Eukaryotes and Eukaryotes and ProkaryotesProkaryotes

Key Differences in Protein Key Differences in Protein SynthesisSynthesis

ProkaryotesProkaryotes

• Prokaryotes lack a nuclear Prokaryotes lack a nuclear membrane so mRNA molecules do membrane so mRNA molecules do not need to be completely formed not need to be completely formed before crossing a nuclear membranebefore crossing a nuclear membrane

• Therefore Therefore translationtranslation begins begins immediatelyimmediately

Coupled Transcription and Coupled Transcription and TranslationTranslation

• The ribosomes are The ribosomes are visible along the visible along the mRNA translating it mRNA translating it into protein into protein immediately upon immediately upon it being producedit being produced

Observe A Immediate Observe A Immediate translation translation

Transcription in ProkaryotesTranscription in Prokaryotes

• Genome is circularGenome is circular

• Coupled with translation Coupled with translation

• Lack of introns (non-coding regions) Lack of introns (non-coding regions) means no excision means no excision

Translation in ProkaryotesTranslation in Prokaryotes

• Ribosomes are smallerRibosomes are smaller• Ribosomes recognize the start of an Ribosomes recognize the start of an

mRNA transcript by a unique sequence mRNA transcript by a unique sequence of purine-rich bases (Shine-Dalgarno of purine-rich bases (Shine-Dalgarno sequence)sequence)

• Commences with tRNA for methionine Commences with tRNA for methionine entering the P site of the ribosomeentering the P site of the ribosome

• The methionine is tagged with a formyl The methionine is tagged with a formyl groupgroup

EukaryotesEukaryotes

• Genome is large and arranged in Genome is large and arranged in chromosomeschromosomes

• Has coding and non-coding regionsHas coding and non-coding regions• No operons presentNo operons present• Ribosomes are larger and recognise Ribosomes are larger and recognise

5’ cap on mRNA as binding site5’ cap on mRNA as binding site• Translation occurs in cytoplasmTranslation occurs in cytoplasm• See chart on page 265 for summarySee chart on page 265 for summary

Endosymbiotic Relationships Endosymbiotic Relationships Between Organelles and CellsBetween Organelles and Cells

• EndosymbioticEndosymbiotic means physical and means physical and chemical contact between one chemical contact between one species and another species living species and another species living within its body, usually beneficial to within its body, usually beneficial to one of the species.one of the species.

Main IdeaMain Idea

• Eukaryotic cells may have evolved Eukaryotic cells may have evolved from an endosymbiotic relationship from an endosymbiotic relationship between the prokaryotic ancestors of between the prokaryotic ancestors of mitochondria and host prokaryotic mitochondria and host prokaryotic cellscells

Mitochondria Resemble Mitochondria Resemble Prokaryotic CellsProkaryotic Cells

• Mitochondria have Mitochondria have circular circular genomes genomes

• SequencesSequences of mitochondrial DNA are of mitochondrial DNA are similar to genomes of similar to genomes of bacterialbacterial cells cells

• Mitochondria divide by Mitochondria divide by fissionfission

• Mitochondria possess their Mitochondria possess their ownown system system of DNA synthesis, transcription and of DNA synthesis, transcription and translationtranslation

Evidence from Red AlgaEvidence from Red Alga

• A similar relationship has been discovered A similar relationship has been discovered between a small alga known as a between a small alga known as a cryptomonad and a structure found within cryptomonad and a structure found within it which is likely derived from a primitive it which is likely derived from a primitive red alga. red alga.

• The structure is called a The structure is called a nucleomorphnucleomorph and is a tiny nucleus containing genomic and is a tiny nucleus containing genomic material found within a eukarytoic material found within a eukarytoic endosymbiotic structureendosymbiotic structure

Gene Organization and Gene Organization and Chromosome StructureChromosome Structure

ChromosomesChromosomes

• Chromosomes have protein and DNA Chromosomes have protein and DNA – chromatin – chromatin

• HistonesHistones – every 200 nucleotides – every 200 nucleotides the DNA is coiled around a core the DNA is coiled around a core group of 8 stabilizing proteinsgroup of 8 stabilizing proteins

