Biology Notes.docx

Mastering Biology Notes-

WEEK #8- ANIMAL DIGESTION + NUTRITION ACQUISTION:

Concept 41.1: An animal’s diet must supply chemical energy, organic building blocks, and essential nutrients
- An adequate diet must satisfy 3 needs:
  - Chemical Energy for cellular processes
    - Carbohydrates, Proteins, Lipids
  - Organic building blocks for macromolecules
  - Essential nutrients
- The process by which an animal takes in and makes use of food to meet these needs constitutes nutrition
- Ingested and digested nutrients can be broken down immediately or stored for future use
- Essential Nutrients- a substance that an organism cannot synthesize from any other material and therefore must absorb in preassembled form
  - Certain amino acids and fatty acids
  - Vitamins & Minerals
  - Key Functions-
    - Serving as substrates of enzymes, as coenzymes, and as cofactors in biosynthetic reactions
- Essential Amino Acids- an amino acid that an animal cannot synthesize itself and must be obtained from food in prefabricated form
  - Example: many animals, including adult humans, require 8 amino acids
    - Isoleucine
    - Leucine
    - Lysine
    - Methionine
    - Phenylalanine
    - Threonine
    - Tryptophan
    - Valine
      - Human Infants need a 9^th: histidine
- Animals require fatty acids to synthesize a variety of cellular components, including membrane phospholipids, signaling molecules, and storage fats
  - Although animals can synthesize many fatty acids, they lack the enzymes to form the double bonds found in certain required fatty acids
    - Essential Fatty Acids- an unsaturated fatty acid that an animal needs but cannot make
- Vitamins- organic molecules that are required in the diet in very small amounts (0.01 – 100 mg per day, depending on the vitamin)
- Dietary Minerals- inorganic nutrients, such as iron and sulfur, that are usually required in small amounts- from less than 1 mg to about 2,500 mg per day
- Variation in Diet
  - Herbivores- dine mainly on plants or algae
    - Cattle, sea slugs, caterpillars
  - Carnivores- mostly eat other animals
    - Sea otters, hawks, spiders
  - Omnivores- don’t in fact eat everything, but they do regularly consume animals as well as plants or algae
    - Bears, crows, cockroaches
  - Animals are opportunistic feeders, broadening their diet when their usual foods aren’t available
    - Deer are herbivores, but occasionally eat insects, worms, or bird eggs
- A diet that lacks one or more essential nutrients or consistently supplies less chemical energy than the body results in malnutrition
  - Malnutrition affects one out of four children worldwide
Concept 41.2: Food processing involves ingestion, digestion, absorption, and elimination
- Ingestion- the first stage of food processing in animals: the act of eating
- Digestion- the second stage of food processing in animals: the breaking down of food into molecules small enough for the body to absorb
  - Mechanical digestion, such as chewing or grinding, breaks food into smaller pieces, increasing surface area
  - Chemical digestion is necessary because animals cannot directly use the nucleic acids, fats, phospholipids, and most carbohydrates in food
    - Chemical breakdown by digestive enzymes reverses the synthesis of a fat or macromolecule by breaking bonds through the addition of water
      - This splitting process is called enzymatic hydrolysis
- Absorption- the third stage of food processing in animals: the uptake of small nutrient molecules by an organism’s body
- Elimination- the fourth and final stage of food processing in animals: the passing of undigested material out of the body
- Complete Digestive tract or Alimentary Canal- digestive tract with two openings, a mouth and an anus
Concept 41.3 Organs specialized for sequential stages of food processing form the mammalian digestive system
- As soon as food enters your mouth, or oral cavity, food processing begins
- Meanwhile, the anticipation or arrival of food in the oral cavity triggers the release of saliva by the salivary glands
  - Salivary glands- a gland associated with the oral cavity that secretes substances that lubricate food and begin the process of chemical digestion
  - Saliva is a complex mixture of materials with a number of vital functions
    - Mucus- a viscous and slippery mixture of glycoproteins, cells, salts, and water that moistens and protects the membranes lining body cavities that open to the exterior
    - The most abundant enzyme in saliva is amylase, which breaks down starch and glycogen
- Pharynx- an area in the vertebrae throat where air and food passages cross
- Esophagus- a muscular tube that conducts food, by peristalsis, from the pharynx to the stomach
- Peristalsis- alternating waves of contraction and relaxation in the smooth muscles lining the alimentary canal that push food along the canal
- Sphincter- a ringlike band of muscle fibers that controls the size of an opening in the body, such as the passage between the esophagus and the stomach
- The Stomach- an organ of the digestive system that stores food and performs preliminary steps of digestion
  - The stomach secretes a digestive fluid called gastric juice and mixes it with the food through a churning action
    - This mixture of ingested food and gastric juice is called chyme
    - Two components of gastric juice help liquefy food in the stomach
      - Hydrochloric Acid (HCl)- disrupts the extracellular matrix that binds cells together in meat and plant material

