DNA and RNA

Chapter 16

Structure of DNA

• DNA (deoxyribonucleic acid) is a very long coiled molecule.

• The structure of DNA can be considered under 3 headings:– nucleotides– base pairs– the double helix

Nucleotides

• A nucleotide consists of a phosphate group, a sugar and a nitrogen-containing base.

• The sugar used in DNA is called deoxyribose.

• There are 4 different bases:– Adenine (A), Thymine (T)– Guanine (G), Cytosine (C)

• Adenine and Guanine are called purines.

• Thymine and cytosine are called pyrimidines.

There are four DNA nucleotides:There are four DNA nucleotides:

P-D-A

P-D-T

P-D-G

P-D-C

(P = phosphate, D = deoxybibose)

Base pairs• Adenine and Thymine both form 2 weak hydrogen

bonds.

• This allows them to join together, they are complementary base pairs.

• Guanine and Cytosine form 3 hydrogen bonds and are another complementary base pair.

• In each base pair there is a purine-pyrimidine link.

• DNA resembles a ladder.

The Double Helix

• Scientists Watson and Crick proposed that DNA was a double helix.

• The complementary strands have the phosphate on the outside; the deoxyriboses are next; with the bases on the inside.

DNA replication

• The sequences of bases on one strand of a double helix determines the sequence of bases on the complementary strand.

• E.g. one side of DNA has the base sequence AGGCCTTA, then the other side must be TCCGGAAT.

DNA Replication

• DNA replication takes place in the nucleus of the cell during interphase.

• It results in the single-stranded chromosome forming two identical strands that are held together at the centromere. Chromosome

(single-stranded)

gene

Chromosome(double-stranded)

DNA replication occurs as follows...1. The double helix unwinds.2. Enzymes break the hydrogen bonds.3. The complementary sands move apart.4. DNA nucleotides move from the cytoplasm

into the nucleus and attach to their complementary bases on the exposed strands.

5. The new strands contain exactly the same sequence of bases. Each new stretch of DNA re-forms into a double helix.

Significance of DNA replication

• DNA is able to produce exact copies of itself.

• This allows exactly the same DNA to be passed to each daughter cell during mitosis.

The Genetic Code

• Genes are composed of long sequences of DNA bases.

• Genes cause a sequence of amino acids to be assembled to form a protein.

• To allow the correct protein to be assembled DNA carries a genetic code.

• This code operates as a sequence of three DNA bases called a triplet or codon.

• It takes many triplets to from a gene.

ATC

This triplet codesfor amino acid 2

CCG

This triplet codes for amino acid 14

Coding and non-coding DNA

• About 97% of the DNA in a human nucleus does not cause the production of proteins.

• This junk DNA has no known function.

• Some of this non-coding DNA is located between the genes, with much of it at the centromeres and at the ends of the chromosomes.

• Much of it is found within the genes.

DNA profiles

• A DNA profile is a unique pattern of DNA from a person.

• It is compared with the DNA profile of another person.

• DNA profiling is also called genetic or DNA fingerprinting.

Preparing DNA profiles.1. Release DNA from cells.

DNA is released from the cell. The cells can

be obtained from saliva, hair roots or semen.

2. Cut the DNA into fragments.

The DNA is cut into pieces using special enzymes (called restriction enzymes). These enzymes cut DNA when they encounter specific base sequences. The DNA sections obtained in this way will vary in length from very small sequences of bases to very long sequences.

3. Separate the fragments. The DNA fragments are separated according to their length. This involves placing the fragments in a gel and pass an electric current through the gel. Small fragments move faster through the gel than large ones. A photograph of the final result is obtained.

Each DNA profile looks like a bar code. No two people have the same DNA profile.

Uses of DNA profiles

• Crime Investigations

• Medical (Paternity tests)

Genetic Screening

• Genetic screening means testing a person’s genes for the presence of abnormal or altered genes.

• This can be very valuable for couples who know particular genetic disorders run in their families.

• They can be advised of the probability of their children having a disorder.

Ethical problems

• If an embryo is tested and shown to have a disorder, it may encourage the couple to have an abortion.

• Should a person be told they have a disorder that will develop later in life and lead to death?

• Should insurance companies be informed of genetic screening results?

RNARNA (ribonucleic acid) differs from DNA as follows

DNA RNA

C, A, T, G (hasthymine)

C, A, U, G (hasurcil)

Double helix Single-stranded

Mainly found inthe nucleus

Found in thenucleus andcytosol

Only one type Three typesmRNA, tRNA,rRNA

Remember…CATGUT for the difference in bases between DNA and RNA, where CATG are the bases in DNA, and U replaces T in RNA!

•RNA bases are complementary to DNA bases.

•Example, if DNA has the base sequence TAGGC, the RNA complementary base sequence will be AUCCG.

Protein Synthesis

• Genes control cells by producing enzymes.

• Enzymes are proteins.

• To make the correct proteins it is important that amino acids are assembled in the correct order in ribosomes.

• Genes work by forming the correct proteins.

• This involves the genetic code in DNA being transcribed to mRNA (messenger RNA).

• This code must then be translated into the correct sequence of amino acids.

• Transcription takes place in the nucleus and translation takes place in the ribosomes.

Remember...

DNA RNA ProteinTranscription Translation

Main steps in protein synthesis...

1.The double helix unwinds at the site of a gene that is to from a protein.

2.The sequence of bases on the DNA (gene) is used for a complementary strand of mRNA. This process is called transcription.

3.The mRNA moves out of the nucleus into the cytoplasm.

4.Ribosomes are made mainly of rRNA (ribosomal DNA).

5.The mRNA move into the ribosomes.

6.There are large numbers of tRNA (transfer RNA) molecules in the cytoplasm. Each tRNA carries a specific amino acid.

7.Triplets or codons on the mRNA strand attract complementary triplets (called anti-codons) in tRNA molecule.

8.Each tRNA carries its own amino acid.

9.The amino acids are detached from their tRNA molecule and bonded together to form new proteins.

10. The protein then folds into the correct 3D shape to allow it to function properly.