular Flashcards
What is the cell cycle clock?
Network of interacting proteins in the nucleus that recieve signals from outside and inside the cell, integrates them and decides the cell’s fate (i.e. proliferate or quiescence?)
What are the 5 stages of the cell cycle?
G1 phase S phase G2 phase Mitosis (prophase, prometaphase, metaphase, anaphase, telophase) Cytokinesis
What are the two major control factors of the cell cycle?
- Cyclins drive cycle forwards
2. Checkpoints stop the cycle
What are the CDK and cyclin pairings throughout the cell cycle?
G1: CDK4/6 --> D cyclins G1 after R point: CDK2 --> E cyclins S: CDK2 --> A cyclins (although later CDK2 replaced by CDC2) G2: CDC2 --> B cyclins M: CDC --> B cyclins G0 to G1: CDK3 --> C cyclin
Why are D-type cyclins an exception?
They are controlled by extracellular signals such as growth factors and integrin-mediated ECM attachment unlike other cyclins which have intracellular signals that coordinate with the cell cycle, activating complexes of the subsequent phase and inhibiting those active in previous phases.
What is the role of CKIs?
They inhibit and regulate CDKs
Where are the three checkpoints and what does each one do?
G1 checkpoint: checks cell size, environment and DNA damage
G2 checkpoint: checks DNA replicated, cell size and environment
Metaphase checkpoint: checks all chromosomes are aligned on the spindle
What are the processes in the G1/S restriction point progression?
The CDK4/D causes phosphorylation of pRb. CDK2/E causes further phosphorylation. pRb releases E2F TF which causes gene transcription.
What are the three places where DNA damage is detected and acted upon to stop the cell cycle?
G1
Entry into S-phase
Entry into mitosis
What processes occur when DNA damage is detected and what happens after this is G1?
ATM/ATR get activated and associate with the site of DNA damage. ATM/ATR will activate other kinases to block the cell cycle. p53 is stabilised and turns on p21 (a CKI).
p21 renders G1/S-CDKs and S-CDKs inactive and repairs DNA. If this is not possible cell will undergo apoptosis.
What extracellular signals determine if a cell will move past the restriction point?
If the serum and growth factors are removed in the final hour of G1 then the cell will proceed. If they are removed before they will revert back to G0.
What is the process of DNA replication in S phase to ensure it is only replicated once and is done so accurately?
In G1 phase, inactive helicases are loaded onto replication origins forming a PreRC. This is called licensing and happens in G1 phase therefore it can only occur once. The helicases are activated in S phase and DNA is unwound and replicated. M phase triggers chromosome segregation.
What are homologous chromosomes and how do they relate to chromatids?
Chromosomes that have the ‘same’ genes arranged in the same order - one inherited from the father, 1 from the mother. Chromatids are the newly copied DNA strands that are still joined to each other via a centromere.
Why is yeast used a genetic model for the cell cycle?
It divides rapidly <1hr
Its control genes are highly conserved
It can be grown as haploids or diploids therefore mutations can be maintained from haploid to diploid as only one parent cell
Why is Xenopus laevis used as a biochemical model for the cell cycle?
Easy to collect eggs
Rapid division rate (~30 mins)
Large size makes purification of proteins easier (lots of cytoplasm)
Can be manipulated by injection of RNAs or chemicals into the oocyte
What genetic tricks can you use to identify mutations?
Diploids can be used to maintain mutations that are studied as haploids i.e. if the haploid cell dies, the diploid will still have the mutation and can be observed
Temperature sensitive mutations allow growth at permissive temperatures but mutation will show at restrictive temperatures
What does M-Cdk cause?
Assembly of mitotic spindle
Each sister chromatid is attached to an opposite pole
Chromosome condensation
Breakdown of the nuclear envelope
Rearrangement of the actin cytoskeleton and Golgi
How does M-cyclin trigger entry into mitosis?
M-cyclin levels increase through G2 and M (by increase of Cyclin B expression) to create a pool of inactive M-Cdk complex. This is activated by CAK and Cdc25. The active M-Cdk acts in a positive feedback loop.
What is APC and what are its targets?
Anaphase-Promoting Complex is a ubiquitin ligase.
It targets S+M cyclins to make sure the previous complexes are inactive
It targets securin (protects protein linkages holding sister chromatids together) by activating a protease which separates the sister chromatids (anaphase)
What is loss-of-heterozygosity and hemizygosity and why could this cause problems?
