1
Q
What is melanogenesis
A
- Formation of melanin
- Oxidation of amino acid tyrosine
- Followed by polymerisation of different molecules
2
Q
Where is melanin found?
A
- Keratinocytes (skin cells)
- Hair
- Cells under iris
- Medulla oblongata
- Part of ear
- Part of adrenal gland
3
Q
What are melanosomes?
A
- Organelle found in animal cells
- Site for synthesis, storage and transport of melanin
4
Q
What is melanin?
A
- Dark pigment found in melanocytes
- Most common light-absorbing pigment found in animal kingdom
5
Q
What is tyrosine?
A
- Non-essential amino acid
- Has polar R group BUT is considered hydrophobic
- Coded for by UAC or UAU in mRNA
6
Q
How does tyrosine form melanin?
A
- Tyrosine oxidised to DOPA by enzyme tyrosinase
- DOPA converted to dopaquinone by enzyme tyrosinase
- Dopaquinone converted to melanin
7
Q
What is the importance of tyrosinase?
A
- TYR gene on chromosome 11 codes for enzyme tyrosinase
- Recessive allele for gene TYR = lack or tyrosinase OR presence of inactive tyrosinase
- No tyrosinase enzyme = tyrosine not converted to melanin
8
Q
What else can tyrosine be converted to?
A
- Tyrosine converted to DOPA and then to melanin
- BUT DOPA an also be converted to noradrenaline and adrenaline
9
Q
Where are melanocytes found?
A
- Melanocytes are cells in skin and eyes
10
Q
What is the choroid?
A
- Thin layer of tissue between sclera and retina
- Richly supplied with blood vessels = supply oxygen and nutrients to retina
11
Q
What is the function of pigmented epithelium in choroid?
A
- PIGMENTED EPITHELIUM = Inner layer of choroid = layer of cells containing melanin
- Absorbs any light that passes through rods and cones so it is not reflected back into eye
12
Q
What is the uveal tract?
A
- Consists of choroid, ciliary body and iris
13
Q
What are the functions of the uveal tract?
A
- Reduced reflected light within eye = improves contract of retinal image
- Absorbs outside light transmitted through sclera (not fully opaque)
- Secreted aqueous humour from ciliary processed
- Controls accommodation via ciliary body (autonomic nervous system)
14
Q
What else does the uveal tract consist of?
A
- Immune cells (particularly lymphocytes)
- Responds to inflammation by developing lymphatic infiltrates
15
Q
What is sympathetic ophthalmia?
A
- Rare disease
- Body unable to distinguish between uveal and retinal antigens
- SO misdirected inflammatory reactions
- SO auto-immune response
16
Q
What are uveal melanocytes?
A
- Pigmented cells which form part of uvea
- Contribute to eye colour phenotypes
- Can develop rare type of eye cancer
17
Q
What is the effect of UV radiation on human skin?
A
- exposure to UV radiation initiates melanogenesis = skin darkens
- One form of melanin dissipates over 99.9% of absorbed UV radiation
= protects skin cells from UVA and UVB radiation damage
= reduced risk of folate depletion and dermal degradation
- More concentrated melanin (darker skin tone) = lower incidence of malignant melanoma
18
Q
What is oxidative stress?
A
- Damage to cell structures can be due to increased conc of certain chemicals
(e.g.: hydrogen peroxide = waste product of respiration)
- Environmental stress (UV/heat exposure) = dramatic increase in levels of hydrogen peroxide and other chemicals
- Cumulative effect = oxidative stress
19
Q
What does melanin protect against?
A
- Protects microorganisms (fungi, bacteria) against stresses involving cell damage (UV radiation from sun, hydrogen peroxide)
- Protects against damage from high temps
- Protects against chemical stresses (heavy metals, oxidising agents)
- Protects against biochemical threats (host defences against invading microbes)
20
Q
What is immune regulation?
A
- Suppression or stimulation of the immune system
- Occurs through gene expression of immune genes
21
Q
How does immune system distinguish between self and non-self antigens?
