WEEK 6 Flashcards
Describe the structure of a molecule of DNA.
Is a polymer
Deoxyribose sugars are joined together by a phosphodiester linkage from carbon 5 on one sugar to carbon 3 on the other
The phosphate backbone is on the outside of the DNA double helix with the sugar inside & the base on the inside
How does base pairing work?
the 2 complementary strands of DNA anneal or hybridise together. The DNA double strand is held together by H bonds formed between the base pairs across the 2 different strands
- adenine forms 2 hydrogen bond with thymine
- guanine forms 3 hydrogen bonds with cytosine
What is the overall charge of DNA? How does this come about?
Overall negative charge due to phosphate groups being on the outside (which are negatively charged)
When/how are chromosomes (i) condensed (ii) spread out?
(i) As cells divide in metaphase, makes them more easily visible
(ii) During interphase, they are spread & dispersed. Although they are often still clustered near their own chromosomes (shown by staining)
How is DNA packaged? How many turns (and in what direction) does DNA make around a single histone?
In multiple layers
The 1st involved 147 bases associated with an octomeric histone core (protein) to form a unit called a nucleosome
1.7 left handed turns around a single histone
Describe the structure of histones, what is special about histone H1?
Histones (H2A, H2B, H3 & H4) are positively charged & have a long N-terminus. This tail protrudes from the nucleosome
Histone H1 has 2 termini (N&C) & sits inside the point at which DNA is coming off the “bead”
Describe how chromatin is formed.
The 1st stage of coilign creates a 10nm fibre which is then coiled again with the help of histone H1 to create a 30nm fibre. Yet more condensing creates a 300nm supercoiled structure.
During metaphase, and more condensing, a 700nm structure is made (a typical chromatid)
How can chromatin be decondensed? What are the 3 ways in which histones can be modified & what does this, in turn, affect?
Using ATP, they are decondensed by a remodelling complex to allow protein access
Histones can be modified post translationally by acetylation, methylation & phosphorylation.
This affects the degree to which the associated genes are transcribed
What are the 2 types of DNA repeats? Describe them both.
INTERSPERSED repeats: SINE & LINE often derived from retroviruses (21% of human genome)
TANDEM repeats: unlike the above, these are adjacent repeats including:
- satellites: telomere TTAGGG
- minisatellites: 7-100 bases repeated ever 40000 bases
- microsatellites: 1-6 bases repeated >500 bases
When is repetitive DNA used? Describe a condition for which it is used.
Can be used in forensics to compare a sample DNA to a number of known samples to find a match
- MYOTONIC DYSTROPHY involves a trinucleotide repeat (microsatellite) that is unstable when transmitted through mitosis
Describe mitochondrial DNA.
circular DNA that is maternally inherited
SIDENOTE: it is through mtDNA that we have discovered the ‘out of Africa’ theory
What are the 6 factors which ensures the skin as a defensive barrier? Describe them.
- EPIDERMIS - stratum corneum (hard horny layer of dead cells) forms a lipid bilayer
- Very DRY surface
- ACIDIC pH 5.5
- SWEAT secretion produces an increase in salt concentration and greater osmotic pressure
- ANTIMICROBIAL substances e.g. fatty acids, sebum, defensins are produced by resident microbial flora
- sebum contains lysozyme, capable of destroying bacterial cell wall - RICH BLOOD & LYMPHATIC SUPPLY ensures rapid specific and non-specific immune responses
What are the 6 ways that pathogens can enter the body?
- Keratinised epithelium (e.g. ringworm)
- Epidermis (e.g. impetigo)
- Hair follicles (e.g. folliculitis)
- Subcutaneous fat (e.g. cellulitis)
- Fascia (e.g. necrotising fasciitis)
- Muscle (e.g. gas gangrene)
What are the 3 routes of infection? Give examples for each.
- SKIN: pores, hair follicles
- WOUNDS: scratches, cuts, burns
- BITES: insects, animals
What are the 4 types of bacterial infection of the skin?
- Staphylococcus aureus
- Streptococcus pyogenes
- Pseudomonas aeruginosa
- Myobacterium tuberculosis
Describe (i) Streptococcus pyogenes (ii) Staphylococcus aureus (iii) pseudomonas aeruginosa.
(i) Cocci morphology, grown in chains, gram positive, group A streptococcus, multiple virulence factors, beta-haemolytic
(ii) Cocci, grown in clusters, gram positive, causes a huge range of illnesses, multiple virulence factors, direct infection, or effect, of toxin
(iii) coccibaccilus, gram negative, opportunistic pathogen, colonises catheters etc, produces exotoxin A
What are the 4 types of skin lesions? Describe each of them.
- Macule = flat, red, local inflammation
- Papule = raised, red, more marked inflammation
- Vesicle = small blister
- Ulcer = disruption of epithelium
Give an example of a fungal infection, describe it. What are certain predisposing conditions of candida albicans?
CANDIDA ALBICANS
- a type of yeast commensal
- however it can lead to candidiasis
Predisposing conditions include Ab treatments, corticosteroid treatment, cytotoxic therapy and diabetes mellitus
What are the 5 types of viral infection? What do they result in?
- Human papilloma virus - warts
- Herpes simplex virus 1 - cold sores
- Human simplex virus 2 - genital warts
- Varicella zoster virus - chicken pox or shingles
- Coxsackie A virus - hand, foot and mouth disease
Give an example of a parasite of the skin. What is causes and how this occurs.
