Biology 067 –Section 15 Gene Expression

A. DNA and RNA Structure and Function

RNA and DNA sectn is basically a review of Section 5: Macromolecules with more detail...

1. DNA (deoxyribonucleic acid) = genetic material found in chromosomes in nucleus

a. Function of DNA:

replicate so can be transmitted to next generation

Stores info, codes so AA can form proteins

Undergoes mutations to increase variability

b. Structure of DNA

Double helix

Composed of base pairs of nucleotides bonded together by hydrogen bonds as ‘rungs’ of ladder

Complementary base pairs A-T, C-G (adenine-thymine, cytosine –guanine)

Nucleotides are composed of:

pentose sugar (5 carbon sugar) + sides of ladder

Phosphate + or backbone

Nitrogen containing base

Purine (2 rings) Pyrimidine (1 ring) - Adenine - Thymine

- Guanine - Cytosine

T

S

P

T

S

P

S

P

P

P

S

P P

S

P

S

S

S

S

S

S

S S Ladder structure

Double helix

P

S

P

S

P

P

T A

G C

T A

C G

A T

c. Replication of DNA

For cell division, each cell needs exact copy of parent

DNA

1. Double stranded helix “unwound” by an

enzyme

-breaks hydrogen bonds between

complementary base pairs

2. new base pairs formed as new complementary

nucleotides bond to each parent strand by DNA

polymerase enzyme

3. another enzyme seals any breaks in the sugar-

phosphate backbone

4. replication is semiconservative – each daughter double helix contains 1 parent strand

Replication error

Either fixed by repair enzymes

Or if it persists = mutation = permanent change =genetic variation

2. RNA (ribonucleic acid)

a. Structure and Function of RNA

RNA = ribonucleic acid

Composed of: ribose -5 carbon sugar

o -phospate

o -nitrogen base – A,C,G and Uracil replaces T

Single stranded

Helps DNA code for proteins

b. Types of RNA

1. Ribosomal RNA (rRNA)

-produced in nucleolus from DNA

-joins with proteins to form large and small ribosome subunits

-subunits leave nucleus through nuclear pores to cytoplasm of cell

-either attach to endoplasmic reticulum or stay free in cytoplasm

-large and small subunits join to form complex –acts as workbench for making proteins

2. Messenger RNA (mRNA)

-produced in nucleus from DNA template

-carries genetic info from DNA to ribosomes

-Linear molecule that acts as the code for order of aa in protein synthesis

3. Transfer RNA (tRNA)

-produced in nucleus from DNA

-in cytoplasm - transfers aa to ribosomes for protein synthesis

-Specific tRNA for each aa (20 aa in total – so at least 20 different tRNA’s)

4. small RNA’s

-there are several types - act as helpers, inhibitors (stop translation), or markers for destruction

(prepares mRA for degredation)

3. RNA vs DNA

Compare and Contrast of DNA and RNA

DNA RNA

Strands Double stranded Single stranded location Found in nucleus and mitochondria Found in Nucleus and cytoplasm Sugar deoxyribose ribose Bases Bases are ACGT ACGU Function

Transcribed to give RNA (RNA synthesis) Translated to give proteins (protein synthesis)

B. Gene expression

DNA provides the cell with a blueprint for making proteins

Basically: DNA acts as a template to make RNA, which in turns acts as a template to make

proteins - and the proteins in turn have specific functions

Requires 2 steps: Transcription and Translation

I. Introduction to gene expression

a) Structure and function of proteins (review)

Proteins:

formed from aa

different proteins have different # and order of aa’s

sequence of aa = certain shape due to bonding between aa

structure dictates function:

1o – linear sequence of aa

2o – helix or pleated sheet

3o – globular structure, 3D shape

b) Gene expression

from Genetic code

each aa matches a triplet code of bases A,G,T,C that were transcribed to mRNA and tRNA to

have 64 potential triplets originated from DNA

each 3 base combination (triplet) = codon of mRNA

61 triplets match an aa > 1 for each aa

3 triplets are stop codons: UAA, UAG, UGA

1 triplet = start codon AUG – start polypeptide formation

2. Transcription

a) Intro

DNA forms mRNA in nucleus

mRNA is complementary copy of DNA strand

DNA forms template for RNA to form – (all RNA is formed this way)

b) mRNA Formation

1. RNA polymerase (enzyme) opens up DNA helix

2. Joins mRNA nucleotides that have complementary base paired to one

DNA strand

3. mRNA processing (in humans) - 1o mRNA = mature mRNA

DNA sequence has introns (part of DNA that are not part of gene

being expressed – this part does not code for a functional protein)

3

5

C C G

T A A T

T A G C

C G A T

C A T C G T A

RNA

polymerase

In between are exons – parts of

mRNA being expressed to result

in a protein

Processing – one end of mRNA

capped and tailed

- Introns removed

- Exons are joined by

mRNA splicing by

ribozyme enzyme (called

ribozyme instead of

enzyme because it is the

RNA(a small RNA) that is

functioning as the

enzyme instead of, as

usual, the protein)

- A mature mRNA is

formed – made up of all

exons (no introns)

3. Translation

a) Intro

Happens in cytoplasm at ribosomes

Synthesizes proteins from sequence of aa according to mRNA sequence of nucleotides.

