Genes Function: genes in action

Chapter 11 Gene FunctionChapter 11 Gene Function 11

Genes Function: Genes Function: genes in actiongenes in action

Genes in ActionGenes in Action

Chapter 11Chapter 11

Pp 385 - 417Pp 385 - 417

TOPICThe ‘role’ of genes:

Case Study: beta Thalassaemia

11.1Pgs: 385 – 388

then

Pgs: 397 - 400

22

Genes ‘n’ action Case Study:beta Thalassaemia

various kinds of haemoglobin are found in red blood cells

each kind consists of four protein chains each with an iron-containing heam molecule

the gene that controls the production of beta chains of haemoglobin a (one of the four chains!) is the hbb gene on chromosome 11

inheritance of the t gene is autosomal

t – normal, t - thalassaemia

absence of beta chains is an inherited disorder known as beta thalassaemia

Task: Complete Quick Check 1-333

HBB gene (Chromosome 11) has 1600bp consisting of Exons (coding for the proteins) and Introns.

In all, 146 amino acids are produced in a normal beta chain

Affected thalassaemia sufferers have a base substitution in which T is replaced by A

Base substituion affects the seventh codon in mRNA

Instead of AAG, UAG is produced. UAG codes for STOP

Quck Check Q’s: 14 & 15

44

TOPIC – PROTEIN SYNTHESISGenes in Action: Transcription & Translation

11.2Pgs: 388 – 402

GENE ACTION

When a gene becomes active (to produce

proteins!), it first makes a mobile copy of the coded

instructions that it contains

.

This occurs by a process known as transcription.

This mobile copy of a genetic instruction can leave

the nucleus and move to the cytoplasm where the

instruction is decoded. This occurs by a process

known as translation. So gene action involves two

processes: transcription and translation

Biozone 202

55

66

TRANSCRIPTION

Defn: Transcription is the process by which the code contained in the DNA molecule is transcribed (rewritten) into a mRNA molecule

transcription involves making a copy of the DNA template. A special molecule called messenger RNA (mRNA) is produced.

Consider a DNA template with the base sequence …

(3’) ATGCCTGAAT (5’)..

This DNA acts as a template to guide the formation of a mRNA molecule with the complimentary base sequence as follows:

(5’) UACGGACUUA (3’) ..

See Biozone pg 205


Steps in TRANSCRIPTION(5’) TACGGACTTA (3’) ..(3’) ATGCCTGAAT (5’)..

Purpose of Transcription!• to make a complimentary strand of mRNA!

At the start …

• DNA is made of two complimentary strands. one is the DNA coding strand (5 – 3)

(5’) TACGGACTTA (3’) ..(3’) ATGCCTGAAT (5’)..

the other is the DNA Template strand (3 – 5)

• At the start of Transcription the double stranded DNA molecule has to unzip and the template copied!

(5’) UACGGACUUA (3’) ..

1: Unzipping DNA…• enzyme known as RNA polymerase attaches to a region of DNA in the upstream region of the template strand

• The double stranded DNA of the gene unwinds and exposes the bases of the template strand.

99

3'

3'

5'

NascentRNA

TemplateStrand

5'

5'

3'

Coding

Strand

Elongation

Site

RNA Polymerase

Unwinding

Rewinding

Movement of polymerase

2. The base sequence of the DNA Template strand guides the building of a complimentary copy of mRNA. The RNA polymerase enzyme moves along the DNA template and as it moves (RNA) nucleotides are brought into place one by one to form a RNA chain

3. The single stranded RNA molecule called pre-messenger RNA (pre-mRNA) is fully complimentary to the original DNA Template molecule.

•the enzyme transcribes only a gene length of DNA at a time•only RNA polymerase is involved in mRNA synthesis •It is common to find more than one enzyme transcribing on the same length of gene

(but, we are not finished with Transcription!)R

Pre-mRNA is modified after Pre-mRNA is modified after transcriptiontranscription

4. The regions of the pre-mRNA that corresponds to the introns in the coding region of the gene are cut out, producing a shorter mRNA molecule.

(Splicing of pre mRNA is carried out by a complex known as splicosome, a complex consisting of RNA and protein)

5. The final mRNA molecule is chemically capped with a a methyl “cap” (AUG)

6. Finally, a long tail of ‘A’ (Adenine’s) called a poly-A tail is added to produce a fully functional mRNA strand that is ready for transport out of the nucleus.

Once mRNA is formed, it leaves as a small single

strand through the nuclear pores. Once in the

Cytoplasm the mRNA will engage ribosomes to begin the

next stage in protein synthesis

1111Quick Check 4 – 8 page 391

1212

TRANSLATIONDefn: The decoding of the genetic instructions occurs

through the process of translation which takes place in the cytoplasm.

by the end of this process the mRNA have been decoded and translated into a protein chain of amino acids.

Key Parts of Translation (see Table 11.3) mRNA Nuclear pore cytoplasms ribosomes tRNA (with anti codon) Amino acids – (forming polypeptides)


tRNA A key player in Translation

tRNA = Transfer RNA

Each tRNA consist of a single strand of 76 coiled RNA nucleotides

At one end of the tRNA molecule are three bases that make up an anti-codon (complimentary to a codon)

At the other end of the tRNA molecule is a region that attached to one specific amino acid

An enzyme, amino acyl tRNA synthetase catalyses the linking of each amino acid to a tRNA

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TRANSLATION – The mRNA leaves the nucleus through the pore and

attaches to sub-microscopic particles known as ribosomes

Each ribosome consists of two sub units

Inside the ribosome, the mRNA codon lines up with the tRNA’s anticodon (they are complimentary

Each loaded tRNA has an amino acid attached

As the mRNA moves through the ribosome, each codon is ‘read’ (translated)

As read, the mRNA moves one codon at a time and with each, a tRNA attaches its amino acid to the growing chain on amino acids!

