DNA

Week 2 Review

1. Draw and label a diagram showing the cell membrane.

2. Define Osmosis3. Define Active and Passive Transport4. Describe the difference between diffusion

and facilitated diffusion

DNA- the basics

• Nucleus of cell• Double Helix• Nucleotides- sugar phosphate group attached to a

nitrogenous base– Guanine– Adenine– Thymine– Cytosine

Gold CoastAlways Together

DNA - the double helix

Wellcome Images – Peter Artymiuk Wellcome Images – Oliver Burston

The ladder model

• If the helix was untwisted it would look like a ladder.

• The side rails of the ‘ladder’ are made of alternating molecules of phosphate and a five-carbon sugar (ribose).

• The rungs of the ladder are paired nitrogen bases.

The DNA backbone

• The two side rails of the ladder run in opposite directions, which can be identified by the end of the rail which is either a three base (3’) end, or a five base (5’) end. [3 & 5 refer to the carbon atoms in the sugar molecule that form a bond with the adjoining phosphate.]

• One strand runs from 3’ to 5’ and the opposite strand, from 5’ to 3’.

Nucleotides are the structural units of DNA and consist of one phosphate molecule, one sugar molecule and one

nitrogen base.

NIH - National Human Genome Research Institute

Nucleotide

Base pair

Backbone

H

H

A H

AT

T

C G

P

P P

P

S

S S

S

SS

G

P

SNucleotide

Nitrogen base

Pentose sugar

Phosphate

P

P

T

P

S

3'

5'

3'

3'

5'

5'

A nucleotide

C

C C

C

CO

5'

4'

3' 2'

1'

OH

The nitrogen bases

• Each “rung” of the DNA ladder is formed from two nitrogen bases.

• There are four bases – adenine (A), thymine (T), cytosine (C), and guanine (G).

The base pairing rule

The base adenine always bonds with thymine (A-T), and cytosine always bonds with guanine (C-G).

The structure of the double helix

Wellcome Images - Pete Jeffs

The base pairsIn DNA, cytosine and guanine are bound together by 3 hydrogen bonds, whereas adenine and thymine are bound by 2 hydrogen bonds.

NIH - National Human Genome Research Institute

The base pairsWellcome Images - Pete Jeffs

DNA- the basics

• Sections contain genes which control the proteins a cell can make

• DNA taken out of nucleus to Ribosomes in cytoplasm- site of protein synthesis

• Proteins made of chains of amino acid

Proteins

• Structure• Transport• Communication• Cell Metabolism• Recognition• Movement

Enzymes

• Metabolic Catalysts• Made of Proteins• Thus controlled by DNA

Protein Synthesis (PS)

• The Genetic Code• Transcription• Translation

PS- The Genetic Code

• Type of Protein cell can make determined by genes

• Different genes activated in different cells• DNA molecules consist of two chains of

alternating sugar and phosphate groups linked by pairs of nitrogen bases

• 4 bases, G, C, A and T- their order determines genetic code

PS- The Genetic Code

• Triplet- three bases that code for a particular amino acid

• Order of triplets determines order amino acids combine thus determining protein

The twenty standard amino acids

Alanine (Ala)Arginine (Arg)Asparagine (Asn)Aspartic acid (Asp)Cysteine (Cys)Glutamic acid (Glu)Glutamine (Gln)Glycine (Gly)Histidine (His)Isoleucine (Ile)

Leucine (Leu)Lysine (Lys)Methionine (Met)Phenylalanine (Phe)Proline (Pro)Serine (Ser)Threonine (Thr)Tryptophan (Trp)Tyrosine (Tyr)Valine (Val)

Write the Genetic code for this protein.

His Met

Phe

His

Glu

Pro

Cys

Cys

M A Glu K

PS- Transcription

• Transcription is the process by which the mRNA is formed using the code in a DNA molecule

• Triggered by chemical messengers that enter the nucleus from the cytoplasm and bind to the DNA at the relevant gene.

PS- Transcription

PS- Transcription

• RNA polymerase then transcribes (copies) the bases on one strand of the DNA to make a complementary molecule of mRNA

• A-U• G-C• Uracil (U) is used on RNA instead of Thymine• Termination sequence tells RNA polymerase to

stop copying- mRNA released

PS- Transcription

• Template strand- strand being copied• Coding strand- strand containing code• Intron- non coding sequence• Exon- coding sequence• Introns must be removed so RNA can be used

to assemble amino acids into proteins• Codon- 3 base pairs that code for a particular

amino acid

PS- Translation

• Translation is the production of a protein using the information that is coded in the mRNA molecule

• Takes place in ribosomes in the cytoplasm• Ribosome attaches to specific binding site on

mRNA• Ribosome moves along mRNA 3 bases at a

time looking for start codon- AUG

PS- Trasnlation

• tRNA bring individual amino acids to Ribosome to be joined to make proteins

• tRNA anticodon- complementary base pairs on tRNA that interact withmRNA• tRNA anticodon determines amino acid carried by tRNA

PS- Translation

• The amino acids carried by the tRNA are joined together so the protein is assembled with the amino acids in the correct sequence.

• For each bond formed between the amino acids, the energy from the breakdown of one ATP molecule is required

• Once the tRNA has delivered its amino acid it detaches from the ribosome and can then pick up another amino acid from the cytoplasm.