BIO SCI 98 Lecture Notes - Lecture 2: Start Codon, Genetic Code, Transfer Rna

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blushmammoth506

19 Jun 2018

School

UC-Irvine

Department

Biological Sciences

Course

BIO SCI 98

Professor

RAZORENOVA/INLAY/MORRISETTE

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What is the Genetic Code?

-Codon- trinucleotide sequence that codes

for an amino acid

How are codons translated into amino acid

sequence?

-Crick predicts existence of an adapter

molecule that reads DNA (or mRNA)

sequences and "translates" to amino acid

sequence

Transfer RNA connects mRNA and protein

-The essential ingredients were

-Cytoplasmic RNA extract

-Microsomes

-ATP/GTP

-Radioactive [14C]-Leucine

Anticodon

-Anticodon: triplet nucleotide sequence on tRNA that base pairs with codon on mRNA

-First position of anticodon binds to third position of codon

-Codons are always 5' to 3', anticodons 3' to 5'

The genetic code is degenerate

-Only 20 amino acids for 61 AA-encoding codons (+3 stop codons)

-Degenerate = multiple codons can encode same amino acid

-Only 20 amino acids but 64 possible codons

-Codon family = when 4 codons specify the same amino acid

-xxU or xxC always = same AA

-Degeneracy optimized to protect against mutation

-Substitution at 3rd base changes AA in only ⅓ of cases = silent

-Most single mutations result in similar amino acid substitutions

-Genetic code is more robust and tolerant of mutation than randomly generated code

Start codons

-Only one start codon = AUG

-Always encodes Met

-Only Met codon so AUG appears within proteins as well

-All proteins initially begin with Met! (often removed)

-Start codon Met is slightly different than other Mets

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Stop codons

-Three stop codons = UAA, UAG, UGA

-Also called termination codons

-Translation will stop if it hits stop codon

-No tRNAs that recognize stop codons

Genetic code is non-overlapping

-Genetic code read in non-overlapping

triplets (3 nucleotides at a time)

-If you analyzed the amino acid sequence of a

mutant with a single base substitution, what

would you expect to see if overlapping

model was true? 3 proteins would be affected

and not one

-Most mutations found in diseases had a single

amino acid substitution

Reading frame

-A nucleotide sequence is read 3 bases at a time

-UUU-AAA-CCC-GGG

-Phe-Lys-Pro-Gly

-mRNA has 3 reading frames, depending on which nucleotide you start at

-But double stranded DNA can be read in either direction

-A DNA sequence has 6 potential reading frames

Open reading frame

-In random nucleotide sequence, a stop codon should appear every 20 codons

-Most proteins are several hundred amino acids long, so a real gene is easily

distinguishable from a random sequence by the absence of stop codons

-A sequence that has a start codon, then a long stretch of codons and then a stop codon all

in the same reading frame is called an open reading frame

How do you tell if a DNA or RNA sequence contains an open reading frame?

1) Does it have a start codon? (AUG)

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Document Summary

Codon- trinucleotide sequence that codes for an amino acid. Crick predicts existence of an adapter molecule that reads dna (or mrna) sequences and translates to amino acid sequence. Anticodon: triplet nucleotide sequence on trna that base pairs with codon on mrna. First position of anticodon binds to third position of codon. Codons are always 5" to 3", anticodons 3" to 5" Only 20 amino acids for 61 aa-encoding codons (+3 stop codons) Degenerate = multiple codons can encode same amino acid. Only 20 amino acids but 64 possible codons xxu or xxc always = same aa. Codon family = when 4 codons specify the same amino acid. Substitution at 3rd base changes aa in only of cases = silent. Most single mutations result in similar amino acid substitutions. Genetic code is more robust and tolerant of mutation than randomly generated code. Only met codon so aug appears within proteins as well. All proteins initially begin with met! (often removed)

Related Questions

Fill in the blank. Elongation during translation does NOT involve ____________.

Question 16 options:

	the translation of codons according to the genetic code
	the formation of bonds catalyzed by the ribosome
	complementary base pairing between RNA molecules
	amino acids being linked together in a polypeptide
	reading the DNA template 3' to 5'

For a given gene, what establishes the reading frame for translation?

Question 17 options:

	the location of the enhancer relative to the gene
	the first three nucleotides at the 5' end of the mRNA
	the first three nucleotides at the 3' end of the mRNA
	the start codon in the mRNA
	the location of the promoter relative to the gene

Which of the following is the LEAST likely direct consequence of a substitution mutation?

Question 18 options:

	changing the length of a protein coded for by a gene
	changing one amino acid in a protein
	creating a stop codon
	eliminating a start codon
	changing the length of the DNA molecule containing a gene

Suppose that the pre-mRNA transcript from a eukaryotic gene is 30,000 nucleotides long, and the gene codes for a sequence of 300 amino acids. What is the best explanation for the relationship between these numbers?

Question 19 options:

	only the first 900 nucleotides of the pre-mRNA transcript are translated
	it takes 100 nucleotides to specify a single amino acid
	300 of the nucleotides in the transcript are important, and the rest are "junk"
	only the last 900 nucleotides of the pre-mRNA transcript are translated
	large portions of pre-mRNA transcripts are cut out during RNA processing

Suppose an individual is born into a population with a novel mutation. Is the new mutation an evolutionary change, and why?