A
- Distinguish between self + non self antigens
- Distinguish between harmful + innocuous foreign antigens
- Deletion of self-reactive clones in thymus (negative selection) = prevents auto-immune responses
- Melanocytes form proteins that can suppress or enhance immune system
22
Q
What is angiogenesis?
A
- Angiogenesis = formation of new blood vessels
- New blood vessels develop from existing cells
- Melanocytes can stimulate angiogenesis
23
Q
What is the importance of angiogenesis?
A
- Process slows in adulthood (but ability to grow new blood vessels is retained)
- Essential for tissue growth and normal development (e.g.: pregnancy, puberty)
24
Q
List the electromagnetic spectrum, low to high frequency
A
- Radio waves
- Microwaves
- Infrared
- Visible Light
- Ultraviolet
- X-rays
- Gamma rays
25
What are pigment granules?
- Small, sub cellular membrane-bound vesicle
- Contains pigment and/or pigment precursor molecules
26
Where are pigment granules formed?
- Multiple sources, including:
- endoplasmic reticulum
- coated vesicles
- lysosomes
- endosomes
27
What happens to melanin when exposed to UV light?
- Melanocytes form temporary pseudopodia = carry melanosomes from centre of cell to edge of cell
- increases cells effectiveness at absorbing light
- process occurs slowly in response to exposure to UV light
- UV light = new melanosomes produced
- Transferred to adjacent keratinocytes in skin
- Melanocytes position themselves by nucleus, closest to UV light = protects genetic material against harmful effects of UV radiation
28
What is circular tissue?
- Radial and circular muscles (antagonistic muscles)
- Control the diameter of the pupil
29
How are radial muscles stimulated?
- Stimulated by sympathetic nervous system
- SNV usually increases activity (fight, fright, flight)
- Post ganglionic NT = noradrenaline
30
How are circular muscles stimulated?
- Stimulated by parasympathetic nervous system
- PNS usually decreases activity (rest and digest)
- Post ganglionic NT = acetylcholine
31
What is a phenotype?
- The physical observable characteristics that are expressed in an organism
- Affected by both genotype and environment
- Includes biochemistry, physiology, morphology and behaviour
32
What is a genotype?
- The combination of alleles present in a cell or organism
- Heterozygous, homozygous recessive or homozygous dominant
33
What is a polygenic characteristic?
- Controlled by several genes
- Usually show continuous variation (e.g.: height, mass, skin colour)
34
What is epigenetic modification?
- Organisms internal or external environment can influence gene expression patterns
- Levels or regulatory proteins or transcription factors affected in response to environmental stimuli (light, chemicals, drugs, hormones)
35
How is melanin production altered by epigenetics?
- Some enzymes are activated in response to UV radiation
- enzymes increase expression of melanin-producing genes
- therefore increase production of melanin
- leads to skin pigmentation
36
What causes the transport of melanin?
- Cytophagocytosis
- Direct Membrane Fusion
37
How does cytophagocytosis cause the transport of melanin?
- A melanocyte dendrite is phagocytosed
- Forms a phagolysosome
- Melanin granules disperse from phagolysosome through the cytoplasm of keratinocytes
38
How does Direct Membrane Fusion cause the transport of melanin?
- Plasma membranes of both cells fuse
- Creates a nanotube which allows the passage of melanosomes
39
What features do homologous chromosomes share?
- Biochemical structure (i.e. DNA)
- Physical shape and size
- Sequence of genes
- Gene loci
- Centromere position
40
What features differ between homologous chromosomes?
- Different origins (one maternal, one paternal)
- Different alleles
41
What is a gene loci?
Position of a gene on a chromosome
42
What is Oculocutaneous albinism?
- A form of albinism involving eyes, skin and hair
43
What are the general causes of Oculocutaneous albinism?
- Caused by mutations in several genes (polygenic) that control synthesis of melanin within melanocytes
- Seven types of Oculocutaneous albinism described
- All caused by disruption of melanin synthesis
- All autosomal recessive disorders
44
What are the causes of Oculocutaneous albinism in humans?