Loss-of-heterozygosity is where both chromosomes will have the same allele e.g. both mutant.
Hemizygosity is where there is a loss of one allele.
This can be a problem as if one allele has a mutation there is no healthy allele to replace its function.
2-hit hypothesis suggests most genes need two mutations to cause a phenotypes.
What is chromosome non-disjunction and when would it occur?
Chromosomes ending up in the wrong daughter cell and occurs in anaphase by a lagging chromosome. Results in too little or too many chromosomes.
What is the structure of the mitotic spindle?
Interpolar microtubules (spindles that overlap) Kinetochore microtubules (attach to chromosomes at kinetchores/centromeres) Astral microtubules (contact cell cortex to position the spindle by connecting to plasma membrane) Centrosome (centriole surrounded by pericentriolar matrix, this acts to nucleate microtubules)
How are inappropriate attachments sensed?
By trial and error. Tension is generated when there is a correct attachment. If the attachment is incorrect there will be lower tension, an inhibitory signal which loosens the microtubule attachment site and it attemps to attach again.
What can cause loss-of-heterozygosity?
Nondisjunction
Mitotic recombination
Gene conversion
What are the processes of meiosis in the ovary and testes?
Repeated mitotic division of diploid cells with growth an differentiation to produce oogonium (ovaries) and spermatogonium (testes).
Meisosis I the primary oocyte/spermatocytes (4n) divide into secondary oocytes/spermatocytes (n).
Meisosis II they divide into (one) mature egg or (four) spermatids (n).
What happens during meiosis I?
Pairing up of duplicated maternal and paternal homologs (prophase I)
Homolog pairs line up on the spindle. (metaphase I)
Segregation of homologs at anaphase I as one complete chromosome (2 chromatids) pulled to separate poles (telophase I)
What happens during meiosis II?
The sister chromatids line up and sperate in Anaphase II.
When do homologues pair up and why is this important?
In meiotic prophase I.
Alligns the chromosomes up ready for anaphase (along with the formation of the synaptonemal complex)
It allows for genetic recombination between paternal and maternal DNA on the same chromosome
What is the synaptonemal complex?
Proteins which facilitate pairing by bringing them 400nm apart.
The axial core (proteins that bind chromatin via cohesin) are cross-linked by transverse filaments.
What is the main difference between meiosis and mitosis and what allows for this difference?
Mitosis = sister chromatids separate
Meiosis = homologs separate
–> In meiosis, both kinetochores attach to the same spindle pole. This is done by protein complex removed after meiosis I by crossing over and during anaphase I the cohesin is removed from the arms but still connects the kinetochores
How is crossing over regulated?
At least 1 crossing over per bivalent but no more than 4
Crossover interference where once one forms it inhibits others close by
What are two chromosome abnormalities?
Abnormalities in chromosome number (aneuploidy, can be monosomy, trisomy or polyploidy)
Chromosome structural rearrangements
How does nondisjunction in meiosis I compare to in meiosis II?
Meiosis I there will be a pair of homologous chromosomes in one daughter cell and a lack of that chromosome in the other. All gametes will be abnormal.
In meiosis II there will be a pair of sister chromatids in one daughter cell and a lack of this chromatid in the other. Half of all gametes will be abnormal.
What are the names for types of numerical chromosomal abnormalities?
Polyploidy –> triploid
Aneuploidy (autosomes) –> nullsomy (missing a pair of chromosomes), monosomy (missing one chromosome), trisomy (one extra chromosome)
Aneuploidy (sex chromosomes) –> additional sex chromosome or lacking a sex chromosome
What are syndromes arising from nondisjunction?
Trisomy 22 –> usually die before birth
Trisomy 18 (Edwards syndrome) –> usually die before birth
45 X Turner’s syndrome (monosomy) –> 1% survive
Why are syndromes arising from nondisjunction lethal?
Haploinsufficiency i.e. the gene is usually expressed from both alleles with dose mattering. If inactivation of one allele then dose not enough and lose expression.
Imprinted genes i.e. monoallelic expression becomes lost
What is the karyotype?
The organised representation of all the chromosomes in a eukaryotic cell at metaphase
What is the structure of a chromosome and how does it differ during interphase?
