MCAT Biology Review 2025-26

1665 Robert Hooke

assembled microscope and found cells

1674 Leewenhock

1st man to view living cells

Tenets of Cell Theory

1. All living things are composed of ccells

2. cell is the most basic unit of life

3. cells arise only from pre-exisiting cells

4. cells carry genetic information in DNA form to be passed onto daughter cells

What tenets of cell theory do viruses violate that make them non-living?

1. cells arise only from pre-exisiting cells

2. cells carry genetic information in DNA form to be passed onto daughter cells

Eukaryotic Cells

• in multi cell orginisms

• nucleus enclosed in membrane

• membrane bound organelles=> compartmentalization of function

• reproduced by mitosis

cytosol

liquid that allows diffusion of molecules through cell

nucleus

Function: holds the genetic material encoded in DNA which is organized into chromosomes

• surrounded by nucleus membrane

• nuclear pores in membrane allow for selective material to enter and leave

• DNA has coding regions called genes wound against proteins called histomes

nucleolus

Function: Rrna Synthesis

• 25% of the colume of nucleus, the darker spot

mitochondria

Function: provides energy for the cell and performs apoptosis

• orginated in prokoyatic cell

• semi autonomous

  • has its own genes

  • replicates independently

  • cytoplasmic inheritance

• outer-membrane → The Matrix → inner-membrane (contains Electron Transport Chain material)

Lysosomes

Function: causes apoptosis when hydrolytic enzymes are released

• works with endosomes to break down many substrates

Endoplasmic Reticulum (ER)

series of interconnected membranes that are actually contiguous with the nuclear envelope

Rough ER

Function: permits the translation of proteins desired for direct secretion into the lumen

• studded with ribosomes

Smooth ER

Function: lipid synthesis and detox, transports proteins to the golgi

Golgi Apparatus

Function: proteins are modified, repackage products in vesicles, exocytosis

Peroxisomes

Function: contains hydrogen peroxide, breaks down fatty acid chains, sythesis of phosphlipids and enzymes

Cytoskeloton

Function: structure of cell and shape maintenance

Microfilaments

Function: cytokinesis and forms cleavage furrow

• made of actin (resistant to compression and fracture) which provides protection for the cell

• actin filaments can also use ATP

Predominant Protein: actin

Microtubles

Function: provides primary pathways for motor proteins kinesin and dynein to carry vesicles (cilla and flagella)

• hallow polymers of tublin proteins

Predominant Protein: tublin

Flagella

projections from cell involved of the movement of material inside the cell

Cilla

projections from cell involved of the movement of material outside the cell

9 + 2 structure

Flagella and Cilla Both have

• 9 pairs of microtubles in outer ring and two microtubles inside

centrioles

found in centrosome (cell region) and organizing cneters for microtubles and have a hallow center

Intermediate filaments

• cell to cell adhesion or maintenance of the cytoskeleton; anchors organelles

Predominant Protein: keratin and desmin

Epithelial Tissue

Function: provides protection against pathogens & desecration, absorbtion, secretion, and sensation (constitutes the parenchyma or the functional part of the organ)

• polarized (one side facing the lumen or hallow cavity and one side facing outside world)

• different classifications

simple epithelia

one layer of cells

stratified epithelia

multiple layers

psedostratified epithelia

appear to have multiple layers but actually have one

cubidol

cube shaped

columnar

long and thin

squamous

flat and scale like

Basement Membrane

the connective tissue that allows epithelial tissue to be joined together

Connective tissue

suppports the body and provides the framework for the epithelial cells to carry out their functions

• forms the extracelleur matrix

prokaryotes

cells that do not contain membrane bound organelles; they contain their genome in a single circular molecule of DNA located in the nucleoid region

archaea

extremophiles (mostly), often use chemosythesis and similar to both eukaryotes (translation, RNA polymerases, histones) and bacteria (single circular chromosomes, divide by binary fission or budding)

bacteria

similar structures to eukaryotes, complex relationships with humans including mutualistic symbiosis and pathogenesis

Eukarya

is the only non prokaryotic domain

Cocci

spherical bacteria

Bacilli

rod shaped bacteria

Spirilli

spiral shaped bacteria

Obligate aerobes

require oxygen for metabolism

obligate anaerobes

cannot survive in oxygen containing enviroments and can only carry out anaerobic metabolism

facultative anaerobes

can survive in environments with or without oxygen containing environment

aerotolerant anerobes

cannot use oxygen for metabolism, but can survive in oxygen-containing environments

Gram positive bacteria

have a thick cell wall composed of peptidoglycan and lipoteichoic acid

Gram Negative Bacteria

thin cell wall composed of peptidoglycan and an outer membrane containing phospholipids and lipopolysaccharides

Diploid (2n)

cells that have two copies of each chromosomes

Haploid (n)

cells have one copy

interphase

G1, S, G2 phases that is where DNA is uncoiled in the form of chromatin

G_{1 stage}

pre-synthetic gap, cells create organelles for energy and protein production, and increase their size