- In humans, OCA2 gene located on long arm of chromosome 15
- Most common type of albinism caused by mutation of P gene
- Mutations in OCA2 cause type 2 Oculocutaneous albinism
45
What are the effects in humans of Oculocutaneous albinism?
- OCA2 codes for human equivalent of mouse p (pink-eyed) gene
- People with OCA2 usually have fair skin
- Usually pale blonde to golden, strawberry blonde or even brown hair
- Usually blue eyes
46
What are the effects in people of African descent with Oculocutaneous albinism?
- Yellow hair
- Pale skin
- Blue, grey, hazel eyes
47
What is P protein and where is it found?
- Coded for by OCA2, pigment can develop in freckles or moles
- Also known as melanocyte-specific transporter protein OR pink-eyed dilution protein
- Found in melanocytes and retinal pigmented epithelium
48
What is the function of P protein?
- intrinsic membrane protein
- Involved in transport of tyrosine into melanocytes
49
What is HERC2?
- A protein ligase involved in DNA repair regulation
- Protein ligases important in cell cycle control, controlling breakdown of cyclins and CDK inhibitor proteins
50
How does HERC2 determine iris pigmentation?
- Mutation in HERC2 gene adjacent to OCA2 = affects OCA2's expression in iris
- Common to nearly all people with blue eyes
- Are 15 single nucleotide polymorphic forms of HERC2 gene (germline single DNA-nucleotide substitutions) which determine iris pigmentation
51
How do you calculate proportion of polymorphic gene loci?
number of polymorphic gene loci
______________________________________
total number of loci
(no units as is a ratio)
52
What are introns?
- Large regions of non-coding DNA which are removed from pre-mRNA before it is translated
- Removed during post-transcription modification (occurs in nucleoplasm = converts pre-mRNA to mature mRNA)
53
What are the steps of gene expression?
- Transcription
- Post transcription modification
- RNA splicing
- Translation
- Post translation modification
54
Why are steps in gene expression controlled?
- Ensures genes are only expressed when are where required for required duration
- Determines cell differentiation, organism development and adaptability of organism
55
What are some important examples of gene expression?
- Control of insulin expression (blood plasma glucose regulation)
- Control of cyclin expression in eukaryotic cell cycle
- Control achieved by DNA methylation and histone coat modification
56
What are barrier tissues?
- Epithelial and epidermal barriers protect the body from pathogens and from other environmental stresses
- Also separate different internal organs with a unique cellular structure and chemical composition
57
Name a barrier tissue that protects body from pathogens and environmental stresses
CILIATED EPITHELIUM
- specialised tissue that lines airways
- Small projections (cilia) sweep mucus, dust and bacteria upwards and away from the lungs with the epithelium itself
58
Name a barrier tissue that separates different internal organs
EPITHELIUM
- A type of body tissues
- Forms the covering on all internal and external surfaces of the body
- Lines body cavities and hollow organs
- Is the major tissue in glands
59
What are mucous membranes?
- Line the gut, airways and reproductive system
- Consists of epithelial cells and mucus-secreting goblet cells
- Contains lots of glycoproteins with long carbohydrate chains (glycocalyces) such as mucin
60
What is the function of mucus?
