Protein Synthesis

Protein Synthesis

3.4.2 DNA and Protein Synthesis

Multiple Choice

What is DNA wrapped around in eukaryotic DNA?

• nucleosomes

• introns

• histones

• proteosomes

Multiple Choice

What parts of a gene do not code for a protein and must be cut out?

• introns

• mRNA

• loci

• exons

Multiple Choice

A different version of the same gene is called a:

• locus

• exon

• genome

• allele

Multiple Choice

What part of a chromosome is found at the ends of the chromosome and protects it from degradation?

• autosomes

• telomeres

• centromeres

• non-coding multiple repeats

Multiple Choice

Which complementary base sequence would match up with this length of DNA? AAT CGA GGT

• AAT CGA GGT

• TTA GCT CCA

• TTA CGA CCA

• CCG ATC TTG

The Problem...

DNA is huge.

It cannot leave the nucleus.

Ribosomes are outside the nucleus in the cytoplasm

They need the DNA code to synthesise proteins.

Need a smaller molecule to carry the code to the cytoplasm.

The Code

• DNA is a degenerate code. This means that multiple triplet codons can code for each amino acid.

• DNA is non-overlapping. Each codon codes for a separate amino acid.

• It is universal:

  This means that it is the code for almost all proteins in almost all organisms on Earth

Transcription

Transcription involves forming a single -stranded mRNA (messenger RNA) from double stranded DNA

A message from the DNA needs to be made to communicate with the ribosomes etc. to tell them what to do

messenger RNA (mRNA)

Carries a complimentary copy (message) from the DNA in the nucleus to the ribosomes in the cytoplasm.

Long, single helix molecule.

Contains codons.

Easily broken down.

Multiple Choice

A triplet of three bases that code for an amino acid is known as a:

• gene

• protein

• nucleotide

• codon

Multiple Select

Which of these described DNA?

• degenerate

• single stranded

• circular

• universal

• linear

Multiple Choice

What does the 'm' in mRNA stand for?

• messaging

• mandatory

• messenger

• malignant

Multiple Choice

Which organelle in the cytoplasm synthesises proteins?

• golgi body

• smooth ER

• mitochondria

• ribosomes

Sense Strand

The strand with the gene we want to copy on it is called the sense strand

It is always read from 5' to 3'

Antisense Strand

The complementary strand is the antisense strand, or template strand

It is always read 3' to 5'

The antisense strand is used as a template so that the mRNA synthesised has the same sequence as the sense strand

DNA Helicase

To access the antisense strand, the DNA must first be unwound using the enzyme DNA helicase

This breaks the hydrogen bonds between the bases and 'unzips' the DNA

It moves along the sugar-phosphate backbone, beginning at a start codon (ATG - methionine)

Making mRNA

The DNA antisense strand then acts as a template for the mRNA as free nucleotides pair with the exposed complementary bases

Uracil will be brought in instead of thymine

The mRNA will have the same sequence as the sense strand

RNA Polymerase

The RNA nucleotides are then joined to adjacent nucleotides by phosphodiester bonds to form the complete mRNA strand

This is catalysed by the enzyme RNA polymerase which travels along the sugar-phosphate backbone in the 3' to 5' direction

DNA Reforms

When transcription ends, the mRNA strand detaches from the DNA and the DNA zips back up

This is important to ensure that the bases are not mutated by the harsh environment of the nucleus

Only the section of the DNA being transcribed is unzipped at any one time

mRNA

The sequence of bases in the mRNA strand is the same as the DNA coding strand, except thymine is replaced with uracil

Just like the coding strand of DNA, the mRNA also contains codons and will be read in groups of three bases

Multiple Choice

Which enzyme is responsible for unzipping the DNA?

• RNA polymerase

• DNA polymerase

• RNA helicase

• DNA helicase

Multiple Choice

What bonds are found between adjacent nucleotides in an mRNA strand?

• phosphodiester

• hydrogen

• peptide

• ester

Multiple Select

Which bases are found in RNA?

• A

• T

• C

• G

• U

Multiple Choice

Which strand is also called the template strand?

• Sense

• Antisense

Multiple Choice

Which strand is read in the 5' to 3' direction?

• Sense

• Antisense

Splicing

In prokaryotes, DNA transcription results in a ready-to-use molecule of mRNA

In eukaryotes, transcription results in the synthesis of pre-mRNA which must be modified to form mature mRNA

Splicing

This is because eukaryotic DNA contains introns and exons

Exons are sections of DNA that code for proteins

Introns are sections of DNA that do not code of proteins

The exons are joined back together to form mature mRNA - this is called splicing

Multiple Choice

Transcription in eukaryotes produces:

• mature mRNA

• pro mRNA

• pre mRNA

• early mRNA

Open Ended

Explain the process of transcription in your own words.