A chromosome is a highly coiled fibre of chromatin. In interphase chromatin is uncoiled and nucleosomes are exposed.
What is the structure of the nucleosome?
DNA is wound round core histones. The histone is made up of 2 copies of 4 subunits and N-terminal tails from the 8 subunits project out of the nucleosome core and are free to interact with other proteins, facilitating regulation of chromatin structure and function.
What are the role of linker histones such as H1 and how do they function?
It stabilises the 30nm formation and facilitates the establishment of transcriptionally silent heterochromatin.
They strap DNA onto histone octamers and limit movement of DNA relative to the histone.
What are fractal globules comprised in interphase chromatin?
Dynamic globules within globules that can reversibly condense and decondense without becoming knotted.
Is transcriptional activation of a gene accompanied by movement from the periphery towards the centre or from the centre to the periphery of the nucleus?
From the periphery towards the centre of the nucleus
What are the different regions of the chromosome, where are they and what is their function?
Telomere - DNA sequences at the ends of linear chromosomes, maintain chromosomal integrity
Replication origin - any sites where replication can be initiated
Centromere - interior location but is not necessarily the midpoint, indirectly attaches to mitotic spindle and mediates chromosome segregation at mitosis and meiosis
Kinetochore - protein complex that binds centromeric DNA sequences and microtubules of mitotic/meiotic spindle
What is the structure of the kinetochore and chromatin formation during cell division?
Centromeres contain alpha-satellite DNA repeats that readily form condensed chromatin with histone octamers containing H3 variant.
The kinetochore inner plate proteins bind to the chromatin containing alpha-satellite DNA and the outer plate proteins bind to microtubules
What percentage of the DNA sequence of eukaryotic genomes encodes information for making cellular proteins?
1.5%
What is increasing biological complexity associated with in terms of DNA?
Increasing number of protein-coding genes
Increasing amounts of non-protein-coding DNA for regulating transcription and organising access to protein-coding genes (e.g. cis-regulatory which determines where and when in the body adjacent protein-coding genes are transcribed)
What are transposons and what are the 3 different types?
Mobile genetic elements that jump around the genome - also called ‘transposable elements’. These take up almost half of the genome.
DNA transposons
Retroviral transposons
Non-retroviral polyA retrotransposons
How do DNA transposons integrate into the genome?
By a cut-and-paste mechanism without self-duplication. Transposase monomers (encoded in the transposon) cleave both ends of the DNA transposon element. The transposome becomes a central intermediate and is integrated into another chromosome.
How do retroviral transposons integrate into the genome?
Entry of the virus into the cell and RNA is released. Self-encoded reverse transcriptase makes DNA/RNA and then DNA/DNA double helix. The DNA is then copied and integrated into the genome, transcripted and translated to make new virus particles.
How do non-retroviral PolyA retrotransposons integrate into the genome?
By a copy-and-paste mechanism. RNA is synthesised from the DNA. Reverse transcriptase/endonuclease are synthesised and bind to the RNA. Cleavage of the first strand as DNA-primed reverse transcription occurs. The multistep DNA synthesis process produces the second DNA strand.
How is DNA always replicated?
In a 5’ –> 3’ direction by formation of phosphodiester bonds.
The leading strand is continuous whereas the lagging strand is discontinuous
What are key enzymes involved in converting Okazaki fragments into a continuous strand of DNA?
DNA primase - makes RNA primer DNA polymerase - extended RNA primer but requires the primer-template junction Ribonuclease H - removes RNA primer DNA polymerase - extended across gap DNA ligase - seals the nick
How is the processivity (= ability to catalyse consecutive reactions without releasing the substrate) of DNA polymerases enhanced?
By their association with a sliding clamp.
It is positioned close to the primer:template junction by a clamp loader and helps move the DNA Polymerase forward.
What is the role of SSBs?
Single-stranded binding proteins expose single-stranded DNA in the replication fork, making it available for templating synthesis of the new DNA strand, easing replication fork progression and enhancing progressivity of DNA polymerase.
How do DNA topoisomerases prevent DNA from becoming tangled during replication and enhance processivity of DNA polymerase?
When helicase unwinds parental DNA strands at the replication fork, superhelical tension is introduced into the rest of the DNA helix which would cause tangling. DNA topoisomerases nick and reseal the backbone of the parental helix.