pH is ~2; causing proteins to unfold increasing exposure of their peptide bonds

- - - - The exposed bonds are then attacked by the second component of gastric juice- a protease (an enzyme that digests proteins by hydrolysis) called pepsin (an enzyme present in gastric juice that begins the hydrolysis of proteins)
  - Pepsinogen- the inactive form of pepsin secreted by chief cells located in the gastric pits of the stomach
  - The breakdown of food by gastric juices is enhanced by muscular activity of the stomach
    - The coordinated series of muscle contractions and relaxations that we call “churning” mixes the stomach contents about every 20 seconds
      - Churning facilitates the action of HCl and pepsin by bringing all the food into contact with the gastric juices secreted by the lining of the stomach

As a result, what began as a recently swallowed meal becomes the acidic, nutrient-rich broth known as chyme

- - - The painful irritation of the esophagus that results from the sphincter at the top of the stomach allows a flux of chyme is commonly called “heartburn” or acid reflux
- Small intestine- the longest section of the alimentary canal, so named because of its small diameter compared with that of the large intestine; the principal site of the enzymatic hydrolysis of food macromolecules and the absorption of nutrients
  - Duodenum- the first section of the small intestine, where chyme from the stomach mixes with digestive juices from the pancreas, liver, and gallbladder as well as from gland cells of the intestinal wall
  - With absorption largely complete, the contents of the duodenum move into the jejunum and ileum, the remaining regions of the small intestine.
    - There, nutrient absorption occurs across the lining of the intestine
      - Villi- a finger-like projection of the inner surface of the small intestine

Microvilli- one of many fine, finger-like projections of the epithelial cells in the lumen of the small intestine that increase its surface area

- - - - Hepatic Portal Vein- a large vessel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood’s nutrient content

Allows the liver to regulate the distribution of nutrients to the rest of the body

Allows the liver to remove toxic substances before they can circulate broadly

- - - - The products of hydrolysis of a fat by lipase are absorbed by epithelial cells and recombined into triglycerides, these products are too large to enter the blood vessels

Chylomicrons- a lipid transport globule composed of fats mixed with cholesterol and coated with proteins

In exiting the small intestine, chylomicrons first enter a lacteal, a vessel at the core of each villus that has large gaps between adjacent cells

- Pancreas- a gland with exocrine and endocrine tissues. The exocrine portion functions in digestion, secreting enzymes, and an alkaline solution into the small intestine via a duct; the ductless endocrine portion functions in homeostasis, secreting the hormones insulin and glucagon into the blood
- Fats present a particular challenge for digestion
  - Fat digestion is facilitated by bile salts, which act as emulsifiers (detergents) that break apart fat and lipid globules.
    - Bile- a mixture of substances that is produced in the liver and stored in the gallbladder; enables formation of fat droplets in water as an aid in digestion and absorption of fats
- Liver- a large internal organ in vertebrates that performs diverse functions, such as producing bile, maintaining blood glucose level, and detoxifying poisonous chemicals in the blood
  - Bile production is metabolically linked to another vital liver function: the destruction of red blood cells that are no longer fully functional
    - Pigments released during red blood cell disassembly are incorporated into bile pigments, which are eliminated from the body with the feces
      - In some liver and blood disorders, bile pigments accumulate in the skin, resulting in a yellowing called jaundice
- Gallbladder- an organ that stores bile and releases it as needed into the small intestine
- The alimentary canal ends with large intestine – the portion of the vertebrate alimentary canal between the small intestine and the anus; functions mainly in water absorption and the formation of feces.
  - The large intestine includes- colon, cecum, rectum
  - The small intestine connects to the large intestine at a T-shaped junction
    - One arm of the T is the colon – the largest section of the vertebrate large intestine; functions in water absorption and formation of feces – which leads to the rectum and anus
      - The colon completes the recovery of water that began in the small intestine. What remain are the feces, the wastes of the digestive system which become increasingly solid as they are moved along the colon by peristalsis