- Mucus in trachea, bronchi and bronchioles traps inhaled viruses, bacteria, pollen and dust
- Particles moved towards back of throat by wafting of cilia
- Mucus and trapped particles swallowed or expelled by coughing up mucus
- Ciliated epithelial cells have motile cilia = beat and move in wave-like manner = moves mucus along airway
61
Outline the role of phagocytes
- WBC produced continuously in bone marrow
- Remove dead cells and invasive microorganisms (non-specific immune response)
- Move in amoeboid movement to site of infection and attach to pathogens
- CSM of phagocyte extends out and around pathogen = forms pseudopodia = engulfs it by endocytosis
- Phagosome fuses with lysosome = phagolysosome
= Hydrolytic enzymes from lysosome digest pathogen = hydrolysis reactions of biochemical components of pathogen)
62
Outline the role of T lymphocytes
- T cells activated when encounter and bind to specific antigen on surface of antigen-presenting cell (clonal selection)
- APC = macrophage, infected body cell or pathogen itself
- Activated T cells divide by mitosis (clonal expansion) = increase in number = genetically identical daughter cells
- Activates T-helper cells bind with complementary receptors on surface membrane of specific B-lymphocytes
- On binding, T-helper cells release signalling proteins = activate B lymphocytes
63
Outline the role of T helper cells
Release cytokines (hormone-like signals) to stimulate:
- Maturation of B-lymphocytes into plasma B-cells
- Production of memory B cells
- Activation of cytotoxic T cells
- Increased rate of phagocytosis
64
Outline the role of T killer cells
- Patrol body in search of APCs
- Attach to foreign substances on cam of infected cells
- Secrete toxic substances = kill infected body cells and pathogen inside
- Perforins secreted by T killer cells punch hole in csm of infected cells = toxins enter
65
Outline the role of B lymphocytes
- B lymphocytes with correct cell surface antibodies recognise antigen and bind to it (clonal selection)
- Binding --> antigen-antibody complex
- Bdining along with cell signalling molecules produced by T helper cells activates B lymphocyte
- Activated = B cells divide repeatedly by mitosis = produces many clones of original activates B cell (clonal expansion)
66
What are the two main types of B cell?
- Effector cells = differentiate into plasma cells = produce specific antibodies to combat non-self antigens
- Memory cells = remain in blood = allow faster immune response to same pathogen in future
67
What is the Cornea?
- Layer under conjunctiva, continuous with sclera
- Focuses light onto retina
- Main structure responsible for refraction of light rays onto retina
68
What is the inflammatory response?
- Localised response to pathogens at site of wound
- Result = pain, swelling, heat and redness of tissue
- Mast cells activated in damaged tissue
- Mast cells release histamine, cytokines, serotonin and prostaglandins
69
What is the role of histamines in the inflammatory response?
- Make arterioles dilate (vasodilation) = increased blood flow to area
- localised redness, increased pain sensitivity and heat (higher temp prevents pathogens reproducing)
- Make arteriole walls more leaky = more plasma forced out = increased tissue fluid formation = soreness, swelling (oedema) and pain
70
What is the role of cytokines in the inflammatory response?
- (e.g.: Interleukin 1 and interleukin 6)
- Attract phagocytes to wound = phagocytosis = destruction of pathogens
- Also stimulate liver to release proteins = bind to surface of bacteria and damaged host cells = promote phagocytosis by macrophages
71
What is the role of serotonin and prostaglandins in the inflammatory response?
- Make arterioles dilate and more leaky
- Some plasma proteins leave blood
72
What is the role of an antigen-presenting cell?
- T lymphocytes produce an immune response when exposed to a specific antigen
- T lymphocytes only bind to a specific antigen if its present on surface of APC
- Once surface receptor of T lymphocyte binds to specific complementary antigen on APC it becomes sensitised
- It starts dividing to produce clones of cells
73
What can be antigen-presenting cells?
- Most common types = macrophages and dendritic cells
- Less common = neutrophils
74
What is the structure of T-lymphocytes?
- Produced in bone marrow, mature in thymus
- Mature T cells have specific cell surface receptors - T cell receptors
- Receptors have similar shape ton antibodies and each is specific to particular type pf antigen
75
What are the 2 types of Herpes Simples Virus (HSV)?
Type 1
- Spreads by oral contact
- Causes infections in or around mouth (oral herpes or cold sores)
- Can cause genital herpes
- Most adults infected with HSV-1
Type 2
- Spreads by sexual contact
- Causes genital herpes
76
What is Herpetic Stromal Keratitis (HSK)?
- An intense inflammatory response, triggered by viral infection of corneal stroma
- If left untreated, chronic inflammatory response --> formation of lesions, scarring and eventually blindness
77
What is the structure of Herpes Simples Virus?
