Genes, DNA, RNA

Nucleic acids carry the genetic code that determines the order of amino acids in proteins

Genetic material stores information, can be replicated, and undergoes mutations

Differs from proteins as it has phosphorus and NO sulphur

DNA Deoxyribonucleic Acid

Nucleotides are smaller units of long chains of nucleic acids. Each nucleotide has

A pentose sugar (deoxyribose in DNA, ribose in RNA)

A phosphate group

An organic base which fall into 2 groups,

Purines (double rings of C and N - bigger)

Adenine or Guanine

Pyrimidines (single ring of C and N - smaller)

Thymine or Cytosine

Base pairing by weak hydrogen bonds

Adenine-Thymine 2 H- bonds

Cytosine-Guanine 3 H- bonds

Chains are directional according to the attachment between sugars and phosphate group

They are antiparallel which is essential for gene coding and replication

DNA molecule has 2 separate chains of nucleotides hold together by base pairing / DNA normally twist into a helix (coil) / forms a double helix

Ribonucleic Acid (RNA)

Ribose instead of deoxyribose

Single chain (shorter than DNA - lower molecular mass)

Base difference: Uracil instead of Thymine. Adenine, Guanine and Cytosine are the same

Ribosomal RNA (rRNA)

Located in the cytoplasm - ER

Reads mRNA code and assembles amino acids in their correct sequence to make a functional protein (translation)

Messenger RNA (mRNA)

Commutes between nucleus and cytoplasm

Copies the code for a single protein from DNA (transcription)

Carries the code to ribosomes in the cytoplasm

Transfer RNA (tRNA)

In the cytoplasm

Transfer amino acids from the cytoplasm to the ribosomes

The Genetic Code

DNA codes for assembly of amino acids / forms a polypeptide chain (proteins - enzymes)

The code is read in a sequence of three bases called

Triplets on DNA              e.g. CAC TCA

Codons on mRNA            e.g. GUG AGU

Anticodons on tRNA        e.g. CAC UCA

(must be complementary to the codon of mRNA)

Each triplet codes for one amino acid / single amino acid may have up to 6 different triplets for it due to the redundancy of the code / code is degenerate. Some amino acids are coded by more than one codon

Same triplet code will give the same amino acid in virtually all organisms, universal code

We have 64 possible combinations of the 4 bases in triplets, 43

No base of one triplet contributes to part of the code next to it, non-overlapping

Few triplets code for START and STOP sequences for polypeptide chain formation

eg START   AUG    and   STOP   UAA UAG UGA

DNA Replication (Semi-Conservative Replication)

Happens during Interphase 'S'

Separate the strands, a little at a time to form a replication fork

Events:

Unwinding / Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds

Semi-conservative replication / each strand acts as a template for the formation of a new strand

Free DNA molecules join up to exposed bases by complementary base pairing

Adenine with Thymine (A=T 2 -H bonding)

Cytosine with Guanine (CΞG 3 -H bonding)

For the new 5' to 3' strand the enzyme DNA polymerase catalyses the joining of the separate nucleotides

"All in one go" → completed new strand

For the 3' to 5' strand DNA polymerase produces short sections of strand but these sections have to be joined by DNA ligase to make the completed new strand. Specific base pairing ensures that two identical copies of the original DNA have been formed

Transcription: DNA to mRNA

DNA in nucleus unzips - bonds break

Single template strand of DNA used for mRNA (triplet on DNA = codon for amino acid on mRNA)

Enzyme RNA polymerase joins nucleotides together

Free RNA nucleotides are assembled according to the DNA triplets (A-U / C-G / T-A)

mRNA bases are equivalent to the non-template DNA strand

Start and stop codons are included

Introns (Non-coding) and exons (coding) DNA sequences are present in the primary mRNA transcript. Introns are removed before the mRNA is translated so that exons are only present in the mature mRNA transcript

[EXAM] Total number of bases in the DNA sense strand and total number of bases in the mRNA are different

mRNA moves into cytoplasm and becomes associated with ribosomes

Translation: mRNA to Protein via tRNA

Translation is the synthesis of a polypeptide chain from amino acids by using codon sequences on mRNA

tRNA with anticodon carries amino acid to mRNA associated with ribosome

"Anticodon - codon" complementary base pairing occurs

Peptide chain is transferred from resident tRNA to incoming tRNA

tRNA departs and will soon pick up another amino acid

Requirement for Translation

Pool of amino acids / building blocks from which the polypeptides are constructed

ATP and enzymes are needed

Complementary bases are hydrogen-bonded to one another

Structure involved in translation

Messenger RNA (mRNA)

Carries the code from the DNA that will be translated into an amino acid sequence

Transfer RNA (tRNA)

Transfer amino acids to their correct position on mRNA strand

Ribosomes

Provide the environment for tRNA attachment and amino acid linkage

DNA and Inheritance

Reactions in cells is referred to as cell metabolism

A sequence of chemical reactions is called a metabolic pathway

Different forms of the same gene are alleles

A gene is the length of DNA that carries the code for a protein (enzyme)

Enzyme effect the cell's metabolism

Visible changes are described with the phenotype

The phenotype is influenced by the metabolic pathway

Therefore

DNA controls enzyme production

Enzymes control metabolic pathways

Metabolic pathways influence the phenotype of an organism

Gene Mutations

Deletion, reading frame shifts

Substitution, one base replaced by another

Duplication, repetition of part of the sequence

Addition, Addition extra base

Change in one or more nucleotide bases in the DNA

Change in the genotype (may be inherited)

Cystic Fibrosis - Defective Gene

Mutation causes the deletion of 3 bases in DNA. One amino acid (phenylalanine) is not coded for in the Cystic Fibrosis Transmembrane Regulator CFTR protein

Faulty CFTR protein cannot control the opening of chloride channels in the cell membrane

Results in production of thick sticky mucus, especially in lungs, pancreas and liver

Organs cannot function normally and infection rate increases

Phenylketonuria (PKU) - Defective Gene

Gene mutation in DNA coding for the enzyme phenylalanine hydroxylase

Phenylalanine hydroxylase not produced

Amino acid phenylalanine cannot be converted to the amino acid tyrosine

Tyrosine is necessary to produce the pigment melanin

Phenylalanine collects in the blood and causes retardation in young children

Managed by controlling diet to eliminate proteins containing phenylalanine

Disease is tested by drops of blood taken from the baby

Normal: IMG 3-12-5

Defect (PKU): IMG 3-12-5

Cells & Molecules

Cell Division

Cell Types

Cell Ultrastructure

Enzymes

Gene Technology

Genes, DNA, RNA

Large Molecules

Plasma Membrane

Respiration

Content

Genes, DNA, RNA

DNA Deoxyribonucleic Acid

Ribonucleic Acid (RNA)

The Genetic Code

DNA Replication (Semi-Conservative Replication)

Transcription: DNA to mRNA

Translation: mRNA to Protein via tRNA

DNA and Inheritance

Gene Mutations

Cystic Fibrosis - Defective Gene

Phenylketonuria (PKU) - Defective Gene