It takes approx. 12-24 hours for material to travel the length of the colon

- - - - Rectum- the terminal portion of the large intestine, where the feces are stored prior to elimination
    - The other arm is a pouch called the cecum – the blind pouch forming one branch of the large intestine- which has an important role in fermenting ingested material
      - In humans, the cecum is small and has an appendix – a small, finger-like extension of the vertebrate cecum; contains a mass of white blood cells that contribute to immunity
Concept 41.4: Evolutionary adaptations of vertebrate digestive systems correlate with diet
- Microbiome- the collection of microorganisms living in or on an organism’s body, along with their genetic material
- Ruminants- a cud-chewing animal, such as a cow or sheep, with multiple stomach compartments specialized for an herbivorous diet

Week #9- The ‘Central Dogma’ of Molecular Biology

Concept 17.1: Genes specify proteins via transcription and translation
- The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins and of RNA molecules involved in protein synthesis
- Gene expression is the process by which DNA directs the synthesis of proteins (or, in some cases, just RNAs)
  - Proteins are the link between genotype and phenotype
  - The expression of genes that code for proteins includes two stages: transcription and translation
- NOT ALL ENZYMES ARE PROTEINS
  - Ex. Keratin + Insulin
- Transcription- the synthesis (production) of RNA using information in the DNA
  - This type of RNA molecule is called messenger RNA (mRNA) because it carries a genetic message from the DNA to the protein-synthesizing machinery of the cell
- Translation- the synthesis of a polypeptide using the information in the mRNA
  - The cell must translate the nucleotide sequence of an mRNA molecule into the AA sequence of a polypeptide
  - The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains
- Transcription occurs in the nucleus, but the mRNA must be transported to the cytoplasm for translation
- Primary transcript: an initial RNA transcript from any gene; also called pre-mRNA when transcribed from a protein-coding gene
- Central Dogma by Francis Crick in 1956-
  - Genes program protein synthesis via genetic messages in the form of messenger RNA.
  - DNA -> RNA -> Protein
  - 1970s discovery that some enzymes us RNA molecules as templates for DNA synthesis
    - RNA -> DNA
- Triplet Code- a genetic information system in which a series of three-nucleotide-long words specifies a sequence of amino acids for a polypeptide chain
- Template Strand- the DNA strand that provides the pattern, or template, for ordering, by complementary base pairing, the sequence of nucleotides in and RNA transcript
  - An mRNA molecule is complementary rather than identical to its DNA template because RNA nucleotides are assembled on the template according to base-pairing rules
- Codons- a three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or termination signal; the basic unit of genetic code
- Coding Strand- contemplate strand of DNA, which has the same sequence as the mRNA except it has thymine (T) instead of uracil (U)
- Reading Frame- on an mRNA, the triplet grouping of ribonucleotides used by the translation machinery during polypeptide synthesis
Concept 20.1: DNA sequencing and DNA cloning are valuable tools for genetic engineering and biological inquiry
- Restriction enzymes protect the bacterial cell by cutting up foreign DNA from other organisms or phages
  - Restriction Site- a specific sequence on a DNA strand that is recognized and cut by a restriction enzyme
  - Restriction Fragments- a DNA segment that results from the cutting of DNA by a restriction enzyme
  - Sticky end- a single-stranded end of a double-stranded restriction fragment
- Gel Electrophoresis- a technique for separating nucleic acids or proteins on the basis of their size and electrical charge, both of which affect their rate of movement through an electric field in a gel made of agarose or another polymer
- Polymerase Chain Reaction- a technique for amplifying DNA in vitro by incubating it with specific primers, a heat-resistant DNA polymerase, and nucleotides

