Chromosomes and DNA

Chromosomes

• Chromosomes are thread-like structures in the nucleus of animal and plant cells.
• They consist of protein and a single DNA molecule.
• DNA provides the unique instructions for each living creature.
• Chromosomes are composed of DNA tightly coiled around proteins called histones, which support their structure.
• When a cell is not dividing, chromosomes are not visible in the cell's nucleus, even under a microscope.
• During cell division, DNA becomes more tightly packed, making chromosomes visible under a microscope.
• Knowledge about chromosomes is mainly derived from observations made during cell division.

DNA

• DNA (deoxyribonucleic acid) carries hereditary information in humans and almost all organisms.
• Nearly every cell contains the same DNA.
• Most DNA is in the cell nucleus (nuclear DNA), but some is in the mitochondria (mitochondrial DNA or mtDNA).
• DNA contains instructions for an organism's development, survival, and reproduction.
• DNA sequences are converted into messages used to produce proteins.
• Proteins are complex molecules that perform most of the work in our bodies.
• Each DNA molecule has two strands forming a double helix.
• Each strand features a backbone of alternating deoxyribose sugar and phosphate groups.
• Each sugar is attached to one of four bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
• The two strands are complementary: A pairs with T, and C pairs with G.
• The sequence of bases along a DNA strand defines the genetic code.
• Genes are composed of DNA.
• Some genes instruct the creation of proteins, but many do not code for proteins.
• In humans, gene sizes range from a few hundred to over 2 million DNA bases.
• The Human Genome Project estimates that humans have between 20,000 and 25,000 genes.

Transcription

• Transcription is the initial step in gene expression, where a gene's information is used to create a functional product.
• This process copies a DNA sequence to produce an RNA molecule.
• RNA polymerase and associated proteins perform transcription.
• RNA polymerase attaches to a DNA region called the promoter.
• The promoter signals the DNA to unwind, allowing the enzyme to "read" the bases on one strand.
• The enzyme then creates an RNA molecule by matching complementary bases.
• The RNA molecule is complementary to the template DNA strand.
• RNA contains uracil (U) instead of thymine (T), so U pairs with A.
• Transcription involves three stages: initiation, elongation, and termination.

Translation

• Translation is the process of decoding the genetic code in messenger RNA (mRNA) to produce a specific sequence of amino acids in a polypeptide chain.
• It occurs in the cytoplasm after transcription and includes three stages: initiation, elongation, and termination.
• Ribosomes, made of RNA and proteins, are the sites of protein synthesis.
• Ribosomes bind to mRNA and move along it, facilitating the attachment of transfer RNA (tRNA) molecules, which carry amino acids.
• The mRNA sequence is read in codons, which are sets of three bases that specify a particular amino acid.
• Each codon corresponds to a specific tRNA molecule carrying the appropriate amino acid.
• The ribosome moves along the mRNA, and each codon is matched by a tRNA molecule that adds its amino acid to the growing polypeptide chain.
• Translation begins when a ribosome binds to mRNA at the start codon (AUG), which codes for methionine.
• Transfer RNA (tRNA) molecules bring amino acids to the ribosome for protein synthesis.
• The process continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA, signaling the end of translation and the release of the polypeptide chain.

Eukaryotic cells

• Eukaryotic cells contain membrane-bound organelles, including a nucleus.
• Eukaryotes can be single-celled or multi-celled, including humans, plants, fungi, and insects.
• Eukaryotic cells are typically larger than prokaryotic cells.
• DNA in eukaryotic cells is stored within the nucleus.
• Eukaryotic cells have complex organelles like the endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria.
• These organelles perform specific functions, such as protein synthesis, energy production, and waste disposal.

Prokaryotic cells

• Prokaryotic cells lack membrane-bound organelles.
• Prokaryotes are always single-celled organisms, including bacteria and archaea.
• Prokaryotic cells are generally smaller and simpler than eukaryotic cells.
• DNA in prokaryotic cells is not stored within a nucleus but resides in the cytoplasm.
• Prokaryotic cells lack complex organelles found in eukaryotic cells.
• They rely on the cytoplasm and cell membrane for functions like protein synthesis and energy production.