DNA Structure and Replication

• DNA holds the instructions for all living things
• DNA is shorthand for deoxyribonucleic acid
• A DNA molecule is a double helix with two strands of nucleotides

Nucleotides

• Each nucleotide consists of a sugar, a phosphate, and a base
• The backbone is alternating phosphate and sugar groups

DNA Bases

• Adenine (A) can pair with thymine (T)
• Guanine (G) can pair with cytosine (C)
• A-T base pairs are connected with hydrogen bonds
• G-C base pairs are connected with hydrogen bonds
• Pointy letters pair together (AT) and curvy letters pair together (CG)
• Each strand of DNA in a double helix is complementary

Key Features of DNA

• Two DNA strands form a double helix
• The bases in opposite strands hydrogen-bond according to the AT/GC rule
• The 2 strands are complementary

DNA Replication

• DNA replication involves the DNA molecule unzipping
• Each strand serves as a template to build a new strand following the base-pairing rules
• Genetic instructions are passed down via DNA replication
• New DNA molecules are made up of one of the original parental strands plus a new half
• DNA replication is called semi-conservative, with DNA replication producing DNA molecules with 1 parental strand and 1 newly made daughter strand

Quick Facts

• DNA codes for all the information needed to make up an organism using only four building blocks (A, T, C, G)
• All the DNA molecules put end to end in the body would reach from the Earth to the Sun and back over 600 times
• Typing 60 words per minute, eight hours a day, would take about 50 years to type the human genome
• Humans and bananas share about 50% common DNA

DNA and RNA Comparison

• RNA directs the production of proteins via an intermediate molecule of RNA
• RNA is also a nucleic acid
• RNA is ribonucleic acid

Three major differences between RNA and DNA

• RNA is single-stranded (not a double helix)
• Sugar in RNA is ribose
• Thymine (T) is replaced by uracil (U)

How DNA Produces Proteins

• DNA can direct the production of proteins
• DNA first directs the production of RNA, which in turn controls the manufacture of proteins
• Proteins then perform the majority of cellular functions and control physical traits
• Within a cell, DNA directs the production of RNA, which in turn directs the production of proteins

Transcription and Translation

• Genetic information flows from DNA to RNA to protein in two steps
• Transcription rewrites the DNA code into RNA, which then leaves the nucleus
• Transcription follows the DNA base-pairing rules with one exception, uracil (U) is used instead of thymine (T)
• The mature molecule that results from transcription is called messenger RNA (mRNA) in eukaryotic cells
• During transcription, the DNA double helix separates
• One strand of DNA is used to generate a molecule of RNA
• The RNA is processed to become messenger RNA, which then exits the nucleus via a nuclear pore
• RNA polymerase binds to a DNA sequence called the promoter
• RNA polymerase reaches the terminator
• There is no simple, agreed-upon definition that accurately describes all known genes.
• A gene is a stretch of DNA that produces a protein
• A gene is a segment of DNA that codes for a trait
• A gene can be defined as a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA.
• At the ribosomes in the cytoplasm, each mRNA codon translates into an amino acid to build a protein Translation involves cytoplasmic ribosomes
• Ribosomes are made from ribosomal RNA (rRNA) and protein
• Ribosomes read messenger RNA (mRNA) and use transfer RNA (tRNA) to produce a string of amino acids

Genetic Coding

• The "language" of nucleotides is in three-letter codons
• Each mRNA codon matches one of 20 amino acids, or signals a stop or start
• One end of a transfer RNA (tRNA) holds an amino acid
• At the other end is an anticodon that matches up with the mRNA
• 20 coded amino acids, three stop codons, & 1 start codon
• There are 444=64 possible codons

Translation

• Translation creates a protein molecule via the genetic code
• Initiations
• Elongation
• Terminations
• Translation begins when two subunits of a ribosome assemble on an mRNA
• A transfer RNA (tRNA) then brings in amino acids that match the codon in the mRNA

Gene Expression

• Gene expression is the process of productions of proteins

Gene Regulation

• Gene regulation is the process of turning genes on and off
• Different cell types express different genes.
• In female mammals, one X chromosome in each body cell is highly compacted and almost entirely inactive
• Special transcription factors must bind to DNA to "turn on" transcription, allowing RNA polymerase binding
• Before leaving the nucleus, the RNA is modified
• A cap and tail are added
• Non-coding regions (introns) may is removed
• Protein-coding exons are connected to each other
• RNA Splicing

Ways RNA is Altered

• Whether translation proceeds
• How proteins are modified after translation
• When proteins are broken down
• The main site of control is in the transcription of a DNA sequence into RNA

Cell Communication

• All the cells in the body contain the same DNA
• Different cells expressing different genes means differences in form, function, and behavior
• Multicellular life depends on cell-to-cell signaling.
• Molecules exit one cell and bind to a receptor protein on the outside of another cell
• This binding triggers a signal transduction pathway
• Development involves frequent cell division that must be carefully coordinated
• Inductive signals can cause cells to change shape, migrate, or even destroy other cells
• Homeotic genes are master control genes which direct the location of the head and body parts