Scarcoptes scabei (mite)
- the mite burrows into the skin with the female laying eggs
- infection is asymptomatic
- hypersensitivity to eggs/their surface proteins may occur
- scratching of the skin may lead to superinfection with streptococcus pyogenes
Describe the concept of semi-conservative replication of DNA.
DNA strands are anti-parallel
- one strand has a free hydroxy group on the 5’ carbon at one end & a free hydroxy on the 3’ carbon on the opposite end. This is base paired to another strand in the opposite direction.
- the strands are copied via the semi conservative model in which each strand of the original DNA is replicated such that the daughter molecule consists of one old strand (template) & one newly synthesised strand
Why is it DNA replication is initiated at many sites? How is this done?
Each eukaryotic chromosome is one linear DNA double helix averaging 10^8 base pairs long. With a replication rate of 2kb/minute, replicating one human chromosome would require 35 days. As a solution, DNA replication initiates at many different sites simultaneously - this occurs by forming replication forks
How is DNA replicated? From the use of the Okazaki fragment to the use of DNA polymerase to complete the lagging of the strand.
Replication is asymmertrical & DNA in short fragments, discontinuously, called okazaki fragments. For DNA to be synthesised in Okazaki fragments an RNA primer is required. A new RNA primer is synthesised by RNA primase
- DNA polymerase ADDS to the new RNA primer to start the okazaki fragment. DNA polymerase finishes the DNA fragment & the old RNA primer is erased & replaced by DNA
- The ‘nick’ between Okazaki’s is sealed by DNA ligase joining them to the growing chain. The exposed single stranded DNA at the replication fork is protected by single-strand DNA binding protein due to the unstable nature of single strands of DNA, the binding protein must be removed to re-expose the DNA=> replication can occur
- at end of new lagging strand is a repetitive telomere sequence. Telomerase binds to the exposed strand of DNA, it has a bound RNA template which can bind with homology to end of DNA fragment. This allows telomerase to add additional repeats to the template strand. The DNA polymerase completes the lagging of the strand
How often do polymerases make mistakes? What is done DURING replication to correct them?
Polymerases make mistakes as often as once every 10,000 polymerisation events. This can lead to mutations if not corrected before the next round of replication.
- errors are removed during synthesis by DNA polymerase. It has two ‘pockets’; an active site where polymerising occurs & as this is complete it twists the template so that as the template passes out of the enzyme, it binds to the editing site
- if the base pair is found to be wrong it is removed & replaced with the correct base pair.
How are mutations repaired post-replicatively? How many errors is there for x number of genomes (approximately)?
By mismatch repair or as defects predispose to disease
- there’s approximately 1 error for every 3 genomes replicated
Describe the technique of PCR (polymerase chain reaction).
The region of DNA to be amplified is heated up to separate the strands by disrupting H bonds. The solution is allowed to cool & primers that are complementary to the single stranded DN are added. There’s a primer to go in each direction for each anti-parallel strand
- DNA polymerase & individual nucleotides are added to allow DNA synthesis from the primers. This process is repeated to produce lots of copies from a small strand
What 3 things is PCR used for?
- Detect the presence of an infectious agent
- Detect inheritance patterns
- Look for changes in the number of copies of a chromosome
What is DNA sequencing? How is it done?
Single stranded DNA is added along with primers, DNA polymerase & nucleotides
- there’s a large proportion of normal precursors & a small amount of dideoxyribonucleoside triphosphate molecules
- the rare incorpporation of the terminal nucleotides by DNA polymerase blocks further growth of the DNA molecule
- the different lengths of strands are separated by electrophoresis to determine the DNA sequence
When is PCR & DNA sequencing used clinically?
Blood sample can be taken from an infected person. Cells removed by centrifugation and the viral RNA genome can be extracted
- reverse transcriptase/PCR amplification is undertaken, this then can be compared to the blood of a non-infected person by electrophoresis
What is the function of the kidneys? (HINT: there are 3 main functions)
- They maintain homeostasis by the production of urine, which is the removal, from the circulating blood, of excess water & electrolytes, and toxic, metabolic waste products such as urea & creatinine
- Also monitor the acid/base balance by excreting H ions during ACIDOSIS, or bicarbonate ions during ALKALOSIS
- They secrete renin which increase BP & erythropoetin which accelerate RBC production
Where, within the body, are the kidneys located?
One on each side, on the posterior abdominal wall, high up, under the diaphragm.
- They are retroperitoneal, encapsulated & protected by layers of fascia and fairly firm renal fat
How many times is blood filtered & cleaned each day?
Up to 60 times
Describe and explain the structure of kidneys, ensuring to mention renal pyramid, calyces, hilum & the ureter.
Has a cortex & medulla, with the renal pyramids (housing the nephrons with their function of filtration & reabsorption) projecting into the calyces of the collecting system
- minor calyces converge on major calyces, which themselves form the renal pelvis at the hilum
- the pelvis becomes the ureter that transports the urine to the bladder
What is the blood supply of the kidneys?
Supplied by the renal arteries which arise directly from the aorta at L2 & enter the hilum along with the renal veins (going to the inf vena cava) & renal pelvis
NOTE: the L renal artery is SHORTER than the R renal artery and vice versa with the veins as the inferior vena cava lies to the RIGHT of the aorta