DNA, RNA & Protein Synthesis

What is DNA?• The hereditary material in the nucleus of

cells that tells the cell when to make proteins and what proteins it should make.

• Regions of DNA that code for proteins are called genes.

• These proteins give organisms their traits, such as eye color.

• DNA stands for deoxyribonucleic acid.

DNA Structure• Discovered in 1953 by James Watson and Francis Crick.• DNA is a double-helix = 2 strands twisted around each

other like a spiral staircase.• DNA is made of repeating subunits called nucleotides.• Nucleotides have 3 parts:

– Deoxyribose sugar

– Nitrogen base

– Phosphate group

•

DNA Nucleotide

DNA Structure continued…• The sides of the DNA ladder are repeating

phosphates and sugars.• The steps of the ladder are the nitrogen

bases.• There are 4 different nitrogen bases in DNA:

– Adenine– Guanine – Cytosine– Thymine

Purines = Have a double ring structure

Pyrimidines = Have a single ring structure

Nitrogen Bases in DNA

Complementary Base Pairing• Watson and Crick discovered that the following

base pairing rule exists in DNA structure:– Adenine is complementary (fits together) with Thymine

• Held together by 2 hydrogen bonds

– Cytosine is complementary with Guanine• Held together by 3 hydrogen bonds

•Each pairing includes one purine and one pyrimidine.

Label the small portion of DNA shown below:

Label the following:•Sugars•Phosphates•Nitrogen bases•Hydrogen bonds

Build a DNA Model• Black= sugar• White hollow= phosphate• White solid= hydrogen bond• Blue= cytosine• Green= thymine• Orange= adenine• Yellow= guanine

Make the sequence for ½ of the DNA

CTGGACT

• If the sequence of half of a DNA strand is TAGGCC, draw the entire DNA strand. Label the phosphate groups, deoxyribose sugars, nitrogen bases, and hydrogen bonds.

Replication • During interphase, chromosomes create new chromatids. This

process of making new DNA strands is called replication.• This process happens in the nucleus of the cell.• Each new DNA produced has ½ from the original strand and ½

that is newly built.• DNA helicase breaks the hydrogen bonds that hold the 2

strands of DNA together. • DNA polymerase forms the new halves of DNA by putting the

correct nucleotides into position. It also proofreads the new DNA built for any errors.

• This process occurs once in a cell’s lifetime.

Let’s Practice Replication!

• Build the DNA model with the sequence GTTCA.

• G=yellow

• T=green

• C=blue

• A=orange

Transcription• Transcription is the making of RNA by using the DNA code as

a template.• RNA is single stranded so only ½ of the DNA is used as a

template. This half is called the sense strand. The other is the non-sense strand.

• RNA polymerase starts at the promoter on the DNA and builds the new strand of RNA until it reaches the termination signal.

• RNA produced leaves the nucleus through nuclear pores to enter the cytosol. Why can it leave?

• 3 types of RNA are produced: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA)

Contrasting DNA and RNA

DNA

• 2 strands• Deoxyribose sugar• Remains in the nucleus• Bases

– ATCG

A pairs with T

G pairs with C

RNA

• 1 strand• Ribose sugar• Exits nucleus• Bases

– AUGC

A pairs with U

G pairs with C

Let’s Practice Transcription!• Build the DNA model with the sequence

CTGGATC.

• G=yellow, T=green, C=blue, A=orange U= purple ribose=purple pentagons

• The side of the DNA listed above is the sense strand.

• If the promoter is after the 1st “C” and the termination signal is before the last “C”, what is the RNA strand produced?

3 Types of RNA1. mRNA (messenger)- carries the

DNA’s instructions for making proteins out of the nucleus to the ribosomes in the cytosol.

• Every 3 nucleotides on mRNA is called a codon. Each codon codes for an amino acid.

– See codon table

2. tRNA (transfer)- carries a specific amino acid on one end and an anticodon on the other end. The anticodon complements the codon on mRNA.

Draw the tRNA structure:

3. rRNA (ribosomal)- Along with protein, this makes up the structure of a ribosome.

Translation (Protein Synthesis)1. mRNA leaves the nucleus and enters the cytosol.2. mRNA attaches to a ribosome.3. At the start codon (AUG), a tRNA brings its amino

acid into position. 4. Adjacent amino acids bond together with a peptide

bond and the tRNA leaves.5. The ribosome then moves onto the next codon.6. This process continues until the STOP codon is

reached.7. The new protein is released.

Draw the translation process

• DNA code TGA ______________ _______________

• mRNA codon _____________ UCU _______________

• tRNA anticodon _____________ ________________ _______________

• Amino acid ____________ ______________ tryptophan

• DNA code CAT ______________ _______________

• mRNA codon _____________ GUU _______________

• tRNA anticodon _____________ ________________ _______________

• Amino acid ____________ ______________ methionine