7 8 9. Control of Gene Expression and dev gen

1. Control of Gene Expression in Prokaryotes (18) and Eukaryotes (19)

Viruses

•

Structure:

– _{Smaller than a ribosome}

– _{Nucleic acid (the genome) enclosed in protein coat}

(called a capsid)

• _{Sometimes they are also enclosed in a membranous}

coat; this is composed of membrane derived from host cell and aids in infection

• _{Genomes may be double or single stranded DNA, or}

•

http://student.ccbcmd.edu/courses/bio141/lecg

uide/unit3/viruses/adlyso.html

Viral life cycles

Even simpler than viruses…

•

Viroids:

– _{circular RNA molecules}

– _{Infect plants; do not code for proteins}

•

Prions

Bacterial Genetics

•

Biggest source of variation: Mutation

– 1 cell to 1,000,000,000 cells in 12 hours

•

Other sources of variation:

1. Transformation: bacteria takes up naked DNA

http://www.phschool.com/science/biology_place/labbench /lab6/concepts1.html

2. Transduction: transfer of DNA by viruses

http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/ genetics/recombination/transduction/spectran.html

3. Conjugation: DNA transferred from donor to recipient cell via conjugation bridge

– _{In prokaryotes, genes for related enzymes often}

controlled together in operons

• _{Regulatory proteins bind to operons and turn them on or off in}

response to environmental changes

Gene Regulation in Prokaryotes: Operons

DNA mRNA DNA Protein mRNA Protein Lactose

Promoter Operator Lactose-utilization genes

Active repressor

RNA polymerase cannot attach to promoter

RNA polymerase bound to promoter

Inactive

repressor Enzymes for lactose utilization OPERON

Operon turned off (lactose absent)

Operon turned on (lactose inactivates repressor) Regulatory gene

virtual cell la c operon

annoying voi ce lac

Chromatin

1. DNA wound around histones

2. Nucleosomes/linker DNA interact

3. Looped domains attach to protein scaffold

(prophase)

4. Further folding (metaphase)

Coordinating Eukaryotic Gene Expression

To make several

proteins for a single

metabolic process,

make all those

proteins at the same

time!

Regulation of Gene Expression in Eukaryotes

– _{Different types of cells make different proteins}

because different combinations of genes are active

Muscle cell Pancreas cells Blood cells

Regulation of Gene Expression: DNA Packaging

– _{DNA packing can block gene}

expression

• _{May prevent access of}

transcription proteins to the DNA

• _{Heterochromatin- ALWAYS too}

tightly compacted to be transcribed

– _{Telomeres, centromeres!!!}

• _{Euchromatin- less tightly packed}

• _{Modification of histones can}

uncoil OR further condense the chromatin

– _{Methylation of DNA can also}

block transcription DNA double helix (2-nm diameter) Histones Linker Nucleosome (10-nm diameter)

Regulation of Gene Expression: Transcription

Factors

• _{Activators bind to}

enhancers

• _{Mediator proteins}

bind activators and proteins at the

promoter

• _{Repressors- affect}

Regulation of Gene Expression: Alternative

Splicing

•

Eukaryotic RNA may be spliced in more than

one way

– _{alternative splicing may generate two or more}

types of mRNA from the same transcript

DNA

RNA transcript

mRNA

Exons

or RNA splicing

Regulation of Gene Expression: Degradation

of mRNA

– _{The lifetime of an mRNA molecule helps}

determine how much protein is made

• _{mRNA may contain info in the 3’ UTR that allows}

Regulation of Gene Expression: microRNA

•

Small pieces of RNA can base-pair to mRNA,

blocking translation OR trigger mRNA decay

– _RNAi

• _{RNA interference}

Regulation of Gene Expression: Initiation of

Translation

• _{Specific proteins can bind to the “upstream” or}

“downstream” UTR

• _{Poly-a tail lengthened only when mRNA needed (else}

too short to initiate translation)- found in some egg cells

• _{The proteins required for translation to occur may be}

Regulation of Gene Expression: Activation of

the Protein

•

After translation is complete

p

olypeptides may

require alteration to become functional

Folding of polypeptide and

formation of S—S linkages Cleavage

Regulation of Gene Expression: Lifespan of

the protein

•

Some proteins

a

re broken down within a few

minutes or hours

– _{Tagged with ubiquitin}

– _{Proteasomes recognize tag; degrade protein}

Eye

Antenna

Leg

S

E

M

5

0



Genetics of development

•

Early cells: Totipotent-

can develop in to ANY

type of cell

– _{Genomic equivalence- all}

cells have the same genes

– _{Pluripotent cells: can}

become a limited number of cell types (adult stem cells)

– _{(multipotent, omnipotent)}

Embryonic stem cells Adult stem cells

Pluripotent cells Totipotent cells Cultured stem cells Different culture conditions Different types of differentiated cells

Liver cells Nerve cells Blood cells

Embryonic development

•

In most organisms, a single-celled zygote gives

rise to cells of many different types

•

Development involves three processes:

1. cell division

2. cell differentiation

– _{Cells become specialized in form and function}

How does differentiation happen?:

Transcriptional

regulation

of gene expression during development

• Cell determination: expression of genes for

tissue-specific proteins

• These proteins allow cells to carry out specific tasks

Nucleus Embryonic precursor cell

DNA

OFF OFF

Master control gene myoD Other muscle-specific genes

mRNA OFF Determination Myoblast (determined) MyoD protein (transcription factor) Differentiation Muscle cell (fully differentiated) mRNA MyoD

mRNA mRNA mRNA

Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation

•

Two sources of info “tell” a cell which genes to

express

1. Cytoplasmic determinants: RNA and proteins

coded for by mom’s DNA found in the egg cell

– _{Unequally distributed around the egg}

2. The environment around the cell

LE 21-11a

Sperm

Molecules of a cytoplasmic

determinant _{Fertilization}

Nucleus

Molecules of another cytoplasmic determinant Unfertilized egg cell

Zygote

(fertilized egg)

Mitotic cell division

Two-celled embryo

Cytoplasmic determinants in the egg

Early embryo (32 cells) Signal transduction pathway Signal receptor Signal molecule (inducer) NUCLEUS

Genetic Analysis of Early Development

•

Study of developmental mutants laid the

I eats you

Pattern formation in animals and plants

Laying of egg Egg shell Multinucleate single cell Early blastoderm Plasma membrane formation Late blastoderm Yolk Body segments Cells of embryo Segmented

embryo _{0.1 mm}

Hatching Larval stages (3)

Pupa

Metamorphosis

Adult fly

Head Thorax Abdomen

0.5 mm BODY AXES Dorsal Ventral Posterior Anterior

•

Pattern formation is the

development of a spatial

organization of tissues and

organs

– _{It occurs continually in plants,}

but it is mostly limited to

embryos and juveniles in animals 1. Maternal effect genes (egg

Pattern Formation in Drosophila

• _{http://bcs.whfreeman.com/t}

helifewire/content/chp19/19 02003.html

(out of class)

2. Bicoid & other egg

polarity proteins then

regulate the expression

of segmentation genes

• Gap genes

• Pair rule genes

3. Homeotic genes,

the

next set of genes

activated, determine

the anatomical identity

of the segments

– Highly conserved

(similar among many members of the

LE 21-14a Head Tail Tail Tail Wild-type larva

Mutant larva (bicoid)

Drosophila larvae with wild-type and bicoid mutant phenotypes

Developing egg cell Bicoid mRNA in mature unfertilized egg

Nurse cells Egg cell

bicoid mRNA

Bicoid protein in early

embryo

Fertilization

Translation of bicoid mRNA 100 m

Anterior end

Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo

Homeotic genes and their evolution

• _{Homeotic genes : master}

regulatory genes in animals

– _{Contain nucleotide}

sequences, called

homeoboxes (hox), that are very similar across species

– _{Code for transcription factors}

– Plants do not appear to have

genes that contain

homeoboxes AND code for major regulatory proteins

Adult fruit fly

• _{A mutation can change a}

proto-oncogene into an

oncogene

– _{Proto-Oncogene: a normal gene}

that promotes cell division

– _{Oncogene: causes cells to divide}

excessively

• _{A mutation can affect a tumor}

suppressing protein

• _{Multiple genetic changes}

usually underlie the development of cancer

– _{Six mutations required for colon}

cancer

The genetics of cancer

Tumor-suppressor gene Mutated tumor-suppressor gene

Normal growth-inhibiting protein Cell division under control Defective, nonfunctioning protein

Cell division not under control

Figure 11.16B

Think Pair Share

• _{Why is it important the gene expression is regulated?}

Come up with at least TWO answers

• _{Without looking at your notes, come up with TEN}

ways gene expression is regulated in eukaryotes

• _{Develop a made up operon for a made up set of genes} • _{Develop a made up virus- describe its structure and}

lifecycle

• _{What two sources of information affect cell}

21.1-21.2 in another ppt- yay!

Genome size/ # genes/ gene density

•

Size: eukaryotic genomes tend to be larger

than prokaryotic

•

# of genes: eukaryotes tend to have more

genes than prokaryotes

•

Density of genes in a given segment of DNA:

“Junk” DNA?

• _{1.5%“coding” DNA in}

animals (mRNA exons, tRNA, rRNA)

• _{Gene fragments and}

pseudogenes- rendered

nonfunctional by mutations

• _{Repetitive DNA} – _{75% of it is}

transposable elements

Transposable elements

• Found in both prokaryotes and eukaryotes

• Transposons

• Cut and paste • Copy and paste

• Retrotransposons (42% of human genome!)

• Move by means of RNA intermediate

Multigene Families

•

rRNA gene: many identical copies

– _{Increases rate of rRNA production}

•

Globin genes: many nonidentical copies

– _{Allows for different forms of globin proteins to be}

made at different stages of development

Big Huge Changes to DNA

•

Polyploidy- often lethal, sometimes a fast track

to a new species

•

Chromosome mutations

– _{Fusion of chromosomes!}

•

Duplication of chromosomal segments

– _{Unequal crossing over}

– _{Can lead to evolution of genes with related functions;}

occasionally one copy acquires a novel function

Conservation of Genes

•

Homeotic genes (Hox genes)

– _{Contain 180 nucleotide sequence called a}

homeobox

• _{60 amino acid homeodomain}

– They’re not flies- they’re little people with wings!

– _{Mostly associated with development}

– _{Small differences in these genes lead to big}

EXCITING DISCOVERY MADE IN THIS

CHAPTER