From DNA To Proteins

Question 1

Bp

Answer 1

“base pairs”

so when describing DNA sizes it refers to both strands

Question 2

kDa

Answer 2

KiloDaltons

One Dalton is the mass of an H atom, or 1/12 of a C 6
atom.

Question 3

S

Answer 3

Svedberg unit, and refers to the mass and shape of cellular organelles.

Question 4

High S means

Answer 4

Larger mass

Question 5

S values are not additive

Answer 5

e.g. bacterial ribosome (70S) consists of 50S and 30S subunits

Question 6

Human DNA is

Double-stranded with a complementary chain

Present in cells at all time

Answer 6

Human RNA is single stranded, and any duoble stranding is usually with itself

Types of RNA mRNA, rRNA, tRNA with different functions

Many mRNA species only accumulate following cell stimulation

Question 7

mRNA

Answer 7

Messenger RNA (mRNA) is printed as a long linear transcript

It then processed to the mature form (in proximity of the nuclear membrane)

It has a 5’CAP and a 3’ Poly A tail

Question 8

Ribosomes are abundant in eukaryotic cytoplasm.

What forms ribosomes?

Answer 8

Four main types of rRNA combine with proteins to form 80S ribosomes.

Question 9

How are all the types of RNA linked?

Answer 9

Ribosome (rRNA) prints transcript (mRNA) with help from transfer RNA (tRNA)

Question 10

tRNA

Answer 10

tRNA carry amino acids to ribosomes, and check that they are incorporated in the right poistion

Each tRNA only carries one aa&raquo_space;> at least 20 tRNA types

Very small molecules

At the anticodon, a triplet sequence pairs with mRNA
»> right aa for the right triplet ( = CODON)

Question 11

The global picture

Answer 11

Slide 18

Question 12

Regions of the gene

Answer 12

Slide 20

Question 13

tRNA binds to a codon on the mRNA

Answer 13

tRNA contains an amino acid at its 3’ end corresponding to the codon on mRNA to which the anticodon of the tRNA can base pair.

Question 14

The code is :

Answer 14

• Degenerate , but unambiguous
– Many amino acids specified by more than one codon – But each codon specifies only one aminoacid

• Almost universal
– All organisms use the same code – Fewer that 10 exceptions

• Non-overlapping and without punctuation – Codons do not overlap
– Each nucleotide is only read once

Question 15

Factors initiating gene expression

Answer 15

• Proteins called “transcription factors” find their way into specific sequences 5’ of the 1st exon (region called “promoter”)

• A “transcription complex” forms around the TATA box 5’ of 1st exon

• Helix opens, DNA strand separation.

• RNA Pol II starts building mRNA
• Etc (as described before)

Question 16

TATA box

Answer 16

In molecular biology, the TATA box (also called the Goldberg–Hogness box)[1] is a sequence of DNA found in the core promoter region of genes

Question 17

Factors turning off expression

Answer 17

• Activation of repressors (inhibitors of RNA polymerase binding)
• Each step of RNA transcription or processing finds no longer actively produced transcription and processing proteins
• Complexes do not form anymore for lack of phosphorylation
• Enzymes no longer activated
• RNA stability
• Many other unknown mechanisms

Question 18

Process of DNA TRANSCRIPTION AND THE PRODUCTION OF PROTEINS

Answer 18

• Proteins called transcription factors (proteins which bind to promoter regions) find their way to specific sequences on the 5’ of the 1st exon (region call the promoter)

• Promoter has a specific sequence of nucleotides - they do not code for proteins but instead act as binding sites - found at the 5’ end.

• A ‘transcription complex’ forms around the TATA box (reads thymine, adenine, thymine, adenine etc.) on the 5’ of the 1st exon:

Question 19

Steps of transcription and protein production

Answer 19

1. Topoisomerase unwinds the double helix by relieving the supercoils. DNA helicase then separates the DNA apart exposing the nucleotides. SSB’s coat the single DNA strands to prevent DNA re- annealing
2. Free mRNA nucleotides line up next to their
   complementary bases on the template strand/ antisense strand of DNA ( U-T & C-G).
3. RNA polymerase (specifically RNA polymerase 2) joins
   the mRNA nucleotides (catalysing phosphodiester bonds between them) to form and antiparallel mRNA strand ( with a 5’CAP head and a 3’Poly A tail) - starting at a promoter (specific sequence that RNA polymerase binds to - initiation of transcription. Transcription is stopped at the stop codon)
4. mRNA leaves the nucleus and attaches to an 80s
   ribosome
5. At ribosome the mRNA (bases on mRNA are read in 3 - codon) sequence is used as a template to bind to complementary tRNA molecules at their anticodon (3 bases complementary to codon on mRNA). Ribosome reads mRNA codon by codon, one codon will code for a particular amino acid. This amino acid is brought by a specific tRNA molecule (carried on it 3’ end) since tRNA molecules are attached to specific amino acids. BASES ARE READ 5’ TO 3’
6. Enzymes remove amino acid from tRNA and amino acids are linked together by a
   peptide bond (created by a condensation reaction), creating a polypeptide chain - a protein

Question 20

How is the ribosome involved in translation

Answer 20

• Ribosome recognises mRNA from its CAP on the 5’ end
• Ribosome is responsible for translation

Question 21

Promoter region

Answer 21

is what RNA polymerase recognises and where it starts

Question 22

mRNA primary transcript

Answer 22

mRNA strand is a complementary cop of original DNA

Question 23

Silencing genes

Answer 23

• DNA can be chemically altered to the methylated form i.e turned on or off (silenced)
• In a macrophage, where immunoglobulins are not produced, but still has normal DNA, the gene for producing immunoglobulins will be in the heterochromatin (can remember as h = hiding i.e. not active) state -
since its not needed to be synthesised thus in the heterochromatin state no transcription of these gene can occur
• Whereas in a B cell, that needs to produce immunoglobulins, that gene will be in the euchromatin state - so it can be transcripted and thus immunoglobulins (proteins) can be synthesised

Question 24

Factors running off gene expression

Answer 24

• Activation of repressors (inhibitors of RNA polymerase binding)

• Enzymes no longer activated

• Transcription and processing proteins required for RNA transcription and or processing are no longer produced