3 steps: initiation, elongation, termination.

tRNA bring aa to ribosome – tRNA has an anticodon end to bind to complementary base in

mRNA and an end to bond one aa

translation happens on a translation complex made of a smaller ribosomal subunit and a larger

ribosomal subunit.

The ribosome has special binding sites called A and P sites.

5 3

3

3

3

5

5

5

DNA

exon exon exon

transcription

intron intron

pre-mRNA

exon exon exon

intron intron

poly-A tail cap intron intron

exon exon exon

exon exon exon

poly-A tail

pre-mRNA

splicing

cap

intron RNA

mRNA

poly-Atail cap

nuclear pore

in nuclear envelope

nucleus

cytoplasm

spliceosome

b) Stages of Translation

1. initiation:

- mRNA binds to the smaller ribosomal subunit – then the larger subunit joins the smaller and

forms the translation complex

2. elongation: occurs in 2 stages

- polypeptide elongates by 1 aa at a time

- tRNA-polypeptide is at the P site and a tRNA amino acid is at the A site – the polypeptide will be

transferred to the tRNA aa.

- Ribosome moves one triplet to the right on mRNA, free A site and new tRNA and polypeptide at

P site.

3. Termination

- Occurs when stop codon on mRNA reached.

- Ribosome breaks apart in 2 subunits and falls off mRNA molecule. Polypeptide released.

4. Coding of DNA to mRNA and tRNA to mRNA: an example:

Triplets on DNA: TAC AGC TGA GGA

Codon on mRNA: AUG UCG ACU CCU

Anti-codon on tRNA: UAC AGC UGA GGA

5. Regulation of Gene expression

All cells have all genes, but only some genes are expressed – ie., nerves cells will not express genes for

skeletal muscle therefore controlled expression of genes occurs to form specialized cells

There are 5 primary levels of control:

1. Pre-transcriptional control: Chromatin organization – before gene can be transcribed

chromosome must be uncondensed and proteins and chemical modification that protects DNA

must be removed before transcription can occur.

2. Transcriptional control: in nucleus – a # of mechanisms regulate which genes transcribed and

rate of transcription including things called transcription factors (or activators) which are DNA

binding proteins - every cell has many different types of these factors and a specific

combination of transcription factors is believed to regulate activity of a particular gene– once

right combination binds to DNA, an RNA polymerase attaches to DNA and begins transcription

by unzipping double helix - As cells mature, they become specialized – specialization determines

which genes are active and therefore which transcription factors are active.

3. Post transcriptional control: in nucleus

- Occurs after DNA is transcribed and after mRNA is formed.

- It is how mRNA is processed before leaving nucleus and how fast it is made which can effect

amount of gene expression.

4. Translational control: in cytoplasm

- Occurs after mRNA leaves nucleus and before protein synthesis begins

- Life expectancy of mRNA varies

- Ability of mRNA to bind to ribosomes varies

- Some mRNA may also need other changes before translation will occur.

5. Post translational control: in cytoplasm

- Occurs after protein synthesis

- Polypeptide may need to go thru other changes before it becomes functional.

6. Diagram of process of gene expression – note next page

Some YouTube to watch - you can google lots more…

https://www.youtube.com/watch?v=41_Ne5mS2ls

https://www.youtube.com/watch?v=rKxZrChP0P4

3.mRNA moves into cytoplasm and becomes associated with ribosomes.

Translation 1. DNA in nucleus serves as a template for mRNA.

2. mRNA is processed before leaving the nucleus.

primary mRNA

mature mRNA

DNA

TRANSCRIPTION

introns

exons

C C G G

G U A

U U U

4. tRNAs with anticodons carry amino acids to mRNA.

nuclear pore

mRNA

peptide

amino acids

5. Anticodon–codon complementary base pairing occurs.

anticodon

codon ribosome

A A A

large and small ribosomal subunits

tRNA

6. Polypeptide synthesis takes place one amino acid at a time.