الحمض النووي وتركيب البروتينات

DNA and Protein Synthesis

• DNA, a crucial molecule, holds the blueprint for proteins.
• Johann Friedrich, in the 19th century, discovered an acidic substance in salmon sperm, initially believed to be the hereditary material of cells; however, this lacked definitive proof.
• By the mid-20th century, the components of DNA became known.
• In 1953, precise DNA structure was determined using X-ray diffraction.
• Watson, Crick, and Wilkins were awarded the Nobel Prize in 1962 for this discovery, but not Rosalind Franklin, who tragically passed away from cancer.

Function of DNA

• DNA's primary role is to code for proteins.
• Structural proteins, enzymes, and hormones are all directed by DNA.
• The sequence of nitrogenous bases within DNA carries the instructions for building proteins.
• Proteins ultimately control an organism's physical and metabolic traits, as well as regulating growth and development.

Chromosome Organization

• Chromosomes are supercoiled strands of DNA.
• Gene-coding sections of DNA make up only about 3% of each chromosome.
• Non-coding or "junk DNA" spans the remaining 97%, and its function may be related to regulation.

Building Blocks of DNA

• DNA's fundamental unit is the nucleotide.
• Nucleotides consist of:
  • A 5-carbon sugar (deoxyribose).
  • A phosphate group.
  • A nitrogenous base (adenine, guanine, thymine, or cytosine).

Nitrogenous Bases

• Purines are double-ringed structures: -Adenine -Guanine
• Pyrimidines are single-ringed structures: -Cytosine -Thymine -Uracil

Composition of DNA

• Chargaff's observations showed the amount of adenine equals thymine and the amount of guanine equals cytosine.
• This pattern is expressed as A=T and G=C.

Watson-Crick Model

• DNA is a double-stranded helix.
• Strands run in opposite directions.
• Bases on opposing strands are complementary (A with T, G with C).
• Hydrogen bonds connect the complementary bases.

DNA Replication

• Before cell division, DNA replicates to ensure each new cell receives a full copy of the genetic material.
• DNA replication happens within the cell nucleus.
• Replication proceeds by separating the DNA strands and creating a new complementary strand for each.
• The outcome is two identical DNA molecules.

Enzymes in Replication

• Topoisomerases relieve tension during DNA unwinding.
• DNA polymerase III adds nucleotides to the newly forming strands.
• DNA ligase joins fragments on the lagging strand.

Replication Fork

• The replication fork is the area where DNA replication is taking place. -Leading and lagging strands are involved.

Protein Synthesis

• Cellular proteins are created based on the instructions in DNA.
• This process involves two steps: -Transcription -Translation

Transcription

• DNA is copied to mRNA in the nucleus.
  • mRNA carries the protein building instructions to the cytoplasm.

RNA Structure

• RNA is a single-stranded nucleic acid.
• RNA differs from DNA in three key aspects:
  • Single-stranded structure.
  • Uracil replaces thymine.
  • Ribose sugar replaces deoxyribose.

RNA Function

• Three types of RNA play crucial parts:
  • mRNA (messenger RNA): carries genetic instructions from DNA.
  • tRNA (transfer RNA): ferries amino acids to the ribosome.
  • rRNA (ribosomal RNA): forms the structure of ribosomes.

Transcription Location

• Transcription takes place in the cell nucleus.
• RNA polymerase is critical to the process, acting as an enzyme that creates mRNA transcripts.

The Promoter

• The promoter is a region on the DNA that indicates the start of a gene.
• RNA polymerase binds to the promoter and begins transcription.

Classes of RNA

• Three main RNA types:
  • mRNA: carries protein instructions.
  • rRNA: structural component of ribosomes.
  • tRNA: brings amino acids to ribosomes for polypeptide chain assembly.

The Genetic Code

• The genetic code is a set of 64 codons (three-base sequences).
• 61 codons specify particular amino acids.
• 3 codons signal the end of a protein sequence ("stop" or termination codons).

Universal Genetic Code

• The universal genetic code is used by all known living organisms.
• Each codon uniquely specifies an amino acid.

Translation

• Translation occurs in several sequential stages:
  • Initiation – the small ribosomal subunit and initiator tRNA combine at the start codon.
  • Elongation – tRNA brings amino acids and assembles the growing polypeptide chain according to the mRNA code.
  • Termination – a stop codon signals the end of translation.

Elongation and Termination

• Ribosomes read mRNA during elongation.
• Elongation involves adding amino acids to a growing polypeptide chain.
• A stop codon ends the process.

Binding sites on Large Ribosomal Subunits

• P site and A site are important binding sites.

Protein Structure

• Involves different types of structures, both local and overall