- DNA virus within nucleocapsid
- Matrix (called the tegument) = made up of proteins and mRNAs = allows virus to evade immune response of host cells and facilitate its replication once its invaded host cells
- Envelope = lipid bilayer = contains virally encoded glycoprotein spikes = important for viral entry
78
What is the role of glycoproteins on surface of HSV?
- At least 12 types of glycoproteins with diverse shapes/sizes reported on surface of HSV
- Facilitate viral entry into host cells by interacting with receptors on host cell surface (viral attachment)
- Glycoproteins also interact with other glycoproteins
- Glycoproteins fuse viral envelope with host cell membrane = enables delivery of viral content into cell
79
What is the function of the spleen?
- Stores blood )to be released in times of hypovolemic shock)
- Removes old erythrocytes from circulation
- Acts in similar way to lymph node = filters blood plasma
- Contains high number of B-lymphocytes and monocytes
- Produces all types of blood cells during fatal life
- Produces opsonins
- Releases neutrophils following myocardial infarction
- Involved in recycling of iron from old erythrocytes and breakdown (hydrolysis) of globin chains of Hb
80
What is the structure of lymph capillaries?
(Some TF drains into lymph capillaries)
- Separate from circulatory system
- Have closed end and large pores = allow large molecules to pass through
- Larger molecules that cannot pass through capillary wall enter lymphatic system as lymph via small, one-way valves in lymph vessel walls
81
What is the role of lymph?
- Lymph moves along larger lymph vessels due to compression (from skeletal muscles contracting)
- Valves prevent backflow
- Lymph re-enters DCS through subclavian vein
- Any plasma proteins that escaped from blood = returned to blood via lymph capillaries
- Plasma proteins not removed from TF = lower WP of TF = prevents reabsorption of of water into blood
- After digestion, lipids transported from intestines to bloodstream by lymph system
82
What happens at the arteriole end in lymph formation?
- Hydrostatic pressure forces small, soluble molecules out of blood in capillary via fenestrations in capillary wall
- Plasma proteins remain in blood = creates WP between capillary and tissue fluid
- Overall movement of water = out of capillaries, into tissue fluid
83
What happens at the venue end in lymph formation?
- Hydrostatic pressure in capillary reduced = less liquid forced out of capillary
- WPG between capillary and tissue fluid remains same as at arteriole end = water begins to flow back into capillary from tissue fluid via osmosis
- Overall = more fluid leaves capillary than returns = tissue fluid left behind to bathe cells
84
What is the effect of hypertension on lymph fluid formation?
- Pressure at arterial end is greater
- Forces larger volume of fluid out of capillary
- Fluid accumulates = causes oedema
85
What is compartmentalisation?
- Occurs in development of specialised cell
- Enables unicellular organisms to develop specialised functions through specific areas of their cell
- E.g.: nucleus region contains DNA molecules, compartmentalisation of ATP = produces area formed by endosymbiosis of mitochondria
86
What is the importance of specialisation?
- Enables the cells in a tissue to function more efficiently as they develop specific adaptations for that role
- Development of distinct specialised features occurs by differentiation
87
What changes occur during differentiation?
- Change in shape of cell
- Change in organelles the cells does or doesn't contain
- Change in chemical composition of cell
(e.g.: presence or increased conc of a specific protein, such as melanin in melanocyte, mucin in goblet cell, haemoglobin in erythrocyte)
88
Why might T-lymphocytes enter cornea?
- Virus-specific T-lymphocytes enter cornea in significant numbers following HSV infection
- Memory T-cells remain in cornea after virus was eliminates (likely to ward off future reinfection)
89
What is a tissue?
- A group of similar cells
- Consisting of one or more than one cell type
- With any extracellular material they secrete
- Which are specialised to carry out a specific function
- e.g.: squamous epithelium, ciliated epithelium, xylem tissue, phloem tissue, nervous tissue
90
Where do T memory cells circulate?
- Plasma
- Lymph
91
What are virus-specific T-lymphocytes?
