MCAT Biology

glands

organs that secrete hormones, signaling molecules that are secreted directly into the bloodstream to distant target tissues 

Classification of hormones by Chemical Structure

• peptide hormones

• steroid hormones

• amino acid derivative hormones

Peptide Hormones

• Made up of variously sized amino acids 

• Effects are rapid but short-lived

• Generally water soluble, travel easily in bloodstream

• Derived from larger precursor polypeptides cleaved during post translational modification

Peptide Hormones Trigger Signaling Cascade

• Charged nature keeps them from passing through extracellular membrane so act as “signal hormones”

  • Uses extracellular receptor to act as first messenger

  • Binding triggers transmission of second messenger 

Signaling Cascade

• describes potential effect from peptide hormone (first messenger) signaling second messenger 

Amplification

•  occurs when large quantities of 2nd messengers are signaled (peptide hormones)

Steroid Hormones

• Derived from cholesterol and produced primarily by gonads and adrenal cortex

• are usually nonpolar and easily cross cellular membrane 

• Undergo conformational change upon binding to a receptor (receptor is usually intracellular)

• Effects are slower but longer lived

• Are not water soluble so require a carrier in the bloodstream

Amino Acid Derivative Hormones

• Less common 

• Hormones derived from one or two amino acids

• Chemistry is considerably less predictable 

• catecholamines  bind to G-protein coupled receptors

  • released by adrenal medulla

• Thyroid hormones bind intracellularly 

Direct Hormones

 secreted and then act directly on target tissue

Tropic Hormones

• require an intermediary to act

  • Usually originate in the brain and anterior pituitary gland

Hypothalamus

• Regulates pituitary gland through tropic hormones 

• Controls pituitary through paracrine release of hormones into a portal system directly connecting the two organs 

• Negative feedback describes how the hypothalamus regulates hormone release

Hypophyseal Portal System

• blood vessel system that directly connects the hypothalamus with the anterior pituitary 

  • Hormones release into portal system and and travel down pituitary stalk to bind receptors in anterior pituitary

Hypothalamus secretes Releasing Hormones

these hormones act upon 4 tropic hormones and 1 direct hormone in anterior pituitary

• Gonadotropin-releasing hormone(GnRH) → follicle-stimulating hormone (FSH) and luteinizing hormone  (LH)

• Growth hormone - releasing hormone (GHRH) → Growth hormone (GH)

• Thyroid releasing hormone (TRH) → thyroid stimulating hormone (TSH)

• Corticotropin-releasing factor (CRF) → adrenocorticotropic hormone (ACTH)

Hypothalamus Directly inhibits the release of one hormone

• Prolactin-inhibiting factor (PIF actually dopamine) → decrease in prolactin secretion

  • Each tropic hormone causes release of another hormone that has negative feedback effects

Hypothalamic Interactions w/ Posterior Pituitary

• Neurons in hypothalamus act directly on posterior pituitary through axons and stimulate hormone release

  • Oxytocin 

  • Antidiuretic hormone (ADH)

Anterior Pituitary

• stimulated by hypothalamus via pituitary stalk

• Secretes and synthesizes seven different hormone products

  • 4 are tropic hormones 

  • 3 are direct hormones 

Tropic Hormones

• work by causing the release of another hormone at the organ level

  • Follicle stimulating hormone (FSH) (acts on gonads)

  • luteinizing hormone (LSH) (acts on gonads)

  • Adrenocorticotropic hormone (ACTH) (acts on adrenal cortex)

  • Thyroid-stimulating hormone (TSH) (acts on thyroid)

Direct Hormones

• Prolactin: stimulates milk production in mammary glands 

• Endorphins: decrease the perception of pain 

• Growth Hormone: promotes growth of bone and muscle

  • Prevents glucose reuptake and age breakdown of fatty acids

Posterior Pituitary

• Contains the nerve terminals of neurons with cell bodies in the hypothalamus 

• Receives and stores ADH and oxytocin from the hypothalamus

• secretes oxytocin

  

Antidiuretic Hormone (ADH)

secreted in response to low blood volume or increased blood osmolarity; increases permeability of water at the collecting duct in nephrons

Oxytocin

• secreted during childbirth and allows for coordinated contraction of uterine smooth muscle 

  • Has a positive feedback loop 

Thyroid

• Controlled by thyroid-stimulating hormone 

• Sets basal metabolic rate via release of triiodothyronine (T₃) and thyroxine (T₄)

• promotes calcium homeostasis through release of calcitonin

Increased T₃ and T₄ lead to higher amounts of cellular respiration

triiodothyronine (T₃) and thyroxine (T₄)

raise metabolic rate by making energy production more efficient via altering use of glucose/fatty acids