What are the two types of topoisomerases and how do they differ?
Type I - nicks and reseals one of the 2 DNA strands, does not require ATP
Type II - nick and reseal both DNA strands, ATP required
What is the origin of replication?
Specific DNA sequences where DNA replication starts by recruitment replication of initiator proteins
When do the two stages of biphasic initiation of DNA replciation in eukaryotes occur?
Replicator selection occurs in G1 phase
Origin activation occurs in S phase
How is the Pre-Replicative Complex formed?
The origin recognition complex binds to the replicator sequence. Helicase-loading proteins Cdc6 and Cdt1 bind to the ORC. The Helicase Mcm2-7 binds to complete formation of pre-RC.
How does increased levels of Cdk in S-phase affect the pre-RC?
When Cdk activity is high, existing pre-RC is activated and formation of new pre-RC is inhibited
How does telomerase fill in the overhang where the RNA primer is removed to prevent chromosome shortening?
Telomerase adds TTAGGG repeats to 3’ end to compensate for the loss of telomere sequences. This means DNA primase can bind and initiate new RNA primer synthesis.
What is the structure of telomerase and how does this allow for addition of multiple TTAGGG repeats?
Is a ribonucleoprotein with an intrinsic RNA component that acts as a template on which telomere repeat sequences are synthesised by the Telomerase Shuffle.
What are examples of things that can cause DNA damage?
Thermal degradation (heat/water) Metabolic byproducts (oxidation) Environmental substances (benzopyrene) Radiation (UV, nuclear fission)
Which DNA bases are purines and which are pyrimidines? What are the differences between these?
The purines are Guanine and Adenine.
The pyrimidines are Cytosine and Thymine.
Purines have a double ring structure.
How many pairs of hydrogen bonds do each DNA bases have?
G - C have 3 pairs of H bonds
A - T have 2 pairs of H bonds
How can deamination of DNA lead to damage?
Deamination of cytosine causes it to change to uracil. This will have higher affinity to adenine and can cause a transition mutation.
Why are transition mutations more likely to occur than transversions and which is less likely to result in amino acid substitutions?
Subsituting a double ring structure for another double ring structure is more likely than substituting a double ring for a single ring and vise versa.
Transitions are less likely to result in mutations.
How can frameshift mutations have a significant impact?
They generate missense proteins or could produce or get rid of stop codon
What can UV light induce (in terms of DNA damage)?
Formation of pyrimidine dimers (these are the primary cause of melanomas)
How is depurination (loss of purine base) resolved by the BER pathway?
The BER (base excision repair) pathway:
Glycosylase recognises and removes incorrect base but leaves backbone
AP endonuclease and phosphodiesterase remove sugar phosphate
DNA polymerase adds new nucleotide, DNA ligase seals the nick
How are pyrimidine dimers resolved by the nucleotide excision repair pathway?
Nucleotide excision repair pathway (NER):
The region around the pyrimidine dimer (~10 nucleotides long) is cut out by excision nuclease
DNA helicase peels away the region
DNA polymerase and DNA ligase fill in the complementary strand
How do translesional DNA polymerases aid replication when there is damage and why are they a last resort?
When sliding clamp and replicative DNA polymerase encounter DNA damage, the sliding clamp undergoes covalent modifications and loses affinity. They are both released from the DNA strand and translesional DNA polymerase is loaded by assembly factors. DNA is synthesised until covalent modifications are removed and normal DNA synthesis can continue.
Translesional DNA polymerases lack precision in template recognition and substrate base choice and exonucleolytic proof-reading activity. They are also a cause of most base substitution and single nucleotide deletion mutations.
How can a double strand break be repaired by non-homologous end joining?
The ends are recognised by Ku heterodimers and additional proteins DNA-PK and ATM PKs are added for processing of DNA ends. There is limited repair synthesis, only ligation therefore repaired DNA usually has a deletion of nucleotides
How can a double strand break be repaired by homologous recombination?
Nuclease digests the 5’ ends of the break. The strands exchange by complementary base-pairing with another double stranded DNA molecule to form a Holliday junction. The undamaged DNA acts as template and can repair breakage by DNA polymerase. This invading strand is then released and DNA synthesis of other strand occurs. DNA is ligated and completely accurate DNA is repaired.
