Molecular Biology -- chapter 9

deoxyribonucleic acid

is a polymer molecular that carries genetic information , which is important for all cellular functions and activities

• a DNA molecule is wrapped around histone proteins to form a single chromatin thread

genetic information is stored in the form of

genes which are used to form polypeptides

polypeptides

used to make proteins , which are responsible for determining the characteristics of an organism

what are DNA and RNA made up of

nucleotides , the building blocks

composition of nucleotide

pentose sugar (5 carbon) , nitrogenous base (A,T,G,C,U) and phosphate group (negatively charged)

pentose sugar in DNA vs RNA

DNA : deoxyribose sugar

RNA : ribose sugar

backbone of DNA

sugar (OH) —> phosphate (P)

5 different nitrogenous bases

Adenine (A) , Cytosine (C) , Thymine (T) — only found in DNA , (U) Uracil — only found in RNA, Guanine (G)

nucleotides are joined together by

phosphodiester bonds to form a polynucleotide chain with a backbone of alternating sugar and phosphate groups , thus forming the sugar-phosphate backbone

polynucleotide chains are

anti parallel since they run in opposite directions , hence they wind together to form a double helix structure

where is hydrogen bonding present

between two polynucleotide strands between complementary bases

• adenine pairs with thymine — 2 hydrogen bonds

• cytosine pairs with guanine — 3 hydrogen bond

gene expression — from DNA to protein

the process by which information from a gene is used in the synthesis of a functional gene product , usually a protein

DNA vs protein synthesis where are they found

DNA — only in nucleus

Protein synthesis — by ribosomes in cytoplasm

Difference between RNA n DNA

1. presence of thymine vs uracil

2. DNA is double stranded (double helix) while RNA is single stranded

3. DNA more stable due to its double strands

4. RNA has one more oxygen atom per nucleotide due to difference in sugars

central dogma of molecular biology

flow of genetic information from DNA to protein

2 processes taking place during gene expression

1. transcription (occur in nucleus , use DNA as a template to make RNA)

2. translation (MRNA used as a template to synthesise protein , occur in cytoplasm

DNA — transcription in nucleus —> RNA —pass through nuclear pore — translation in cytoplasm—> Protein

template strand

chain sued for protein synthesis

genetic code

message stored by gene

triplet code/ Condon

every 3 bases in the template strand codes for 1 amino acid

transcription

process by which the DNA template is used to make a single - stranded RNA , occurring in the nucleus

• catalysed by RNA polymerase

3 main types of RNA molecules

1. messenger RNA — contains codons that determine the order of amino acid in the protein

2. Transfer RNA — transfer specific amino acid to the ribosome during translation

3. Ribosomal RNA — component of ribosome

nitrogenous base pairing in mRNA and DNA

DNA : A—>T (2 h bonds) , G—>C (3 h bonds) ,

RNA : A—> U (2 h bonds) , G —> C (3 h bonds)

template strand

only one in each gene + have genetic code , it is from 1 DNA strand

transcription diagram

Translation

• after transcription , the mRNA formed will enter the cytoplasm for translation by ribosomes

• the ribosomes attach to and move along the mRNA

• specific tRNA carrying different amino acids will bind to codons on mRNA via complementary base pairing

• a growing polypeptide chain compromising of amino acids linked by peptide bonds is produced

translation diagram

stop / start codon

start : AUG
stop : UAA , UGA , UAG

differential gene expression

important to determine cell differentiation and give rise to different cell types with specific structures and functions

• Different cells use different genes, even though they contain the same DNA.

• 
  

  Cell type	Genes expressed
  🧠 Nerve cell	Genes needed for nerve function
  💪 Muscle cell	Genes needed for muscle contraction
  🩸 Red blood cell	Genes needed for its specialised function
  🥞 Pancreatic cell	Genes involved in insulin production