- A target therapy generated for specific viral antigens
- Antigens identified as immune target presented by APCs to T-lymphocytes along with other molecules that stimulate their growth and activation
- Those T-cells then isolated from donor and expanded in culture to generate more VSTs
- Some cases = cells created from blood samples of patients who have recovered from viral infections
92
What do the letters stand for in Hardy Weinberg equation?
P = frequency of dominant allele
Q = frequency of recessive allele
P^2 = frequency of homozygous dominant genotype
Q^2 = frequency of homozygous recessive genotype
2pq = frequency of heterozygous genotype
93
What is allele frequency?
Relative frequency of an allele )variant of a gene) at a particular locus in a population, expressed as a fraction of percentage
94
What is a gene?
- A section of DNA that codes for a single polypeptide chain
95
What determines genotype?
- Phenotypic variation due to combination of genetic and environmental factors
- e.g.: recessive allele causing sickle cell anaemia = high frequency in populations where malaria present as heterozygous recessive individuals are resistant to malaria
- Complete phenotype of an organism determined by expression of its genotype and interaction of environment
Phenotype = Genotype + environment
96
How can the environment affect a phenotype?
- Levels of regulatory proteins or transcription factors can change in response to environmental stimuli (e.g.: light, chemicals like drugs and hormones)
- Some enzymes activates in response to UV radiation = increase expression of melanin-producing genes
97
What are epigenetics?
- Control of gene expression by factors OTHER THAN an individual's DNA sequence
- Involves switching on and switching off genes WITHOUT changing the actual genetic code
98
How can chromatin be genetically modified?
- Methylation of DNA = chemical addition of -CH3 groups to the cytosine NCBs
- Histone modification = Via acetylation of amino acid tails
- These modifications are called epigenetic tags
99
What is the epigenome?
- All the epigenetic tags in an organism
- Epigenome can undergo changes due to smoking, stress, exercise ad diet
- Internal signalling fro, body's own cells can also cause modifications to occur
- Epigenetic modification is independent (e.g.: DNA methylation or histone modification in one area is NOT linked to modification in another)
100
What is the effect of winding in epigenetic changes?
- Chemical modification of histones and DNA controls how tightly the DNA is wound around them
(as intermolecular bonding between histones and DNA changes)
- DNA wound more tightly in certain area = genes on this section of DNA are 'switched off'
(as gene and promoter regions more hidden from transcription factors and RNA polymerase)
- Histone modification is reversible = can be different in different cell types and can vary with age
101
What is the effect of DNA methylation?
- Methyl groups directly added to DNA to alter gene expression
- Usually involves direct addition of methyl group to cytosine bases
- Methylation of DNA suppresses transcription of affected gene by inhibiting binding of transcription factors and enzymes needed for transcription (e.g.: RNA polymerase)
- Cells use mechanism to lock genes in 'off' position = gene repressed / inactivated
- DNA methylation affected by many environmental, lifestyle or age-related factors
102
What is the effect of lysine in gene expression?
- Acetyl groups added to lysine amino acids on histone proteins
- Lysine has positively charged R group
- Forms ionic bonds with negatively charged sugar-phosphate backbone of DNA
- Helps DNA coil tightly around histone protein core
103
What is the effect of acetylation of histones in gene expression?
- Adding acetyl groups to lysine residues removes positive ion = removes bond between histone protein and DNA
- SO DNA less tightly wrapped around histones
- RNA polymerase and transcription factors can bind more easily = gene expression can occur
- Gene = activated
104
What is the effect of deacetylation on gene expression?
- Removal of acetyl groups
- Makes lysine return to its [ositively charged state
- So stronger attraction to DNA molecule
- So inhibits transcription
- So stops gene expression
105
Why can epigenetic changes be passed onto daughter cells?
