topic 1.1 central dogma

Question 1

what is a gene?

Answer 1

segment of DNA that encodes a polypeptide chain or RNA molecule

Question 2

what is a sequence of each gene?

Answer 2

provides information to form a product (typically a protein, but NOT ALL genes code for proteins)

Question 3

what is central dogma?

Answer 3

• flow of genetic information in cells
• replication
• transcription
• translation

Question 4

what is replication?

Answer 4

• 2 strands of nucleotides separate (double helix/1 strand of DNA)
• the strands act as “templates” for other free nucleotides to bind to
• 1 strand = 1/2 parent DNA and 1/2 new DNA
• creating 2 identical daughter strands

Question 5

what is transcription?

Answer 5

• RNA is synthesised
• 1 strand of DNA acts as the “template” for the synthesis of a complementary strand of RNA
• RNA polymerase “reads” the bases of the DNA template to form a RNA transcript (a string of RNA nucleotides/mRNA)

Question 6

what is translation?

Answer 6

• the mRNA strand is like a set of “instructions” denoting how to line up a chain of AA to obtain a certain protein.
• ribosomes “read” mRNA nucleotides in groups of 3 (each group of 3 ribonucleotides = 1 codon)

Question 7

what is 1 codon?

Answer 7

• a string of 3 consecutive ribonucleotides (triplet codon)
• “codes” for only 1 type of amino acid (but 1 type of AA can be “coded” by many codons)

Question 8

unambiguous

Answer 8

• each codon specifies a single amino acid only

Question 9

degenerate

Answer 9

• a given amino acid can be specified by more than 1 codon
• this is the case for 18 out of 20 standard AA. only 2 AA are specified by only 1 codon

Question 10

start and stop codons

Answer 10

• for initiation/termination of a translation (does not code for any AA)

Question 11

DNA complementary base pairing

Answer 11

• Guanine (G) and Cytosine (C) - 3 H bonds
• Adenine (A) and Thymine (T) - 2 H bonds
• in RNA, Thymine (T) is changed to Uracil (U)
• each base can only bond with a specific base partner.

Question 12

chromosomal proteins

Answer 12

• histones
• nonhistone chromosomal proteins (not the focus of this syllabus)
• chromosomal protein + DNA = chromatin

Question 13

first level of chromosome organisation: nucleosome

Answer 13

• “beads on a string”
• involves histones
• 1 strand of DNA (double helix/2 complementary strands of nucleotides) wraps around 8 histone molecules + 1 more histone molecule attached on the outside = nucleosome

Question 14

Linker DNA

Answer 14

• DNA strand that links 1 nucleosome to the next
• can vary in length depending on the need of the cell at the time
• Shorter linker DNA = more tightly packed nucleosomes
• Longer linker DNA = less tightly packed nucleosomes

Question 15

How does DNA wrap around the core?

Answer 15

• the sugar-phosphate backbone of DNA and AA histones are able to form H bonds
• AA are the building blocks of protein/polypeptides - some chains have hydrogen atoms which allows for the formation of a H bond

Question 16

Further levels of organisation: nucleosomes packed on top of one another

Answer 16

• histone proteins (chains of AA) have “tails” that extend out from the DNA-histone cores
• allows the nucleosomes to interact with one another
• linker DNA connects the nucleosomes
• other nonhistone chromosomal proteins also help with packing the DNA

Question 17

Forms of chromatin in interphase (resting phase of nuclei, no cell division)

Answer 17

Heterochromatin and euchromatin

Question 18

what is heterochromatin?

Answer 18

• highly condensed form of chromatin
• 10% of genome in mammalian cells
• present in many locations along chromosomes
• concentrated in centromere (cell division) and telomere (DNA replication)
• does not contain genes as they are very tightly compact and resistant to gene expression (hard to tear apart to access the gene sequences)

Question 19

Why is there a need for heterochromatin if the gene sequences cannot be accessed?

Answer 19

• Centromere and telomeres play important roles
• prevent them from being damaged by the surroundings/mutated
• heterochromatin is hard to tear apart and acts as protection for the important sequences

Question 20

what is euchromatin?

Answer 20

• Less condensed form of chromatin
• majority of genome in eukaryotic cells
• Contains genes
• Can be easily unfolded and transcribed into RNA products

Question 21

Why can’t DNA remain compacted all the time?

Answer 21

• access to the DNA is required during replication, repair and gene expression

Question 22

Euchromatin can transform to heterochromatin if needed by the cell. Why?

Answer 22

• At different times, due to the fluctuating external environment, the cell’s needs may change
• thus, different sets of genes must be exposed to be expressed
• Euchromatin is able to transform to heterochromatin to control gene expression

Question 23

Why must nucleosome structures be re-modeled when a certain gene needs to be expressed?

Answer 23

• To better expose the section of gene needed to be expressed, the nucleosome structures can be re-modeled

Question 24

What are chromatin remodelling complexes?

Answer 24

• PROTEINS
• loosens DNA-histone contacts
• changes positions of nucleosomes along DNA (depending on whether DNA needs to be expressed/accessed by other proteins)
• Re-form nucleosomes when access to DNA is no longer required
  *comes into play when euchromatin transforming to heterochromatin / nucleosomes are being re-modelled