Week 4 Flashcards
What are the functions of skin
Physical protection -acts as a barrier
Thermoregulation
Sensation - lots of sensory receptors in skin
Metabolic functions- fat, adipose tissue, Vit D production
Indicator of general health
Shock absorption
Maintenance of fluid balance
What are the three main layers of skin
Epidermis
Dermis
Hypodermis/ subcutis
What is the epidermis
Keratinised stratified squamous epithelium
Mostly filled with keratinocytes
Four/five discrete layers
Cuboidal cells at the basal layer (stratum basale)
Squamous layer at top
What is the stratum basale/basal layer
Cuboidal cells
Most deep layer
Sat on the basement membrane
Mitosis occurs to replenish above layers
Melanocytes present
What is the stratum spinosum/ prickle layer
8-10 cell layers- biggest layer in epidermis
Connected by desmosomes
Gives prickly appearance on dehydration
Produces cytokeratin- intermediate filament of cytoskeleton
What is the stratum granulosum/ granular layer
3-5 cell layers
Become squamous cells
Granules contain keratohyalin- one of steps in keratin maturation
Stains deeply with H&E
Keratin maturation
Keratin is produced as intermediate filaments- cytokeratin, which matures into packed extracellular keratin
The epidermis is constantly being replenished every 25-30 days in normal skin
Process of keratin maturation in layers
Stratum corneum- keratin
Stratum granulosum- keratohyaline granules
Stratum spinosum- cytokeratin- tonofibrils
Stratum basale- mitosis, some cytokeratin
What is the stratum lucidum
Layer between the stratum granulosum and corneum
Only present in thick skin
What is the stratum corneum/cornified layer
Mature keratin= cytokeratin & keratohyaline
Dead/dying keratinocytes cells ‘squames’
No cytoplasm/nucleus/organelles
what aremelanocytes
Produce melanin
Found in basal layer
Can’t see with H&E
UV protection
Pigment
What are Merkel cells
In stratum basale
Associated with free nerve endings
Sensory- light touch
What are Langherhan’s cells
Found in all layers and dermis
Immune cell, like a macrophage
Specialised immune cell
Antigen presenting cell
Involved in immune and allergic reactions
What is the papillary layer
In dermis
Above the reticular dermis
The dermal papillae (elevations) increase surface area, diffusion nutrients
What is the reticular layer
Dense irregular collagenous tissue- more collagen
Elastin present throughout dermis
This elastic reduces age
Neurovascular supply in the dermis
Complex of vessels present
Subpapillary and cutaneous plexus with shunting vessels between
Important in thermoregulation
Controlled by ANS
Types of skin mechanoreceptors
Meissner corpuscle - papillary layer
Pacinian corpuscle - dermis & hypodermis
Ruffini’s corpuscles
Merkels disks
Free nerve endings
What are free nerve endings for
Pain, nociception (feeling pain)
Merkel disks
Perception of shape , texture
Meissners corpuscle
Motion detection
Grip control
Ruffini ending
Skin stretch
Tangential force
Pacinian corpuscle
Perception of distant events through vibrations
What are the unencapsulated mechanoreceptors
Merkel
Free nerve endings
What are encapsulated mechanoreceptors
Pacinian
Ruffinis
Meissners
What type of skin are hairs found
Thin skin
What is hair made of
Keratin, densely packed
What are sabaceous glands
Secrete oily sebum on upper part of hair follicle
Sweat glands
Found in the superficial hypodermis
Watery fluid
Secrete sweat through exocytosis- termed merocrine secretion
Sebaceous glands
Attached to hair follicles
Secrete sebum
Cells fill with secretory vesicles then dissociates- termed Holocrine secretion
What are the two types of sweat glands
Eccrine
Apocrine
Both are merocrine secretion
Eccrine sweat glands
Directly on skin
Function in heat loss (ANS)
Less viscous
Found everywhere
Apocrine sweat glands
Open into hair follicle
Regulated by hormones
More viscous
Limited to axilla/ genitals and areola
Secrete through merocrine secretion
burns classification
Superficial epidermal
Partial thickness- superficial dermal
Partial thickness- deep dermal
Full thickness- burn extends through all layers of skin to subcutaneous tissue
What is an autograft
Layer of own skin used
What is a allograft
Donor skin cadaver
What does central dogma mean
A theory stating that genetic info flows only in one direction from DNA to RNA to protein
Structure DNA
Double stranded
High molecular weight
Structure RNA
Single stranded
Heterogeneous in size, smaller, vary in size
What does a nucleotide consist of
Phosphate group
Pentose sugar
Base
Sugar used in RNA
B-D-ribose
Sugar used in DNA
B-D-2-deoxyribose
What bonds join nucleotides