- E.g.: during gamete production - DNA in parent cell underuse de-methylation BUT often methyl groups not removed = present in DNA in sperm of oocytes
- MAYBE AS epigenetic changes occur in response to environmental factor
= beneficial for epigenetic changes to also occur in daughter cells (e.g.: gametes)
= adapted for environmental factor
- Modifications to epigenome in one generation passed onto next generation at cellular or whole organism level
= Epigenome is heritable
106
Give some examples of membrane proteins
- Receptors (e.g.: hormone receptor such as insulin)
- Immobilised enzyme (e.g.: maltase)
- Channel proteins (e.g.: sodium ion channels)
- Voltage-gated channels (e.g.: potassium ion channels)
- Electron carriers (e.g.: cytochrome)
- Carrier-Protein Pump (e.g.: sodium-potassium ion pump)
- Cell-to-cell recognition (e.g.: glycoprotein-antigen)
- Anchor proteins
- Proteins enabling cell adhesion
107
What are the roles of uveal melanocytes?
- Light absorption
- Regulation of oxidative stress
- Immune regulation
- Formation of new blood vessels
108
What rays does melanin absorb?
- Infrared light
- Visible light
- Ultraviolet radiation
(in front of eye, melanocytes block 99.9% of these wavelengths)
109
What are the 2 types of melanin?
- Eumelanin (brownish black)
- Pheomelanin (reddish yellow)
110
Why do amino acids act as buffers?
- Amphoteric = can donate or accept electrons
- Zwitterion formed (as COO- group or NH3+ group) = can be either positively or negatively charged amino acid
- Amphoteric and zwitterions = act as buffers as add or remove protons into environmental solution = alter pH
111
General recap of amino acids
- 20 naturally occurring amino acids
- Amino / Amine group = NH2
- Carboxyl group = COOH
- R group (or side chain) varies between amino acids
112
Overview of tyrosine
- Codes for by UAC or UAU in mRNA = example of degenerate codon
- Recessive allele for tyrosine --> albinism
- Residual group likely specific for tyrosinase active site
- R group has 'OH' = can form hydrogen bonds
- R-group is polar = R-group is hydrophilic (but not the amino acid itself)
- Means tyrosine is soluble in water
113
What is the definition of primary protein structure?
Number and sequence of a chain of amino acids joined by a series of condensation reactions, forming peptide bonds
114
What is the definition of secondary protein structure?
Local folding of the polypeptide chain into alpha helices and/or beta pleated sheets, held in place by hydrogen bonds
115
What is the definition of tertiary protein structure?
Three-dimensional further folding pattern of a polypeptide chain due to residual groups, resulting in a globular or fibrous protein
116
What is the definition of quaternary protein structure?
2 or more polypeptide chains, OR 1 polypeptide chain and a prosthetic group
117
Describe the bonds in primary protein structure
- Peptide
- Covalent (which are peptide bonds)
118
Describe the bonds in secondary protein structure
- Peptide
- Hydrogen bonds between NH of amino group of one amino acid and C=O of carboxyl group of another amino acid
- Covalent (which are peptide bonds)
119
How are alpha helices and beta-pleated sheets formed?
- Hydrogen bonds form between NH of amino group in chain and C=O of carboxyl group (usually 4 amino acids further down polypeptide chain)
120
Describe the bonds in tertiary protein structure
- Peptide
- Additional hydrogen bonds between functional groups which form dipoles in a single ppc
- Hydrophobic interactions between non-polar R-groups in same ppc
- Disulphide bonds between 2 cysteine R-groups of same ppc
- Ionic bonds between polar R-groups of same ppc
- Covalent (which are peptide bonds)
121
Describe the bonds in quaternary protein structure
- Peptide
- Hydrogen bonds can form between same ppc or different ppc's
- Hydrophobic interactions between non-polar R-groups between same or different ppc's
- Disulphide bonds between 2 cysteine R-groups of same or different ppc's
- Ionic bonds between polar R-groups in same or different ppc's
- Covalent (which are peptide bonds)
122
How does melanocyte show differentiation: Change in number of organelles?
- presence of melanosomes
- higher number of mitochondria
- increased RER (size and number) = more extensive and more cisternae
- Increased number of Golgi apparatus
- Higher number of 80s ribosomes as is a eukaryotic cell
123
How does melanocyte show differentiation: Change in chemical composition?