1616

TRANSLATION – DECODING INSTRUCTIONS

Biozone 206

Task: Translating Code

1717

Using the table on the previous page….

1. Write the amino acid sequence from the following DNA template strand

tac gga gag gca cct act

_____ _____ _____ _____ _____ _____

2. Write the amino acid sequence if a spontaneous mutation occurred at the sixth nucleotide and cystosine was transcribed onto the pre mRNA strand instead of uracil _____ _____ _____ _____ _____ _____

What kind of mutation is the above?:

• Substitution or frameshift?• Silent, Nonsense or Missense

3. List the amino acid sequence that would result if an insertion of G occurred between the 11th and 12th base

_____ _____ _____ _____ _____ _____

1818

Protein Synthesis Summary

Biozone: – Review of Protein Synthesis

25 000 genes but more than 25 000 characteristics?

‘Alternative Splicing Theory’ 394-395

1919

How might one gene produce different proteins at different stages of development and in different tissues?

Alternative splicing: involves the retention of some introns and or juggling of exons in the prioduction of mRNA resulting in the production of different proteins.

1.Intron retention:

2.Exon Juggling

Quick Check 9 - 13

COMPARING PROKARYOTES & EUKARYOTES


Similar with some differences:

Characteristic ProkaryotesI.E. Bacteria

Eukaryotes

Location of genes Cytoplasm nucleus

age of mRNA Survives only a few minutes

Can survive for days

Post-transcriptional modification

Doesn’t occur Occurs (introns cut out, cap added, poly A tail added

Effect of tetracycline on transcription

Very effective. Prevents

transcription

No effect

(Nucleus protects!)

2121

TThalassaemia

Biozone– Inherited Metabolic Disorders

Case Study p397-399

Quick Check 14 - 15

2222

rRNA A key player in Ribosomes

rRNA = Ribosomal RNA

Key component of ribosome structure

Stored in nucleus as a nucleolus

Need lots of it

Genes on short arms of chromosomes 13, 14, 15, 21 and 22 code for the production of rRNA

The loci of the above are called nucelar organiser regions (NORs)

A secondary constriction or narrowing, marks the position of NOR on each of these chromosomes (see Figure 11.20

2323

Structural and Regulator genes - Page 402

- Genes vary in function:

- Production of RNA All genes produce RNA of some kind (mRNA, rRNA,

tRNA!)

Structural Genes: produce proteins that become part of the structure

and function of organisms

Regulator Genes Produce proteins that control the action of other

genes

Example: Homeotic Genes in insects; HOX genes in

Mammals.

Actions of Regulator Genes

1. Some produce DNA – Binding Proteins (directly switch genes on or off)

2. Some produce proteins that bind to receptor sites on membranes and function as ‘signalliny proteins

TOPICDNA REPLICATION

11.3Pgs: 403- 415

then

Pgs: 397 - 400

2424

2525

DNA Replication

Defn: The process by which DNA makes exact copies of itself is called DNA Replication

When: must occur prior to Interphase

Where: for Eukaryotes – nucleus

What Cells: Somatic cells (prior to Mitosis) Germ line cells (prior to meiosis)

Outcome: Two Double stranded DNA molecules from each one Each ‘new’ molecule consists on one strand of original DNA and

one copy! This is known as the semi-conservative model

2626

PROCESS OF DNA REPLICATION

Step One: Unwinding DNA Unwinds to form a region of single stranded DNA Unwinding controlled by enzyme called DNA helicase Occurs at many points along the DNA

Step Two: Copying Short RNA Primers attach to the open strands and initiate replication Each ‘open’ strand acts as a template for a new DNA strand Leading strand is formed one nucleotide at a time; Individual DNA nucleotides are added at the 3’ end according to base

pairing rules DNA polymerase is responsible for catalysing this reaction Lagging strand formedby fragments (chuncks) aided by enzyme DNA

Ligase

Result Each new copy of DNA is an exact copy of the original Each newdouble stranded molecule contains one of the original DNA

strands and one new DNA strand (semi conservative)

Biozone: 207 - 208

Quick Check 18 & 19

ACTIVE GENES

When are genes active?

Many genes remain active throughout the life of a person, eg genes controlling enzymes for cellular respiration.

Some genes are not expressed on a phenotype until a person is well into adulthood. Eg. Huntington disease.

Identifying active genes

Defn: Microarrays (DNA Arrays): technique used to Recognise ‘active’ genes using single stranded DNA

2929

Switching genes off!

Taken From Page 410

RNA interference (RNAi) provides a means of selectively targeting and silencing genes.

Small interfering RNAs (siRNAs) produced in cells are the active molecules in gene silencing.

How Does RNA Interference Work? p410

Case Study – Blue Roses at last! p411

Quick Check Questions 20-24 p412

Review Biozone – 203-204 Gene Expression


Genes in Action Review

Biochallenge Questions 1-5 p413

Chapter Review Questions

Add Key Words to Glossary List

Complete Questions 2-14: p414-417

Genes Function: genes in action

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