Week #10- Transcription and Translation: How DNA is used to make mRNA, and mRNA is used to make proteins

Concept 17.2: Transcription is the DNA-directed synthesis of RNA
- RNA polymerase- an enzyme that links ribonucleotides into a growing RNA chain during transcription, based on complementary binding to nucleotides on a DNA template strand
- Promoter- a specific nucleotide sequence in the DNA of a gene that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place
- Terminator- in bacteria, a sequence of nucleotides in DNA that marks the end of a gene and signals RNA polymerase to release the newly made RNA molecule and detach from the DNA
- Transcription Unit- a region of DNA that is transcribed into an RNA molecule
- RNA polymerase Binding and Initiation of Transcription
  - Start Point- in transcription, the nucleotide position on the promoter where RNA polymerase begins synthesis of RNA
  - Based on interactions with proteins (transcription factors), RNA polymerase binds in a precise location and orientation on the promoter
    - This binding determines where transcription starts and the direction it will travel, thus which strand of DNA is used as the template
  - Transcription factors- a regulatory protein that binds to DNA and affects transcription of specific genes
  - Transcription initiation complex- the completed assembly of transcription factors and RNA polymerase bound to a promoter
    - TATA box- a DNA sequence in eukaryotic promoters crucial in forming the transcription initiation complex
Concept 17.3: Eukaryotic cells modify RNA after transcription
- Enzymes in the eukaryotic nucleus modify pre-mRNA in specific ways before the genetic message is dispatched to the cytoplasm
  - RNA processing- modification of RNA primary transcripts, including splicing out of introns, joining together of exons, and alteration of the 5’ and 3’ ends
    - The 5’ end, which is synthesized first, receives a 5’ cap- a modified form of guanine nucleotide added onto the 5’ end of a pre-mRNA molecule
    - At the 3’ end, an enzyme then adds 50-250 more adenine (A) nucleotides, forming a poly-A tail
    - FUNCTIONS:
      - Facilitate the export of the mature mRNA from the nucleus
      - Help protect the mRNA from degradation by hydrolytic enzymes
      - Help ribosomes attach to the 5’ end of the mRNA once it reaches the cytoplasm
    - UTRs- at the ends of mRNA; not translated into protein, but have other functions, such as ribosome binding
    - RNA Splicing- after synthesis of a eukaryotic primary RNA transcript, the removal of portions of the transcript (introns) that will not be included in the mRNA and the joining together of the remaining portions (exons)
      - Introns- a noncoding, intervening sequence within a primary transcript that is removed from the transcript during RNA processing; also refers to the region of DNA from which this sequence was transcribed
      - Exons- a sequence within a primary transcript that remains in the RNA after RNA processing; also refers to the region of DNA from which this sequence was transcribed
      - Spliceosome- a large complex made up of proteins and RNA molecules that splices RNA by interacting with the ends of an RNA intron, releasing the intron and joining the two adjacent exons
    - Ribozymes- an RNA molecule that functions as an enzyme, such as an intron that catalyzes its own removal during RNA splicing
    - Alternative RNA splicing- a type of eukaryotic gene regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns
    - Domains- a discrete structural and functional region of a protein
Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: A Closer Look
- Transfer RNA (tRNA)- an RNA molecule that functions as a translator between nucleic acid and protein languages by picking up a specific amino acid and carrying it to the ribosome, where the tRNA recognizes the appropriate codon in the mRNA
- Anticodon- a nucleotide triplet at one end of a tRNA molecule that base-pairs with a particular complementary codon on an mRNA molecule
- Aminoacyl-tRNA synthetases- an enzyme that joins each amino acid to the appropriate tRNA
- Wobble- flexibility in the base-pairing rules in which the nucleotide at the 5’ end of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position (3’ end) of a codon
- Ribosomal RNAs (rRNA)- RNA molecules that, together with proteins, make up ribosomes, the most abundant type of RNA