• released by thryroid

• act as feedback inhibitors on hypothalamus reducing TRH, and as a result, TSH

Calcitonin

hormone released by thyroid that decreases calcium resorption and thus lowers blood calcium levels

Parathyroid Hormone

serves as an antagonistic hormone to calcitonin, raising blood calcium levels

• decreasing excretion of calcium by the kidneys

• increasing absorption of calcium in the gut via activation of vitamin D

Parathyroid Glands

Four small glands located on the posterior surface of the thyroid gland; responsible for regulating calcium levels in the blood and bone metabolism

• secrete parathyroid hormone (PTH),

Adrenal Anatomy

• Adrenal glands: located on top of the kidney 

  • Adrenal cortex: secretes corticosteroids

  • Adrenal Medulla: secrete catecholamins

Corticosteriods

• have 3 classes 

  • glucocorticoids

  • mineralcorticoids

  • cortical sex hormones

Glucocorticoids

• steroid hormones that regulate glucose levels and affect protein metabolism 

  • Cortiasol and cortisone 

Cortisol and Cortisone

• glucocortiocids that raise blood glucose by increasing gluconeogenesis and decreasing protein synthesis 

• Decrease inflammation and immunologic responses

• released under control of ACTH hormone

Mineralcorticoids

• Used in salt and water homeostasis 

• Have profound effect on the kidney 

• Aldosterone

• Renin-angiotensin-aldosterone system

Renin-angiotensin-aldosterone system

• Controls release of aldosterone 

• Blood pressure restoration acts as negative feedback on renin release 

• 

Aldosterone

• increases sodium reabsorption in the distal convoluted tubule and collecting duct of the nephron ; decreases reabsorption of potassium and hydrogen ions promoting their excretion

Cortical Sex Hormones

• Androgens and estrogens 

• Adrenal testosterone plays relatively small role in male physiology due to secretion from testes

• Females are especially sensitive to disorders of cortical sex hormones since the ovaries secrete little androgen

Adrenal Medulla

• Responsible for production of sympathetic hormones epinephrine and norepinephrine (are amino acid derivatives part of a broader class of catecholamines)

• Have diverse effects centered around fight or flight response 

• Catecholamines release is also stimulated by cortisol

Pancreas Function

• Exocrine function: tissue secretes directly into ducts (digestive enzymes)

• Endocrine function

  • Islets of Langerhans: hormone-producing groups of cells littered throughout pancreas 

    • alpha cells

    • beta cells

    • delta cells

Islets of Langerhans

• Contain 3 cell types and exhibirt endocrine function of pancreas

  • Alpha cells:

  • Beta cells:

  • Delta cells: ns 

Alpha Cells (islet of langerhans)

• secretes glucagon

  • Glucagon is secreted when glucose levels are low, inhibited when high

Beta Cells (islet of langerhans)

• secrete insulin

  • Insulin is secreted when glucose levels are high and inhibited when low

  • Insulin induces glycogen storage

Delta Cells

• secret somatostatin 

  • Somatostatin inhibits both insulin and glucagon secretoin 

  • Stimulated by high blood glucose and amino acid concentrartio

Pineal Gland

• Secretes melatonin hormone 

  • Involved in circadian rhythms and maybe sleepiness

Fundamental Concept of Genetics

• Genes = DNA sequences that code for heritbable traits 

• Chromosomes = contains all genes; assures genetic material is passed to daughter cells

• Genotype: individual’s genetic combination

• Phenotype: observable traits

• Each gene has a locus on a chromosome

• Each chromosome homolog possesses an allele of a particular gene 

  • Can be homozygous, heterozygous, or hemizygous

  • Hemizygous = only one allele is present for a given gene

Patterns of Dominance

• Complete dominance 

• Codominance:

• Incomplete dominance:

Complete Dominance

• presence of one dominant allele will mask the recessive apple

Codominance

• when more than one dominant allele exists for a given gene 

Incomplete Dominance

• occurs when a heterozygote expresses a phenotype intermediate between two homozygous genotypes

Penetrance

• proportion of individuals in the population carrying the allele who actually express the phenotype 

  • Full …. 

  • High……

  • Low …..