Phosphodiester bonds between phosphate group and sugar
Comparison of RNA and DNA
RNA has a OH group at ribose C2 DNA doesn’t
RNA uses uracil where DNA used thymine
Why is RNA unstable
Because the lone pair of electrons on OH group on C2 can react with phosphate
What are purines
Adenine and guanine
Pyrimidines
Cytosine and thymine
How is DNA wound up
Double helix with antiparallel strands
Helices interact via hydrogen bonding between complimentary bases G&C and A&T
DNA strands have 5’ and 3’ ends
The base sequence of one strand determines the sequence of the others
What does 5’ and 3’ mean
The number of carbon atoms in a deoxyribose sugar molecule to which phosphate group bonds
Is C&G or A&T stronger
Guanine-cytosine base pair
What is a chromosome
A single molecule of DNA
What is a gene
Sequence of nucleotides that codes for a protein
What are mitotic chromosomes formed from
Tightly packed chromatin
Where is DNA found
In the nucleus and in mitochondria
How is DNA packaged into chromatin
By histones and other chromosomal proteins
How can DNA sequence differences be used
To paint chromosomes
Specific probes of different colours reveal each chromosome pair
What are exons
Coding DNA
What are introns
Non coding DNA
Removed from primary RNA by splicing machinery
Human genome contains repeated sequences
Often occur in blocks of tandem repeats
Called ‘satellite DNA’ - highly repetitive DNA consisting of short sequence so repeated a large number of times
Types of satellite DNA
Mini satellite - highly polymorphic up to 1000 copies in one block also found at telomeres
Micro satellite- small arrays of simple sequence repeats
Usually in intergenic/ intronic (non coding) DNA
Telomeres
Allow replication to tip of chromosomes
Long tracts of repeats can be unstable= deletions immediately below telomeres
Centromeres
Essential for segregation during cell division
Proteins involved in cell division bind to centromeric sequences
Mini satellite
No known purpose
Can cause mispairing during cell division to give
Large scale duplication/deletion between homologous chromosomes
Translocation of DNA between non-homologous chromosomes
What is the fundamental unit of chromatin
The nucleosome- this is DNA wound a histone core
How are decondensed chromosomes structurally organised in non-dividing cells
Attachment to nuclear skeleton
Has a functional role e.g. clustering of ribosomal genes in the nucleolus
What is nucleolus
Large structure within the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled
What is euchromatin
Transcriptionally active form of chromatin
Prevalent in gene rich areas less compact allowing access for proteins involved in transcription
What is heterochromatin
Highly condensed region of an interphase chromosome generally gene poor and transcriptionally inactive
Can exist in active (decondensed) or inactive (condensed) forms
Or can always be inactive and condensed
Found at telomeres and centromeres
When does chromatin structure change
During the cell cycle
Also modified during gene regulation (allowing access to regulatory proteins and RNA polymerase)
DNA replication is
Semi conservative
What end are nucleotides added
at the 3’ end dNTPs (deoxyribonucleotide triphosphate) required
Where is DNA replication initiated
At replication origins
The double helix opened with aid of initiator proteins, single stranded DNA templates ready for DNA synthesis
DNA polymerase
Mediates DNA replication
Proceeds in a 5’ to 3’ direction
Adds 1000 bases/ second to the chain
Requires dNTPs
Must have template and RNA primer
Has ‘proof reading’ activity
What is a mutation
Is a permanent change in the sequence of DNA
Why does DNA polymerase have a high fidelity
Stability of base pairing
Proof reading by DNA polymerase
How do DNA repair mechanisms work
Minimise introduction of mutations
DNA mismatch repair proteins bind
Removal of newly synthesised DNA strand
Repair of gap by DNA polymerase and ligase
What is depurination
Lose a base
What is deamination
Cytosine becomes uracil
During which phase of cell cycle does DNA replication occur
S
Phases of mitosis
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
What is the purpose of mitosis
Cell division
Organismal growth
Genetically identical products
Prophase
Chromosomes condense
Centrosomes move apart
Prometaphase
Nuclear membrane breaks down
Spindle microtubules attach to the kinetochores and move actively
Metaphase
Chromosomes align at the equator of the spindle
Kinetochores from paired sister chromatids attach to opposite poles of spindle
Anaphase
Sister chromatids synchronously separate. Fast