- P site (peptidyl-tRNA binding site)- one of a ribosome’s three binding sites for tRNA during translation. The P site holds the tRNA carrying the growing polypeptide chain
- A site (aminoacyl-tRNA binding site)- one of a ribosome’s three binding sites for tRNA during translation. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain
- E site (exit site)- one of a ribosome’s three binding sites for tRNA during translation. The E site is the place where discharged tRNAs leave the ribosome
- The Initiation stage:
  - Brings together an mRNA, a tRNA bearing the first amino acid of the polypeptide, and the two subunits of a ribosome
- The Elongation stage:
  - Amino acids are added one by one to the previous amino acid at the C-terminus of the growing chain
- The Termination stage:
  - Elongation continues until a stop codon in the mRNA reaches the A site
- Signal Peptide- a sequence of about 20 amino acids at or near the leading (amino) end of a polypeptide that targets it to the endoplasmic reticulum or other organelles in a eukaryotic cell
- Signal-recognition particle- a protein-RNA complex that recognizes a signal peptide as it emerges from a ribosome and helps direct the ribosome to the endoplasmic reticulum (ER) by binding to a receptor protein on the ER
- Polyribosomes- a group of several ribosomes attached to, and translating, the same messenger RNA molecule
Concept 16.2: Many proteins work together in DNA replication and repair
- DNA replication- the process by which a DNA molecule is copied; also called DNA synthesis
- Semiconservative Model- type of DNA replication in which the replicated double helix consists of one old strand, derived from the parental molecule, and one newly made strand
- The replication of chromosomal DNA begins at particular sites called origins of replication- short stretches of DNA that have a specific sequence of nucleotides
- Replication Fork- a Y-shaped region on a replicating DNA molecule where the parental strands are being unwound and new strands are being synthesized
- Helicases- an enzyme that untwists the double helix of DNA at replication forks, separating the two strands and makings them available
- Single-Strand Binding Proteins- a protein that binds to the unpaired DNA during DNA replication, stabilizing them and holding them apart while they serve as templates for the synthesis of complementary strands of DNA
- Topoisomerase- a protein that breaks, swivels, and rejoins DNA strands. During DNA replication, topoisomerase helps to relieve strain in the double helix ahead of the replication fork
- Primer- a short nucleotide with a free 3’ end, bound by complementary base pairing to the template strand and elongated with DNA nucleotides during DNA replication
- Primase- an enzyme that joins RNA nucleotides to make a primer during DNA replication, using the parental DNA strand as a template
- DNA Polymerases- an enzyme that catalyzes the elongation of new DNA by the addition of nucleotides to the 3’ end of an existing chain. There are several different DNA polymerases; DNA polymerase 3 and DNA polymerase 1 play major roles in DNA replication in E. coli
- Leading strand- the new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5’ to 3’ direction
- Lagging strand- a discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5’ to 3’ direction away from the replication fork
  - Okazaki Fragments- a short segment of DNA synthesized away from the replication fork on a template strand during DNA replication. Many such segments are joined together to make up the lagging strand of newly synthesized DNA
- DNA ligase- a linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3’ end of one DNA fragment to the 5’ end of another DNA fragment
- Mismatch Repair- the cellular process that uses specific enzymes to remove and replace incorrectly paired nucleotides
- Nuclease- an enzyme that cuts DNA or RNA, either removing one or a few bases or hydrolyzing the DNA or RNA completely into its component nucleotides
- Nucleotide excision repair- a repair system that removes and then correctly replaces a damaged segment of DNA using the undamaged strand as a guide
- Telomeres- the tandemly repetitive DNA at the end of a eukaryotic chromosome’s DNA molecule. Telomeres protect the organism’s genes from being eroded during successive rounds of replication; nucleotide sequences, usually noncoding, that are present in many copies in a eukaryotic genome. The repeated units may be short and arranged tandemly or long and dispersed in the genome.

