1
Q
General structure of eukaryotic and prokaryotic cell (6)
A
- Cell size (10-100μm vs 0.5-5μm)
- Nucleus (Present/Absent)
- Genetic material (Linear/Circular DNA)
- Ribosome (80S/70S)
- Organelles (Many/few)
- Cell walls (Cellulose/chitin/peptidoglycan/murein)
2
Q
Structure of genome of eukaryotic and prokaryotic cell (7)
A
- Size (Larger/smaller)
- Appearance (Multiple, linear/single, circular)
- Molecule (Double helix DNA)
- Association w proteins (Histones/DNA-binding protein)
- Level of DNA packing/coiling
- Location (Nucleus/Nucleoid region)
- Extrachromosomal DNA (Mitochondria/chloroplast or plasmids)
3
Q
Organisation of genome of eukaryotic and prokaryotic cell (4)
A
- Number of genes (25000/4500)
- Non-coding regions (Common/not common)
- Presence of operons (Few/many)
- Origin of replication (Many/one)
4
Q
Non-coding regions (4)
A
Does not code for proteins or RNA products
- Introns
- Promoters
- Enhancers/silencers
- Telomere
- Centromere
5
Q
- Introns structure
A
- Non-coding sequences within a gene
- Between exons (coding regions)
- Present in mRNA
- Only in eukaryotes
6
Q
- Introns function
A
- No involvement in translation
- Splicing of pre-mRNA → introns excised, exons joined → mature mRNA
- Spliceosome (small nuclear RNA)-protein complex
- Precise points of excision
- Alternative RNA splicing of single pre-mRNA → produce different mature mRNA
- 1 gene can code for >1 type of polypeptide
7
Q
Non-coding regulatory sequences (Control elements) (2)
A
- Proximal → promoter
2. Distal → enhancer/silencers
8
Q
- Promoters structure
A
- Located just upstream of the transcription start site of a gene
- Critical elements:
1. TATA box at -25 sequence (i.e. located 25bp upstream of transcription start
2. CAAT and GC boxes → not critical in determining transcription frequency
9
Q
- Promoters function
A
- Recognition and binding site for general transcription factors which then recruits RNA form transcription initiation complex which initiates transcription
- TATA box determines precise location of transcription start site
10
Q
- Enhancers and silencers structure
A
Usually located far away from the promoter (usually much further upstream or downstream)
11
Q
- Enhancers and silencers function
A
- Recognition and binding site for activators/repressors (specific transcription factors)
- Promotes/prevents assembly of TIC (with the help of DNA bending proteins that bend spacer DNA for enhancers)
- ↑/↓ frequency of transcription
12
Q
Non-coding Repetitive DNA (2)
A
- Telomeres
2. Centromeres
13
Q
- Telomeres structure
A
- found at both ends/terminals of linear, eukaryotic chromosomes
- Non-coding DNA made up of a series of tandem repeat sequences (a specific sequence of nucleotides occurring many times in a row), 5’ TTAGGG 3’ in human beings
- Single stranded region at their 3’ ends known as the 3’ overhang (due to a limitation of DNA polymerase, this region of DNA does not have a complementary strand)
14
Q
- Telomeres function (3)
A
a) Ensure genes are not eroded and vital genetic info is not lost with each round of DNA replication due to end replication problem
b) Protect and stabilise terminal ends of chromosomes
c) Allow their own extension
15
Q
a) Ensure genes are not eroded and vital genetic info is not lost with each round of DNA replication due to end replication problem
A
- DNA pol requires a free 3’OH of a pre-existing strand to add nucleotides
- Last RNA primer on the lagging strand with DNA cannot be replaced with DNA
- Creates 3’ overhang
- DNA molecule shortens with each round of replication
- Telomeres, which are non-coding sequences at the ends of linear chromosomes will be lost before any vital genetic information is
- Since they are non-coding, they can be lost without any deleterious effect
16
Q
b) Protect and stabilise terminal ends of chromosome
A
- Form a loop using the 3’overhang
- Prevents single-stranded terminal end of one chromosome from annealing to a complementary single-stranded terminal end of another chromosome, prevent fusing of 2 chromosomes.
- Prevents cell’s DNA repair machinery from detecting the chromosome as damaged DNA (i.e. double stranded breaks) and trigger apoptosis
17
Q
c) Allow their own extension
A
- 3’ overhang provides an attachment point for the correct positioning of the enzyme telomerase
- Although telomeres shorten with every round of DNA replication, telomerase activity in germ cells, embryonic stem cells and cancer cells can maintain telomere length
18
Q
How telomerase works
A
- A short 3-nucleotide segment of RNA within telomerase binds to part of a DNA repeat in the 3’overhang by cbp
- Adjacent part of the RNA within telomerase used as template to synthesise short complementary 6-nucleotide DNA repeat
- Telomerase catalyses formation of phosphodiester bonds between existing 3’OH group of existing DNA overhang and 5’ incoming deoxyribonucleotide
- After repeat is made, telomerase translocates 6 nucleotides to the right in the 5’ to 3’ direction of the DNA overhang and begins to make another repeat → repeated such that a series of tandem repeats are made, elongating the telomere.
- Primase makes an RNA primer near the end of the telomere → DNA pol adds nucleotides to 3’OH end of primer and hence synthesises a complementary strand → nick sealed by ligase → RNA primer is eventually removed.
19
Q
- Centromeres structure
A
- Constricted region on chromosome where kinetochore microtubules attach during nuclear division
- Non-coding DNA made up of a series of tandem repeat sequences
20
Q
- Centromeres functionhttps://www.brainscape.com/packs
A
- Allow sister adhere to each other
- Allow kinetochore proteins to attach and which in turn allow spindle fibres to attach so that sister chromatids/homologous chromosomes can align along the
metaphase plate and subsequently be separated to opposite poles.
Ultimately, allow proper alignment and segregation of chromosomes
Biology Core Idea 2
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