Kinetochore microtubules shorten
Spindle poles move apart
Telophase
Chromosomes arrive at poles
Nuclear envelope reassembles giving 2 new nuclei
Nucleoli reappear- RNA synthesis begins
Initiation of plasma membrane cleavage
Cytokinesis
Contractile ring is formed and mediates division of the cell in two
Function of meiosis
Reduction division (23 chromosomes per gamete)
Re assortment of genes: independent segregation of chromosomes, crossing over
Mechanism of meiosis
Each homologue replicates to give two chromatids
Homologous pair
Exchange of material between non-sister chromatids
Chiasmata (visible cytologically) are the physical manifestations of crossing over
What generates genetic variation
Independent assortment of maternal and paternal homologues at meiosis I
Re-assortment of genes by crossing over
What are chiasmata
Hold chromosomes together first phase of meiosis
Typical bivalent has at least one chiasma
Larger chromosomes show more
Meiosis metaphase I
Kinetochore microtubules of sister chromatids point in same direction
Kinetochores of sister chromatids function as a unit
Meiotic anaphase 1
Arms of sister chromatids become unglued
Meiotic metaphase II
Kinetochores point in opposite directions (as in mitosis)
Meiotic anaphase II
Cohesions in centromere degraded; sister chromatids separate
Difference between mitosis and meiosis
Mitosis designed to generate identical daughter cells
Meiosis designed to make haploid gametes and generate genetic diversity
Genome
Complete set of DNA
DNA sequence
Transcriptome
Complete set of RNA transcripts
Proteome
Complete set of proteins produced
Transcription
RNA synthesis needs a DNA template (RNA polymerase as enzyme)
Nucleoside Triphosphates
RNA is synthesised in a 5’ to 3’ direction
Only one strand is copied
RNA polymerase adds 50 bases per second
The ‘primary’ RNA transcript binds proteins involved in RNA processing ‘spliceosome’
Splicing requires RNA-protein complexes called snRNP
Some genes have alternative splicing leading the production of different mRNA and proteins from same gene
What is tRNA
An adaptor molecule
3 ‘hairpin’ stem-loop structures
Stabilised by base pairing
Distinct functional regions
Specific aminoacyl-tRNA synthetases
Contains an anti codon
Polyribosome/polysome
More than one ribosome attaches to a single mRNA
How many possible reading frames does DNA sequence have
3
Defining start of coding region is important
The genetic code
Code is read in groups of 3 bases
Read in a 5’ to 3’ direction
3 possible reading frames
Each amino acid is coded for by a codon
Some amino acids have more than one codon some only have one
3 codons do not encode amino acids (UAA, UGA and UAG are stop codons)
Code is universal
Mutations can affect protein sequence
Deletion or insertion of one or more nucleotides
Substitution or point mutations- alteration of a single base position
Consequences depend on nature and position of mutation- biggest impact if in coding or regulatory region of gene
What is the name of the fascia that connects skin to body
Superficial fascia is found directly under the skin and superficial adipose layers
Epidermis definition
Keratinised stratified squamous epithelial cells. Located superficially to the dermis
Avascular
Dermis definition
Deep to the epidermis and made of connective tissue
Contains blood vessels and nerves
Split into papillary and reticular dermis
Hypodermis definition
Loose connective tissue which connects skin to underlying tissues
Burns definition
Damage from heat, chemicals, electricity or radiation
A scald is normally caused by water or steam
Skin appendages definition
Specialised structures associated with the skin such as hair, nails, glands
What is dermatitis
A medical condition in which skin becomes red, swollen and sore sometimes with small blisters
Resulting from direct irritation of skin by an external agent or an allergic reaction
Eccrine sweat gland definition
A type of simple sweat gland that’s found in almost all regions of skin, these glands produce watery substance, sweat directly on skin surface
Help to maintain homeostasis, primarily by stabilising body temperature, cools skin
Apocrine sweat gland definition
Open into the hair follicle leading to surface of skin
Develop in areas with many hair follicles such as scalp, armpits and groin
Tubular secretory glands
Secrete a more viscous, odorous product
Sebaceous glands definition
Secrete sebum, which is an oily substance that helps skin/hair to be water-proof. Mostly associated with hair follicles
Not found in palms and soles and are found in higher concentrations in face and scalp