Question 20 options:

	no, because it is not a big enough change to count
	yes, because new mutations are always adaptive
	yes, because the appearance of a new genetic variant is a genetic change in a population
	no, because not enough individuals have the mutation for it to matter
	no, because most mutations are not adaptive

Question 9

Codominance is a form of inheritance in which two different allelesfor a gene are both expressed, and neither allele is dominant overthe other. One example of codominance is fur color in cattle; thealleles for red fur and white fur are codominant. If a homozygousred cow and a homozygous white bull mate, what would you expect tofind in their offspring?.912.l.16.2>

They will all have white fur.

Their fur will be a mixture of red and white hairs.

Their fur will be a lighter shade of red.

They will all have red fur.

Question 10

If a tRNA molecule has an anticodon which reads AUG, what will itmatch up with and what amino acid is it carrying?
.912.l.16.5>

	U		C		A		G
	UUU	Phenylalanine	UCU	Serine	UAU	Tyrosine	UGU	Cysteine	U
U	UUC	Phenylalanine	UCC	Serine	UAC	Tyrosine	UGC	Cysteine	C
	UUA	Leucine	UCA	Serine	UAA	Stop	UGA	Stop	A
	UUG	Leucine	UCG	Serine	UAG	Stop	UGG	Tryptophan	G
	CUU	Leucine	CCU	Proline	CAU	Hisitidine	CGU	Arginine	U
C	CUC	Leucine	CCC	Proline	CAC	Hisitidine	CGC	Arginine	C
	CUA	Leucine	CCA	Proline	CAA	Glutamine	CGA	Arginine	A
	CUG	Leucine	CCG	Proline	CAG	Glutamine	CGG	Arginine	G
	AUU	Isoleucine	ACU	Threonine	AAU	Asparagine	AGU	Serine	U
A	AUC	Isoleucine	ACC	Threonine	AAC	Asparagine	AGC	Serine	C
	AUA	Isoleucine	ACA	Threonine	AAA	Lysine	AGA	Arginine	A
	AUG	Methionine/Start	ACG	Threonine	AAG	Lysine	AGG	Arginine	G
	GUU	Valine	GCU	Alanine	GAU	Aspartate	GGU	Glycine	U
G	GUC	Valine	GCC	Alanine	GAC	Aspartate	GGC	Glycine	C
	GUA	Valine	GCA	Alanine	GAA	Glutamate	GGA	Glycine	A
	GUG	Valine	GCG	Alanine	GAG	Glutamate	GGG	Glycine	G

It is carrying arginine and will match with a CGU codon on themRNA.

It is carrying methionine and will match with a TAC codon on theDNA.

It is carrying methionine and will match with a UAC codon on themRNA.

It is carrying tyrosine and will match with a UAC codon on themRNA.

Question 11

While the DNA in a human skin cell was being replicated, a singlebase was miscopied. What will be the most likely result of this forthe cell in which it happened?.912.l.16.3>

All the proteins the cell creates from the miscopied strand willdo different jobs than the old ones.

If the new sequence codes for the same amino acid as theoriginal cell, it will function normally.

Both new DNA strands will end up together in a new cell, and theinaccurate one will be discarded.

Any miscopied DNA will be replaced with an accurate DNA copyonce the cell divides.

Question 12

A ferret's haploid number of chromosomes is 20. How would thenumber of chromosomes in the ferret's body cells compare to thenumber of chromosomes in its gametes?.912.l.16.17>

Its body cells would have 20 chromosomes, and its gametes wouldhave 40 chromosomes.

Its body cells and gametes would both have 40 chromosomes.

Its body cells and gametes would both have 20 chromosomes.

Its body cells would have 40 chromosomes, and its gametes wouldhave 20 chromosomes.

Question 13

A body cell is in the longest stage of its life cycle. The cellgrows, synthesizing proteins and increasing in size. Eventually,the cell will grow too large to carry out normal activities. Whichof the following is the best conclusion you can make about the lifecycle of this cell?.912.l.16.14>

The cell is close to the end of its life cycle, and a chemicalsignal will initiate cell death.

The cell is ready to undergo mitosis, and a chemical signal willsend the cell to prophase.

The cell is in the G1 phase of interphase, and a chemical signalwill send the cell to the S phase.

The cell is in the S phase of interphase, and a chemical signalwill move the cell to the G2 phase.

Question 14

While mRNA strands are being created a sequence is sometimesmiscopied. What is the best possible outcome for the cell shouldthis take place?.912.l.16.5>

The miscopied sequence codes for the same amino acids as theoriginal sequence.

The new sequence creates a protein that serves a differentfunction from the original.

The ribosomes will correct the mistake before the tRNA matchesan amino acid to it.

The mRNA will only be used to create non-critical proteins forthe cell.

Question 15

Over the last several decades, the scientific community hasgathered a large amount of information regarding genetics andgenetic variation. What are two main sources that lead toincreased genetic variation?.912.l.15.15>

Selective breeding

Gamete mutations

Recombination

Genetic drift

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BIO SCI 98 Lecture Notes - Lecture 2: Start Codon, Genetic Code, Transfer Rna

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