3.4.3 - Cell Differentiation

Question 1

How Can Stem Cells Become Specialised?

Answer 1

Through differential gene expression, where only certain genes in the DNA of a the stem cell are activated and get expressed.

Question 2

Describe The Steps Of Differentiation In Gene Expression
(6 Points)

Answer 2

~ Some genes are activated, some aren’t.

~ mRNA is transcribed from the active genes only.

~ mRNA is then translated to form proteins.

~ These proteins modify the cell, determining the cell structure and control cell processes.

~ As these proteins continue to modify the cell, the cell gets increasingly specialised.

~ These changes are irreversible, so the cell stays specialised.

Question 3

What Controls Gene Expression?

Answer 3

Transcription factors.

Question 4

What Are Transcription Factors?
(2 Points)

Answer 4

~ Protein that controls transcription of genes by binding to a specific region of DNA.

~ Activators and repressors are transcription factors.

Question 5

What Are Activators?

Answer 5

Transcription factors, that increase the rate of transcription, by helping RNA polymerase bind to the DNA and begin transcription.

Question 6

What Are Repressors?

Answer 6

Transcription factors, that decrease the rate of transcription, by preventing RNA polymerase from binding.

Question 7

What Do Transcription Factors Bind To If It Is A Eukaryotic Cell?

Answer 7

Promotor region of a gene.

Question 8

Describe The Process Of Cell Differentiation In A Eukaryotic Cell
(3 Points)

Answer 8

~ Binding of transcription factors to the promoter region, can either allow or prevent the transcription of the gene.

~ Transcription factors interact with RNA polymerase, either by assisting RNA polymerase binding to the gene. Or by preventing it by inhibiting gene expression.

~ Presence of the transcription factor will either increase or decrease the rate of transcription.

Question 9

What Do Transcription Factors Bind To If It Is A Prokaryotic Cell?

Answer 9

Operons.

Question 10

What Is An Operon?

Answer 10

Section of DNA.

Question 11

The Operon Is A Section Of DNA, What Does This Contain?
(3 Points)

Answer 11

~ Cluster of structural genes that are transcribed together, coding for useful proteins e.g. enzymes.

~ Control elements, including a promotor region (RNA polymerase binds to) and an operator region (Transcription factors binds to).

~ Regulatory genes, coding for activators and repressors.

Question 12

Describe The Lac Operon
(3 Points)

Answer 12

~ Controls the production of the enzyme lactase.

~ Lactase breaks down the substrate lactose, so that it can be used as an energy source in the bacterial cell. E.g. In the case of e.coli.

~ Lactase is only synthesised when lactose is present, helping bacteria avoid wasting energy and materials.

Question 13

Describe The Structure Of The Lac Operon
(4 Points)

Answer 13

~ Promoter for structural genes (SG).

~ Operator.

~ Contains 3 structural genes -> lacZ, lacY and lacA -> produce proteins that help the bacteria digest lactose.

~ Left of the lac operon is promotor for regulatory gene and regulatory gene lacl that codes for the lac repressor protein.

Question 14

Describe The lac repressor Protein
(3 Points)

Answer 14

~ Has two binding sites that allow it to bind to the operator in the lac operon and lactose.

~ When it binds to the operator, it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter.

~ When it binds to the lactose, the shape of the repressor protein distorts and the repressor protein can no longer bind to the operator.

Question 15

Describe What Happens When Lactose Is Absent, In The Lac Operon
(4 Points)

Answer 15

~ Regulatory gene is transcribed and translated to produce lac repressor protein.

~ lac repressor binds to the operator region.

~ Due to the presence of the repressor protein, RNA polymerase is unable to bind to the promotor region.

~ Transcription of structural genes does not take place, no lactase enzyme is synthesised.

Question 16

Describe What Happens When Lactose Is Present, In The Lac Operon
(4 Points)

Answer 16

~ Lactose binds to the repressor protein, distorting its shape so that the repressor protein cannot bind to the operator region.

~ RNA polymerase is then able to bind to the promoter region and transcription takes place.

~ mRNA from all three structural genes is translated.

~ Enzyme lactase is produced and lactose can be broken down and used for energy.