What are some examples of human diseases caused by failures in DNA repair systems?
Breast and ovarian cancer (e.g. BRAC1 and 2) Colon cancer Skin cancer (e.g. XP variant)
What are characteristics variations involved in communication?
The number of cells which see and then actually respond to signal
The duration of signal
The method of signal
How far signal can travel
What are the different types of signals?
Contact-dependent
Synaptic
Paracrine
Endocrine
How do cells respond to an extracellular signal molecule?
Receptor protein detects signal
Intracellular signalling proteins (signalling cascade)
Effector proteins e.g. metabolic enzyme, gene regulatory protein, cytoskeletal protein)
Effect (e.g. altered metabolism, althered gene expression, altered cell shape or movement)
Why is signalling important and what are two examples of when signalling can go wrong?
If signalling goes wrong it can lead to diseases such as cancers
- Mutation JAK2 V617F of JAK2 means its signalling is more active leading to myeloproliferative neoplasms (blood cancers)
- Mutation BCR-Abl causes sustained Abl tyrosine kinase expression leading to chronic myeloid leukaemia
How do positive and negative feedback loops regulate signalling and what is an example of each?
Positive feedback works to increase the response quickly and to the max response (i.e. A –> B –> A –> B etc, switch mechanism). e.g. Notch pathway lateral inhibition
Negative feedback has a slower response so max response can be reached before it feeds back to inhibit the pathway (i.e. A –> B –| A, button mechanism). e.g. JAK/STAT pathway down regulation by SOCS negative regulator
What are two examples of modifications that can act as switches on existing proteins and what enzymes modulate these?
Phosphorylation - protein phosphatase (OFF) and protein kinase (ON)
GTP-binding - GAP (OFF) and GEF (ON)
What are two major groups of protein kinases in eukaryotes?
Serine/threonine kinase (S/T) Tyrosine kinases (Y)
What are the 4 types of kinase inhibitors and how does each of them function?
Type I - binds to the active conformation of kinase with aspartate residue of the DFG motif pointing into the ATP-binding pocket
Type II - bind and stabilise inactive conformation of kinase with flipped aspartate residue facing outward of binding pocket
Type III - occupy an allosteric pocket that is adjacent to the ATP-binding pocket but does not overlap with it
Type IV - binds to an allosteric pocket remote to the ATP-binding pocket
What are key features that differ between ion channels?
Gating mechanism
Ligand or voltage
Ion selectivity of pore (defined by physical size of filter and amino acids lining the pore)
What is the P loop of an ion channel and what is it important for?
A hydrophobic loop between two TMs that does not go all the way through membrane.
Important for selectivity as acts as a filter
How does the voltage-gated ion channel become inactivated?
An inactivation peptide can swing in and block the open pore
What do TRP channels sense and which channel type is the structure similar to?
Chemicals and physical (e.g. temperature) stimuli.
Structure similar to voltage-gated ion channels
What are ligand-gated ion channels gated by and what are different types (including how their structural components differ)?
Chemical transmitters (intercellularly generated or extracellular).
Cys-loop type - pentameric assembly e.g. nAChR, GABAa, 5-HT3
Ionotropic glutamate type - tetrameric assembly e.g. NMDA
P2X type - trimeric assembly e.g. P2XR
How do voltage-gated ion channels and ligand-gated ion channels work together to control the excitability and function of muscle and neurons?
Voltage-gated ion channels are responsible for AP generation and release of NT.
Ligand-gated ion channels detect NTs leading to the response in the cell.
What are three families of glutamate receptors caused by different genes, alternative splicing and RNA editing?
AMPA
Kainate
NMDA
What happens when nAChR is mutated or lost e.g. by autoimmune diseases?
nAChR receptors are present on skeletal muscle therefore will cause myasthenia gravis (muscle weakness).
They are also present in the CNS therefore overstimulating mutations can cause ADNFLE (epilepsy)
What are structural features of GPCRs?
The characteristic 7TMs (alpha helices) of all GPCRs are packed in a similar way
TM3 is centrally located next to binding pocket as is crucial for transduction of ligand binding
Other TMs and extracellular N’ terminus also contribute to ligand binding (extended N’ terminus = big peptide recognition)
What are PAR receptors activated by?
Protease cleaving (e.g. thrombin)
What are the basic principles of GPCR signalling?
