Each cell can be thought of as a “protein factory” making the many proteins that build and run the cell (and build other kinds of molecules like carbohydrates and lipids)
We begin with a review of DNA, then move on to the process of DNA synthesis itself.
Structure of DNA
Nucleotides
Nucleotide is the subunit, made of three components:
Sugar (ribose in RNA; deoxyribose in DNA)
Phosphate
Nitrogen base
Different types of nitrogen base: cytosine, guanine, adenine, thymine (DNA only), uracil (RNA only)
First letters are used as abbreviations: C, G, A, T, U
Bases may link C-G or G-C and T-A or A-T (in RNA, U-A or A-U)
This rule is complementary or obligatory “base-pairing”
Nucleotides combine to form a double helix structure
Sugar and phosphate groups link together to form the “backbones” of the double helix
The nitrogen bases link in pairs to connect the two backbones
Linked nucleotides
Nucleotides link to form DNA.
Sugars and phosphates link to form the backbones and bases link in pairs to hold both helices together.
(Click image to enlarge it and view source)
dna structure
DNA double helix model.
(click image for larger view)
Genetish—language of genetics
Genetish is a made-up word introduced by author Matt Ridley to describe the coding used by cells in DNA/RNA
Codon
Every three bases make up one “word” or codon in genetish
Each different codon represents a different amino acid
Amino acids combine to form polypeptides or proteins
Gene
A sequence of codons containing the information needed to make one polypeptide/protein is called a gene (gene = “recipe” for one protein)
Genes are found at various locations along a DNA molecule (chromatin/chromosome)
All genes on all chromosomes is a set of information called the genome
codon table
Codon table. This table is a type of “phrasebook” for the language of “genetish” –showing the meaning of different possible codons.
Function of DNA
The information molecule
Master code for proteins made by the cell
Proteins perform functions that regulate the cell, make up parts of the cell, and regulate the synthesis (and breakdown) of other types of molecules (lipids, carbohydrates, so on)
In humans, temporary “working copies” of specific genes from DNA are in the form of RNA (review Cell Structure & Function)
Information “between the genes” formerly called “junk DNA” may also have functions in regulating gene expression, etc.
Acronym Table
rRNA
ribosomal RNA
Forms ribosomes
mRNA
messenger RNA
Unfolded strand contains gene (code for one polypeptide)
tRNA
transfer RNA
Brings specific amino acids to ribosome and places them according to code on mRNA
nuclear DNA
“Master” genetic code in the nucleus
mDNA or mtDNA mitochondrial DNA
Additional “master” genetic code in the mitochondrion
DNA unzips at one gene, and other side “fills in” with RNA nucleotides with bases that complement the exposed bases of the DNA strand
Promoter – sequence of bases that tells the cell where to start transcribing the gene
RNA polymerase – enzyme that facilitates formation of mRNA strand
transcription
Transcription
(click image to enlarge)
Editing
mRNA transcript is edited before leaving nucleus
Introns – parts of sequence that are deleted (“nonsense”)
Exons – parts of sequence the remain in the final mRNA molecule
A structure called the spliceosome forms on the mRNA, facilitating the splicing of transcript (removal of introns, gluing together of exons)
transcription
DNA editing (simplified)
(click image for larger view)
Other forms of RNA (tRNA, mRNA) produced in a similar manner
The edited transcript then usually folds into a complex shape, unlike the simple strand of mRNA
mRNA leaves nucleus via nuclear pores
Translation of RNA (protein synthesis)
Location
Translation occurs in the cytoplasm outside the nucleus
Process
Initiation – mRNA associates with rRNA of ribosome
Elongation – tRNA brings amino acids into place (anticodons on tRNA complement codons on mRNA) elongating string of amino acids
Termination – protein is released
translation
Translation
EAch tRNA (blue) brings a specific amino acid to ribosome (green) and links to the complementary codon on mRNA strand. As amino acids link together with peptide bonds, a polypeptide grows in length.
Mutations
Definition
Chromosomal mutations – additions, deletions to chromosome
Point mutations – change in one or few nucleotides in a gene sequence
Cause
Mutations can be spontaneous (no known cause)
Can be caused by radiation (x-ray, UV, etc.), chemicals, mechanical damage, extreme temperature, and other factors
Regulation of gene expression
Chromosome level
Packing – DNA wraps around histone proteins, forming “beads” called nucleosomes
Packing can prevent certain genes from being activated (transcribed)
Transcription level
Master genes turn on a group of other genes
Enhancer genes change the rate of transcription of other genes
Regulatory chemicals, such as steroid hormones, may activate certain genes
mRNA can be edited in different ways (exons can be linked in different ways)
mRNA can be “made ahead of time” and then “masked” until needed later
Translation level
Translation can be halted by regulatory mechanisms in the cell
Interferon released by nearby virus-infected cells can trigger the activation of genes that produce translation-inhibiting proteins
This causes the cell to be more careful in checking mRNA and will halt translation of viral RNA that has infected the cell
RNA interference (RNAi)
Mechanism by which translation of mRNA can be disrupted (interference)
May help regulate gene expression
May help defend against mutations and viral RNA infection
Also called gene silencing
Location of DNA
Nucleus
46 chromosomes (diploid number) within nucleus (except during cell division)
DNA structure and location
DNA location and structure.
Mitochondrion
Single, ringlike strand of DNA comprising one chromosome
mitochondrial dna
mitochondrial dnaMitochondrial DNA (mtDNA).
This ringlike strand of DNA, similar to that seen in bacteria, contains genes that code for enzymes needed in mitochondrial function. Click on image to see larger source image, which is labeled with gene locations.
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to describe the coding used by cells in DNA/RNA
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