• The positively charged proteins are The positively charged proteins are attracted to the negative DNAattracted to the negative DNA

Nucleosome – complex of Nucleosome – complex of histones enveloped by coiled histones enveloped by coiled DNADNA

SupercoilingSupercoiling

• DNA is folded into DNA is folded into a higher level of a higher level of coiling than is coiling than is already present in already present in nucleosome nucleosome

Non-coding DNANon-coding DNA

• More than 95% of human DNA is non-More than 95% of human DNA is non-codingcoding

• Variable number tandum repeats Variable number tandum repeats (VNTR’s)(VNTR’s)

• Repetitive sequences of DNA that Repetitive sequences of DNA that vary among individiuals vary among individiuals

VNTR’s Vary from Individual to VNTR’s Vary from Individual to Individual Individual

• Shown to the left are Shown to the left are the chromosomes of the chromosomes of two individuals. The two individuals. The first individual has one first individual has one chromosome with 4 chromosome with 4 repeated sequences repeated sequences and one chromosome and one chromosome with 6 repeated with 6 repeated sequences. The other sequences. The other individual has one individual has one chromosome with 3 chromosome with 3 repeated sequences repeated sequences and one chromosome and one chromosome with 5 repeated with 5 repeated sequences. sequences. 

TelomeresTelomeres

• Telomeres are long Telomeres are long sequences of repetitive sequences of repetitive noncoding DNA on the noncoding DNA on the end of chromosomesend of chromosomes

• Teleomeres protect Teleomeres protect cells from losing cells from losing valuable genomic valuable genomic material during DNA material during DNA replicationreplication

• VNTR’s are also found VNTR’s are also found in centromeresin centromeres

PseudogenesPseudogenes

• Pseudogenes have a sequence similar Pseudogenes have a sequence similar to a functioning gene but are never to a functioning gene but are never transcribed transcribed

• Two types of pseudogenes:Two types of pseudogenes:

• LINE’s LINE’s long interspersed nuclear long interspersed nuclear elementselements

• SINE’s SINE’s short interspersed nuclear short interspersed nuclear elements elements

SINES and LINES are used to SINES and LINES are used to show Evolutionary show Evolutionary RelationshipsRelationships(see page 612)(see page 612)

ReviewReview

• Why does translation begin immediately in Why does translation begin immediately in prokaryotes?prokaryotes?

• They lack a nuclear membrane.They lack a nuclear membrane.• What is the genome like in prokaryotes?What is the genome like in prokaryotes?• It is circular and lacks non-coding regions.It is circular and lacks non-coding regions.• How are ribosomes different in Prokaryotes?How are ribosomes different in Prokaryotes?• Ribosomes are smaller and recognize start Ribosomes are smaller and recognize start

of mRNA by a unique sequence of purine-of mRNA by a unique sequence of purine-rich bases.rich bases.

ReviewReview

• What is an endosymbiotic What is an endosymbiotic relationship?relationship?

• It is one in which one species lives It is one in which one species lives within another.within another.

• How do mitochondria resemble How do mitochondria resemble prokaryotic cells?prokaryotic cells?

• They have circular genomes, DNA is They have circular genomes, DNA is similar to bacterial cells, can divide, similar to bacterial cells, can divide, have own system of DNA synthesis. have own system of DNA synthesis.

ReviewReview

• What is a histone?What is a histone?

• A stabilizing protein associated with A stabilizing protein associated with DNADNA

• What is a nucleosome?What is a nucleosome?

• DNA is coiled around 8 stabilizing DNA is coiled around 8 stabilizing proteins.proteins.

• What is a VNTR?What is a VNTR?

• A non-coding region filled with repeats.A non-coding region filled with repeats.

ReviewReview

• What are telomeres?What are telomeres?

• Long sequences of repetitive non-Long sequences of repetitive non-coding DNA at the ends of the coding DNA at the ends of the chromosomes.chromosomes.

• What is a pseudogene?What is a pseudogene?

• A nucleotide sequence similar to a A nucleotide sequence similar to a functioning gene but one that does functioning gene but one that does not make RNA or protein.not make RNA or protein.