Resting (three inactive components)
Activation by ligand binding causes conformational change, movement of TMDs opens cleft for alpha subunit to bind and exchange of GDP for GTP
GTP binding reduces affinity for beta and gamma subunits so they dissociate and begin intracellular signalling
How is duration of trimeric G protein signalling regulated?
By rate of GDP hydrolysis by Ga
How is the local concentration of 2nd messengers (e.g. 3’5’-cyclic AMP and 3’5’-cyclic GMP) determined?
Rate of production (e.g. by adenylyl cyclase)
Rate of removal (e.g. by phosphodiesterase)
Rate of diffusion from site of production
What are examples of effectors of G proteins?
Adenylyl cyclase (inhibit or stimulate)
Phosphodiesterase
Rho (or other small molecule proteins)
Phospholipase
–> enzymes which create 2nd messengers and ion channels
What is desensitisation?
Where the agonist may still be present but the receptor will not respond. Prolonged or frequent activation by stimulus can induce receptor desensitisation.
How is the beta 2 adrenoreceptor for regulation of metabolism in liver and skeletal muscle activated and what are its effects?
Stimulated by a ligand e.g. epinephrine or norepinephrine. Receptor activates trimeric GTPase and this activates adenylyl cyclase. This converts ATP to cAMP which activates PKA. This phosphorylates PK which is then activated by Ca2+. PK phosphorylates phosphorylase which makes glucose-6 P.
What are some mechanisms which can switch off signalling?
Agonist dissociating from receptor GTPase activity of Gas cAMP breakdown by phosphodiesterase Dephosphorylation of enzymes Negative feedback via PKA, B-arrestin, GRK
What are examples of activating mutations in GPCRs that can cause disease?
Parathyroid Ca2+ sensor –> hypoparathyoidism
Rhodopsin –> night blindness
Thyroid hormone receptor –> hyperthyroidism, thyroid cancer
How are lipid-derived second messengers produced?
Receptor regulated lipases (e.g. GPCRs activate PLC) target membrane lipids. Two types of 2nd messengers can be generated:
- water soluble and so can diffuse through cytoplasm e.g. IP3
- hydrophobic so remain in membrane e.g. DAG
Lipid kinases then add phosphate groups to lipids
What are PKCs, what activates them and what occurs when they are activated?
Protein Kinase C
These are Ser/Thr kinases and are activated by DAG (C1 domain) and Ca2+ (C2 domain - not present in all forms).
Substrate binding site becomes exposed as DAG causes dissociation of intramolecular pseudosubstrate domain from active site. Can then bind to ser/thr and transfer terminal phosphate of ATP.
What is an example of a downstream effect of GqPCRs (involving Munc13)?
GqPCRs and synthesis of DAG in plasma membrane recruits Munc13 (a C1 domain containing protein) to the membrane and stimulates secretory vesicle docking to the plasma membrane, preparing it for fusion
How does mutations in signalling cause Lowe Syndrome?
Mutations in OCRL which codes for inositol polyphosphate 5 phosphatase alter PIP2 levels which are normally tightly regulated by many enzymes
How are calcium ER levels refilled and maintained?
When Ca levels fall below critical level it leads to conformational change in STIM (from dimer to oligomer) as it moves to the plasma membrane and joins with Orai channels. These open and Ca is pumped from the ECF into the ER.
How can desensitisation of G protein coupled receptors occur and what does this mean?
Phosphorylation of receptors, used for negative feedback, can contribute to desensitisation as become uncoupled and unresponsive to agonist.
What is adrenaline, where is it produced and what is its role?
Adrenaline is a water-soluble, non-steroid hormone produced by the adrenal glands (mostly).
It acts via adrenergic receptors to produce a wide range of physiological responses including the fight or flight response.
What are the two structures that play key roles in the maintenance of homeostasis and act as gateways linking the neuronal and endocrinological systems?
Hypothalamus and pituitary gland
What is cholesterol and what is it a precursor of?
A lipid with an -OH group therefore is an alcohol. It makes up around 30% of all cell membranes.
Is a precursor of a range of steroid hormones e.g. cortisol, estradiol, testosterone, vitamin D3
What is the structure of steroid hormones and what does this allow them to do?
They have both hydrophilic (-OH group) an hydrophobic (lipid) properties.
They are therefore able to penetrate through the BBB
