14-unit4_chromatin_transcription_oct7_post.pptx

8/10/2019 14-Unit4_Chromatin_transcription_Oct7_post.pptx

1/28

Welcome to Biology 200 Section 101

Dr. Marcia GravesMWF 10-10:50

Wesbrook 1001


2/28

Unit 3: Nuclei and Chromatin

Three topics:

1. The interphase nucleus

2. Nucleic acids review (self-study)3. Chromatin and chromosomes

2


3/28

1. Naked DNA (2nm)

Levels of DNA packing in the nucleus

2. Nucleosomes (10-11nm)

3. Chromatin (30nm)


4/28


3. Chromatin (30nm)

Fig. 5-21

A chain of nucleosomes can be arranged in a 30 nm fiber

formation of the 30nm fiber is dependent on the presence

of the H1 histone.

H1 pulls nucleosomes together to form a regular repeating

array of stacked nucleosomes

There are competing ideas about how the 30nm is arranged

This is the level of packing found in

the interphase nucleus


5/28

This is one model of the 30nm fiber

Where every 6th-8thnucleosome curl on themselves to

form a compacted structure

This interphase chromatin structure can be loosened by

remodeling complexes to allow expression of genes


3. Chromatin (30nm)


6/28

1. Naked DNA (2nm)



3. Chromatin (30nm)

Interphase chromatin canform higher-ordered loops

of the 30nm fiber


7/28

1. Naked DNA (2nm)



3. Chromatin (30nm)

Additional folding beyond the30nm loops, only happens

when chromatin condenses into

chromosomes during mitosis

4. Chromosome condensation


8/28



Recall: The structure of chromatin varies along a single interphasechromosome

Loops of 30nm chromatin Formed by non-histone

chromatin proteins that

form a scaffold


9/28



Mitotic chromosomes: After DNA replication, 2 copies of each

chromosome

It is poorly understood how mitotic

chromosomes become so compact

10 000 fold shorter than its length


10/28

Important structural components of every eukaryotic

chromosome

Most important function of chromosomes is to carry genes

Mitosis produces genetically identical daughter cells


11/28

Levels of organization:

From DNAgene-chromosome-genome

Human Genome: ~25000 genesonly

10% of total DNA!

23 pairs of

chromosomes

Multiple genes per

chromosomes Genes are a segment

of DNA that encodes

for a functional

productthrough

transcription!


12/28

Unit 4: From Gene to Protein

Three topics:

1. Nuclear FunctionRNA transcription

2. Nuclear FunctionRNA Processing

3. Genetic Code and Tranlation

12


13/28

Unit 4: Topic 1Nuclear Function RNA Transcription

Learning Objectives:


14/28Figure 7-1 Essential Cell Biology ( Garland Science 2010)

Biological Information Flow


15/28

RNAs as Intermediates in the Flow of Genetic Information


16/28

Anatomy of a Transcription Unit (gene): codes for RNA

A Transcription unit Consists of

TWO major parts:

Regulatory region Coding region

16


17/28

TranscriptionStart site

Transcription

Stop site5

53

3

Template strand

Coding strandEnhancer

regionPromoter

Regulatory region

Promoter: Regulatory region of DNA near the

transcriptional start site Binds RNA polymerase, transcription

factors

Enhancers/repressors : Bind activators and repressors to

control transcription

may be far away from the actual gene

Transcribed region

Coding region

A Transcription Unit Consists of TWO major parts:

Upstream Downstream


18/28

Fig. 7-2

Regulatory regions regulate gene expression:

Some genes are expressed a lot, some only a little

Different genes aretranscribed at different

rates

mRNA can be used to

direct the synthesis of

many identical protein

molecules (amplifies)

A cell can change how

much gene expressiondepending on its needs

Different tissues express

different genes, defines

cell specialization

(differentiation) 18

Regulatory region


19/28


Transcription

Stop site5

53

3

Template strand


regionPromoter

Regulatory region

Promoter: Regulatory region of DNA near the

transcriptional start site Binds RNA polymerase, transcription

factors

Enhancers: Bind activators and repressors to

control transcription

may be far away from the actual gene

Transcribed region

Coding region

A Transcription Unit Consists of TWO major parts:

Gene sequence

Code is transcribed with RNA

Polymerasemoving along the DNAtemplate 3 to 5

RNA is the gene product of

transcription

New ribonucleotides are added to

the 3 end of the growing RNA

Upstream Downstream


20/28

Type of polymerase Genes transcribed

RNA polymerase I Most rRNA genes

RNA polymerase II All protein-coding genes, plus some

genes for small RNAs (ie. those in

spliceosomes)

RNA polymerase III tRNA genes

55 rRNA gene

Genes for some small structural RNAs

The three RNA Polymerases in Eukaryotic Cells

20


Transcription

Stop site5

53

3

Template strand


regionPromoter Transcribed region

Table 8-1


21/28

RNA Polymerase reads the template strand

moving from 3 to 5

RNA

Polymerase II

5 end of RNA is

synthesized first

New ribonucleotidesare added on the 3 end

h h l h h h h


22/28

1. 3 end of transcript

2. 5 end of transcript

3. 3 end of template strand

4. 5 end of template strand

Match the letters on the micrograph with the

numbered terms.

A. D, C, B, AB. B, A, D, C

C. B, A, C, D

D. C, D, A, B

C

A

B D

DNA


23/28

A single gene (Transcription unit) visualized with TEM


Transcription

Stop site5

53

3

Template strand


regionPromoter Transcribed region

C

A

B D35


24/28

RNA polymerase reads 3 to 5 on the template DNA strand

The 5 end of the RNA is made first (due to complimentary base pairing)

Promoter region determines which DNA strand is the template

Template

strand


25/28

bl l h d ( ) b


26/28

Problem 4.1.2: Explain how adjacent transcription units (TU) can be

transcribed in different directions relative to the chromosome.

(Template strand always read 3 to 5)

A. Genes can invert due to viruses.

B. mRNA is usually produced from both strands

simultaneously within each transcription unit.

C. Genes can be encoded on either strand of the DNA.

D. mRNA synthesis can occur in either direction on the

template strand

E. Genes on either strand are complementary

Transcription can occur in both directions onProblem 4 1 2


27/28

Transcription can occur in both directions on

chromosomes.

Genes are transcribed on both DNA strands, but

are always read 3 to 5 Any one transcription unit is always transcribed in

the same direction.

RNA polymerase

Problem 4.1.2

How does transcription occur?


28/28

For next day.

1. Compare and contrast prokaryotic and eukaryotic

gene transcription

Give it some thought before class on

Wednesday

2. We will finish transcription and begin discussing

RNA processing

Pre-read on-line notes for Unit 4: Topic 2

14-unit4_chromatin_transcription_oct7_post.pptx

Documents