- Increased concentration of melanin
- Higher quantity of mRNA for the melanin
124
How does melanocyte show differentiation: Change in shape
- Extensions / protrusions of csm
- Projections of cytosol to form dendrites
- Irregular shape
125
How is a melanosome formed?
- Membrane-bound vesicle containing melanin
- Buds off Golgi vesicle
126
What is the function of the cornea?
- Protective layer, refracting light rays as they enter the eye
127
What is the function of the iris?
- Consists of radial muscles and circular muscles (antagonistic)
- Controls intensity of light that passes through pupil
PUPIL REFLEX:
- When pupil constricts, circular muscles contract
- When pupil dilates, radial muscles contract
128
What is the function of the lens?
- Transparent disc that can change shape to focus light rays on the fovea of the retina
- Carries out accommodation, controlled by ciliary muscles and suspensory ligaments
129
What is the function of the retina?
- Light rays are refracted onto the retina
- Contains photoreceptors (rod cells and cone cells) to absorb light
- Fovea has highest concentration of cone cells
130
What is the function of the optic nerve?
- Collection of neurones that carries nerve impulse between photoreceptors on retina and occipital lobe
131
What is the function of the pupil?
- Point at which light enters eye
- Constriction or dilation of pupil controlled by radial and circular muscles in iris = pupil reflex = accommodation
132
Outline rod cells
Pigment = rhodopsin
Type of vision = black and white
Number of cells that synapse with 1 bipolar neurone = 2 or more (multiple)
Sensitivity to light = higher
Visual acuity = lower
133
Outline cone cells
Pigment = iodopsin
Type of vision = colour
Number of cells that synapse with 1 bipolar neurone = 1
Sensitivity to light = lower
Visual acuity = higher
134
What are the different substances that can result in environmental stress?
- UV
- Heat (damages bonds)
- Hydrogen peroxide = apoenzyme (needs calcium ions), broken down by catalase
135
Why are gamma rays the most dangerous of the EM waves?
- Highest energy as highest frequency = more likely to cause damage
136
How can a melanosome be transferred from one cell to another?
Different types of bulk transport:
- Phagocytosis (adapted)
- Spontaneous annealing of membranes (adapted)
- Endocytosis (adapted)
- Exocytosis and phagocytosis (adapted)
137
What is bulk transport?
- Active process
- Uses ATP as cytoskeleton involved
- More tubulin produced = more protein synthesis needed = more ATP needed
138
What is the definition of a gene?
A short length of DNA that codes for the production of a specific polypeptide chain
139
What is the definition of an allele?
A gene variant that codes for the variation of the gene
140
What is the definition of a dominant allele?
An allele which is always expressed if present in the genotype, as only 1 copy is needed in the genotype to be expressed in the phenotype
141
What is the definition of a recessive allele?
An allele that is only expressed in the phenotype if two of them (2 recessive alleles) are present in the genotype
142
What is the definition of genotype?
The combination of all alleles present in a cell of individual
143
What is the definition of phenotype?
Observable characteristics expressed due to the organism's genotype
144
What is the definition of homozygous?
A genotype made up of 2 alleles of the same type
145
What is the definition of heterozygous?
A genotype made up of 2 different alleles, one dominant and one recessive
146
What is the definition of monogenic?
A phenotype that is controlled by a single gene
147
What is the definition of polygenic?
A phenotype that is controlled by 2 or more genes
148
What is the function of OCA2?
- Gene located on chromosome 15
- Crucial role in production melanin (especially brown-black eumelanin)
149
What is the effect of mutated OCA2?
- OCA2 gene codes for protein that regulated melanosomes which produce melanin
- Mutation = affects quantity and distribution of melanin in iris
- 2 functional copies of OCA2 = brown eyes
- 2 non-functional copies of OCA2 = blue of green eyes (as less eumelanin produced = underlying blue structural colour more visible)
150
How is HERC2 involved in eye colour?
- Specific region of HERC2 gene regulates expression of OCA2
- Mutations in HERC2 affect expression of OCA2
- A variant of HERC2 more common in people with blue eyes
Biology Extras
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