Question

1) Describe the experiments that showed all differentiated cells contain all the genetic instructions to direct the formation of a complete organism. 2) Describe the different DNA-binding motifs of gene regulatory proteins 3) What are the three different types of RNA and type of genes do they transcribe.

Answer 1

1) a) experiment- The nucleus from skin cell of an adult frog is transplanted into enucleated egg which can give rise to an entire tadpole.

Skin cells from an adult frog is aken into culture dish and its nucleus is injected into second enucleated egg using a pipette which turns into a normal embryo and later a tadpole. An unfetilized egg can also be taken and its nucleus is destroyed by uv light and is injected into egg which again becomes a normaal embryo and tadpole later. This is one experiment.

b) In many types of plants differentiated cells retain the ability to dedifferentiate so that a single cell can form a clone of progeny cells that give rises to an entire plant.

c) A differentiated cell is introduced from an adult cow into an enucleated egg from a different cow can give rise to a calf. calves produced are different from the same differentiated cell donor and are genetically identical and hence becomes clones of one another. ( either the epithelial cells from oviduct or an unfertilized egg removed from meotic spindle and associated chromosomes which forms the donor cell is placed into a second ennucleated egg and electric pulses causes the donor cell to fuse with enucleated egg cell. The reconstructed zygote divides and becomes embryo, and this embryo is placed to foster mothers and calf clones are obtained.

2) different DNA binding motifs include-

a) helix turn helix b) the homeodomain c) leucine zipper d) helix loop helix e)zinc fingers of several types

a) - most common motif found in prokaryotes

- present in transcription repressors and activators

-dna recognition helix gets inserted in the major groove of DNA.

-helix turn helix proteins are dimeric with 2 recognition helices recognizing 2 adjacet dna sequences.

-dimeric because it binds to dna stronger than monomer

-by changing relative positions of monomers dimeric activity can be turned on and off.

b) -homeobox genes contain 180 base pairs of dna sequences that provide instruction for makin string of 60 amino acids protein building blocks called homeodomain

-they act a stranscription factors

-binds to dna and control transcription of other genes in cell

-they initiates patterns of gene expression.

c) - forms amphipathic alpha helix and coiled coil dimer

-they dimerize as homo or hetero dimers

-basic recongnition is the dna binding site

-basic region is a pair of helices that wraps around major groove.

d) - protein structural motif that forms a family of transcription fcators

-one helix is smaller and due to flexibility of loop it allows dimerization by folding and packaging against another helix.

-larger helix contains dna binding regions. They typically bind to consensus sequence called E box. -CACGTG

-their transcription factors are dimeric each with 1 helix containing amino acid residues that facilitate dna binding.

- eg- 1-CLOCK, N-Myc etc..

e) - 3D structural motif of protein

- characterised by coordination of one or more Zn ions

-Zn ion is kept by 2 cysteine and 2 histidine R group

-present as tandem repeats

-interacts with dna and rna

-zinc ion holds the structure together

-each motif consists of 30 amino acid and folds into alpha and beta structure.

-each finger binds to triplet within dna substrate.

3) a) m RNA- messenger RNA- encodes amino acid sequence of a polypeptide

b) t-RNA- transfer RNA- it brings amino acid to ribosomes during translation

c) r-RNA- Makes ribosomal proteins and make sup the ribosomes the organelles that translate the mRNA.