  • Non-……

Expressivity

• when there are varying phenotypes despite identical genotypes 

  • Constant….: all individuals with a given genotype express same phenotype

  • Variable ….: individuals with the same genotype may have different phenotypes 

Mendel”s First Law of Segregation

• Genes exist in alternative forms (alleles)

• An organism has two alleles for each gene –one inherited from each parent 

• The two alleles segregate during meiosis, resulting in gametes that carry only one allele for any inherited trait 

• If two alleles of an organism are different, only one will be fully expressedThe inheritance of one gene does not affect the inheritance of another 

  • During prophase I, recombination allows for novel combinations of alleles that were once not present before

Mendel”s Second Law of Independent Assortment

• The inheritance of one gene does not affect the inheritance of another 

  • During prophase I, recombination allows for novel combinations of alleles that were once not present before

Frederick Griffith’s Experiments

• discovered the transformation principle

  • live, nonvirulent bacteria must have acquired the ability to form smooth capsules from dead virulent bacteria 

Gene Pool

all of the alleles that exist within a species 

• Genetic variability is essential for the survival of a species because it allows it to evolve and adapt 

Mutaitons

• Change in DNA sequence that results in a mutant allele 

• Contrasted with the wild type or natural allele 

• Transposons: can assert and remove themselves from a genome to induce a mutation 

• Nucleotide Level Mutaions

  • point mutation

  • frameshift mutation

Point Mutations

• Silent mutation: nitrogenous base inserted codes for same amino acid

• Missense mutation: new base codes for new amino acid 

• Nonsense mutation: codes for stop codon 

Frameshift Mutation

• shift the reading frame, usually result in either changes in the amino acid sequence or premature truncation of the protein  

  • Insertion

  • Deletion 

Chromosomal Mutations

• Deletion mutations 

• Duplication mutations

• Inversion mutations

• Insertion mutations

• Translocation:

Deltion and Duplication Mutaitons (chromsome)

• Deletion mutations: occur when a large segment of DNA is lost from a chromosome 

• Duplication mutations: a segment of DNA is copied multiple times in the genome 

Insertion and Inversion Mutatoins (chromosomes)

• Inversion mutations: occur when a segment of DNA is reversed within the chromosome 

• Insertion mutations: occur when a segment of DNA is moved from one chromosome to another 

Translocation Mutation

• occur when a segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome 

Consequences of mutations

• Advantageous mutations: confer a positive selective advantage that may allow the organism to produce fitter offspring 

• Deleterious mutations: when the mutation is detrimental 

  • Inborn errors of metabolism; class of deleterious mutations that cause defect in genes required for metabolism 

Genetic Linkage

a flow of genes between species 

• In some cases, individuals from different species can mate to produce hybrid offspring 

Genetic Drift

 refers to changes in the composition of the gene pool due to chance 

• Tend to be more pronounced in smaller populations 

Founder Effect

a more extreme case of genetic drift where a small population of a species finds itself in reproductive isolation from other populations as a result of natural barriers, catastrophic events or other bottlnecks

• bottlenecks, drastically and suddenly reduce the size of the population available for breeding 

Inbreeding

mating between two genetically related individuals, may occur in later generations if a breeding group is small 

• Encourages homozygosity 

Reduce genetic diversity 

• Genetic drift 

• Founder effect 

• Inbreeding 

Punnett Squares

•  predict the relative genotypic and phenotypic frequencies that will result from the crossing of two individuals 

  • Monohybrid cross: cross in which only one trait is being studied 

    • P generation: individuals being crossed

    • F generation: offspring

  • Test cross: used to determine an unknown genotype by crossing with an organism known to be homozygous recessive 

Monohybrid Cross

Dihybrid Cross

• Extension of a punnett square to account for inheritance of two different genes 

• Takes in account Mendel’s 2nd law of inheritance 

  • Holds true for “unlinked” genes 

  • Crossing individuals heterozygous for both traits produces a 9:3:3:1 ratio

Sex-linked Crosses

• Carrier: describes an X chromosome with a defective allele, denoted by  subscript 

• Females may be homozygous or heterozygous for a condition because they possess two X chromosomes 

• Males have one X chromosomes and are susceptible to even recessive sex-linked diseasees

Gene Mapping

• utilizes Recombination frequency: the likelihood that two alleles are separated from each other during crossing over 

  • The further apart two genes are, the more likely it is that there will be a point of crossing over, called a chiasma 

  • Tightly linked genes have lower recombination frequencies

Hardy Weinberg Equilibrium

• When  the gene frequencies of a population are not changing, the gene pool is stable and evolution is ostensibly not occurring

Five criteria of Hardy-weinberg Equilibrium

1. The population is very large (no genetic drift)

2. There are no mutations that affect the gene pool 

3. Mating between individuals in the population is random 

4. There is no migration of individuals into or out of the population 

5. The genes in the population are all equally successful at being reproduced 

Hardy Weinberg Equilibrium formula

allele frequency: How often an allele appears in a population

Conclusions from Hardy-weinberg Equations

• First equation tells allelic frequency

• 2nd equation tells genotypic and phenotypic frequency 

• If evolution is not occurring, allele frequencies will remain constant from generation to generation 

Natural Selection

• theory that certain characteristics or traits possessed by individuals within a species may help those individuals have greater reproductive success 