• Restriction point: during which the DNA is checked for quality, must be passed for the cell to move into the S stage

S (synthesis) stage

further cell growth and replication of organelles in preparation for mitosis, quality checkpoint before mitosis

M stage

mitosis and cytokinesis occurs

G₀ stage

the cell performs its functions without preparing for divsion

p53

plays a role in the two major checkpoints of the cell cycle

Cyclins and Cyclin Dependent Kinases (CDKs)

rise and fall during the cell cycle, cyclins bind to CDKs which activate transcription factors for next stage of cycle

Cancer

when cell cycle control becomes deranged which allows damaged cells to replicate; these type of cells will release factors that allow them to delocalize and invade nearby tissues (metastasize)

Somatic Cell

where mitosis produce two genetic identical diploid daughter cells from a single cell

Prophase

the chromosomes condense, the nuclear membrane dissolves, nucleoli dissapear, centrioles migrate to opposite sides of the cell and the spindle appratus begins to form

— kinetochore of each chromosome is contacted by a spindle fiber

Metaphase

chromosomes line up along the metaphase plate

Anaphase

sister chromatids are separated and pulled to opposite poles

Telophase

the nuclear membrane reforms, spindle apparatus disappears, and cytosol and organelles split between daughter cells through cytokinesis

mieosis

occurs in gametocytes and produces up to four haploid gametes

Meiosis

one round of replication and two rounds of division

Meiosis I

homologous parts of chromosomes are separated from each other

Homologues

are chromosomes that are given the same number, but are of opposite parental origin

Prophase I

same events occur EXCEPT:

• homologues come together and intertwine in a process called synapsis

• Four chromosomes called tetrad have crossing over that occurs to exchange genetic materal

• Mendels Second Law of Independent Assortment

Mendels Second Law of Independent Assortment

crossing over that occurs in prophase I

Metaphase I

homologous chromosomes line up on opposite sides of the metaphase plate

Anaphase I

homologous chromosomes are pulled to opposite pole of the cell; account for Mendels First Law of Segregation

Mendels First Law of Segregation

during the formation of reproductive cells (gametes) of Anaphase I , the two alleles for each trait separate, so that each gamete receives only one allele

Telophase I

the chromosomes may or may not fully decondence and the cell may enter interkinesis after cytokinesis

Meiosis II

sister chromotids are seperated from each other in a process that is functionally identical to mitosis

Sister Chromotids

are copies of the same DNA held together at the centromere

X Chromosome

• X linked genes are sex linked disorders

• males are hemizygous with respect to the unpaired genes on the X chromosome so they will express sex linked disorders even if they have one recessive allele while females will not

Y chromosome

carries little genetic information, but does contain SRY gene which cause the gonads to differentiate into testes

Sperm

develop in the seminferous tubiles in the testes; norished by sertoli cells

interstitial cells of Leydig

in the testes, secrete testosterone and other male sex hormones

scrotum

where the testes are located which hangs outside of the abdominal cavity and has a temperature 2 to 4 degrees celcius lower than the rest than the rest of the money

epididymis

sperm gain mobility here once formed and are stored there until ejaculation

Path of ejaculation

Vas Deferens —→ ejaculatory duct —→ uretha —→ penis

seminal vesicles

contribute fructose to nourish sperm and produce alkaline fluid

prostate gland

produces alkaline fluid

bulbourethral glands

produce a clear viscous fluid that cleans out any remnants of urine and lubricates the urethra during sexual arousal

semen

is composed of sperm and seminal fluid from the glands above

spermatogenesis

four haploid sperm are produced from a spermatogonium

Primary Spermatocytes

after S stage, the germ cells are called….

secondary spermatocytes

after meiosis I, the germ cells are called…

spermatids

after Meiosis II, the germ cells are called…

spermatozoa

after maturation, the germ cells are called…

head of sperm

contains the genetic material and is covered with an acrosome (modified Golgi apparatuss that contains enzymes that help sperm fuse with and penetrate the ovum)

mid-piece of sperm

generates ATP from fructose and contains many mitochondria

flagellum of sperm

promotes motility

Ova

are produced in follicles in the ovaries

Ovulation

once each moth, the eggs are ovulated into the peritoneal sac and is drawn into the fallopian tube or ovidact

Uterus

fallopian tubes are connected; the bottom is called the cervix

Vaginal Canal

lies below the cervix and is the site where sperm are deposited during intercourse; site of childbirth

Vulva

the external parts of the female genital

Oogenesis

one haploid ovum and a variable number of polar bodies are formed from an oogonium

Primary oocytes

at birth, all oogonia have already undergone replication and are considered…

Secondary oocytes

the ovulated egg each month and is arrested in metaphase II

Polar Body

Cytokinesis is uneven in oogenesis; the cell receiving very little cytoplasm and organelles