Week #11- Regulating Gene Expression: How we control whether or not we make a protein

Concept 18.1: Bacteria often respond to environmental change by regulating transcription
- First, cells can adjust the activity of enzymes already present.
  - The activity of the first enzyme in the pathway is inhibited by the pathway’s end product
  - Thus, if tryptophan accumulates in a cell, it shuts down the synthesis of more tryptophan by inhibiting enzyme activity
  - Such feedback inhibition, typical of anabolic (biosynthetic) pathways, allows a cell to adapt to short-term fluctuations in the supply of a substance it needs
- Second, cells can adjust the production level of certain enzymes via a genetic mechanism; that is, they can regulate the expression of the genes encoding the enzymes.
  - Example: If the environment provides all the tryptophan the cell needs, the cell stops making the enzymes that catalyze the synthesis of tryptophan
    - In this case, the control of enzyme production occurs at the level of transcription, the synthesis of messenger RNA from the genes that code for these enzymes
- A key advantage of grouping genes of related function into one transcription unit is that a single “on-off switch” can control the whole cluster of functionally related genes; in other words, these genes are coordinately controlled
  - Operator- in bacterial and phage DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach. The binding of the repressor prevents RNA polymerase from attaching to the promoter and transcribing the genes of the operon
- Operon- a unit of genetic function found in bacteria and phages, consisting of a promoter, an operator, and a coordinately regulated cluster of genes whose products function in a common pathway
- By itself, the trp operon is turned on; that is, RNA polymerase can bind to the promoter and transcribe the genes of the operon.
  - The trp operon can be switched off by a protein that is called the trp repressor
    - Repressor- a protein that inhibits gene transcription. In prokaryotes, repressors bind to the DNA in or near the promoter. In eukaryotes, repressors may bind to control elements withing enhancers, to activators, or to other proteins in a way that blocks activators from binding to DNA
      - A repressor protein is specific for the operator of a particular operon
- Regulatory Gene- a gene that codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes
- The binding of repressors to operators is reversible
  - An operator alternates between two states: one with the repressor bound and one without the repressor bound
- The trp repressor, like most regulatory proteins, is an allosteric protein, with two alternative shapes: active and inactive
  - The trp repressor is synthesized in the inactive form, which has little affinity for the trp operator
- Only when a tryptophan molecule binds to the trp repressor at an allosteric site does the repressor protein change to the active form that can attach to the operator, turning the operon off
- Corepressor- a small molecule that binds to a bacterial repressor protein and changes the protein’s shape, allowing it to bind to the operator and switch an operon off
- Inducer- a specific small molecule that binds to a bacterial repressor protein and changes the repressor’s shape so that it cannot bind to an operator, thus switching an operon on
- Activator- a protein that binds to DNA and stimulates gene transcription. In prokaryotes, activators bind in or near the promoter; in eukaryotes, activators generally bind to control elements in enhancers
Concept 18.2: Eukaryotic gene expression is regulated at many stages
- Differential gene expression: the expression of different set of genes by cells with the same genome
- In all organisms, gene expression is commonly controlled at transcription; regulation at this stage often occurs in response to signals coming from outside the cell, such as hormones or other signaling molecules
- The structural organization of chromatin not only packs a cell’s DNA into a compact form that fits inside the nucleus, but also helps regulate gene expression in several ways
  - Genes within heterochromatin, which is more densely arranged than euchromatin, are usually not expressed
  - In euchromatin, whether or not a gene is transcribed is affected by the location of nucleosomes along a gene’s promoter and also the sites where the DNA attaches to the protein scaffolding of the chromosome
- Histone Acetylation- the attachment of acetyl groups to certain amino acids of histone proteins
- DNA methylation the presence of methyl groups on the DNA bases (usually cytosine) of plants, animals, and fungi.
- Epigenetics- the study of the inheritance of traits transmitted by mechanisms that do not involve the nucleotide sequence
- Control elements- a segment of noncoding DNA that helps regulate transcription of a gene by serving as a binding site for a transcription factor. Multiple control elements are present in a eukaryotic gene’s enhancer
- Enhancers- a segment of eukaryotic DNA containing multiple control elements, usually located far from the gene whose transcription it regulates
Concept 16.3: A chromosome consists of a DNA molecule packed together with proteins
- Euchromatin: the less condensed form of eukaryotic chromatin that is available for transcription
- Heterochromatin: eukaryotic chromatin that remains highly compacted during interphase and is generally not transcribed

Week #12- DNA and Disease: How our genetic ‘instruction manual’ can include mistakes