Tenets of Natural Selection

1. Organisms produce offspring, few of which survive to reproductive maturity 

2. Chance variations within individuals in a population may be heritable

   1. If variations give an organism an advantage, that variation is termed favorable 

3. Fitness, or reproductive success, is directly related to the relative genetic contribution of this individual to the next generation 

Neo-Darwinism

• adds knowledge of genetic inheritance and changes in the gen pool to Darwin’s original theory;

  • focus on differential reproduction and inclusive fitness

Differential Reproduction

• When mutation or recombination results in a change that is favorable to the organism’s reproductive success, that change is more likely to pass on the to the next generation; the opposite is also true 

Inclusive Fitness

a measure of of an organism’s success in the population

• based on the number of offspring, success in supporting offspring, and the ability of the offspring to then support others

Punctuated Equilibrium

• Contrasts Darwin;s theory to suggest changes in some species occur in rapid bursts rather than evenly over time 

Modes of Natural Selection

• Stabilizing selection:

• Directional selection:

• Disruptive selection

Stabilizing Selection

• keeps phenotypes within a specific range by selecting against extremes 

Directinoal Selection

• adaptive pressures can lead to the emergence and dominance of an initially extreme phenotype 

Disruptive Selection

• two extreme phenotypes are selected over the norm 

  • Facilitated by the existence of polymorphisms – naturally occurring differences in form between members of the same population 

Speciation

•  the formation of a new species through evolution 

  • Result from evolutionary pressures leading to adaptive changes 

  • Species: the largest group of organisms capable of breeding to form fertile offspring

Isolation

• when the progeny of populations can no longer freely interbreed

• Prezygotic mechanisms: prevent formation of the zygote completely 

  • Ex: temporal, ecological, behavioral, reproductive isolations

• Postzygotic mechanisms: allow for gamete fusion but yield either nonviable or sterile offspring 

  • Ex: hybrid inviability, hybrid sterility, hybrid breakdown

Divergent Evolution

refers to the independent development of dissimilar characteristics in two or more lineages sharing a common ancestor

Parallel Evolution

• refers to the process whereby related species evolve in similar ways for a long period of time in response to analogous environmental selection pressures 

Convergent Evolution

• refers to the independent development of similar characteristics in two or more lineages not sharing a recent common ancestry 

Molecular clock model

The more similar the genomes, the more recently the two species separated from each other

• Molecular evolutionists correlate the degree of genomic similarity with the amount of time since two species split off from the same common ancestor 

Skeletal Muscle

• Responsible for voluntary movement and is innervated by somatic nervous system 

• Actin and myosin arranged in repeating units called sarcomeres, appears striated

• Multinucleated

posseses two types of muscle fibers

• red fibers

• whiter fibers
  

Red Fibers (skeletal muscle)

• Slow-twitch fibers 

• High myoglobin content 

  • Myoglobin: oxygen carrier that uses iron in a heme group to bind oxygen 

• Derive energy aerobically

• Muscles contract slowly, fatigue slowly

White Fibers (skeletal Muscle)

• Fast-twitch fibers 

• Contain much less myoglobin 

• Muscles contract rapidly fatigue quickly

Smooth Muscle

• Responsible for involuntary action, innervated by autonomic

• Single nucleation

• Capable of more sustained contraction then skeletal muscle

  • Tonus: constant state of low-level contraction (ex. Blood vessels)

Can exhibit myogenic activity: muscle cells contract in direct response to stretch or other stimuli

Cardiac Muscle

• Characteristics of both smooth and skeletal muscle 

• Primarily uninucleated, striated  

• Involuntary and innervated by autonomic 

• myocytes (cardiac muscle cells) are connected by intercalated discs that contain gap junctions

  • Allows for rapid and coordinated depolarization of muscle cells and efficient contractions of cardiac muscle

Sarcomere

• Basic contractile unit of skeletal muscle 

  • Thick and thin filaments 

    • Thick filaments: organized bundles of myosin

    • Thin filaments:  actin bundled with troponin and tropomyosin 

Titin

• acts as spring and anchors actin and myosin filaments preventing excessive stretching 

Sarcomere (diagram)

• boundary of each sarcomere 

• A Band: thick filaments and their overlap w/ thin filaments

• I band: exclusively thin filaments 

• H zone: exclusively thick filaments 

Myofibrils

•  formed from sarcomeres attached end to end and surrounded by a covering known as the sarcoplasmic reticulum 

  • Modified endoplasmic reticulum that contains high concentration of Ca2+

Sarcolemma

• cell membrane of a myocyte capable of propagating an action potential through calcium ions of its sarcoplasmic reticulum 

  • Transverse tubules: distribute action potential through all all sarcomeres in a muscle 

Organization of muscle structure

• Myocyte (muscle fiber): parallel units make a muscle 

• Muscle Fiber: made up of many myofibrils 

• Myofibrils: made of many many sarcomere units