Biology: DNA, Cell Cycle, Mitosis, Meiosis

Friedrich Miescher’s scientific discovery (1868)

investigated the chemical composition of the nucleus and concluded that the nucleus contains material that is not a protein but another phosphorus containing material

• used cells from the pus from hospital bandages

• isolated an organic acid that was high in nitrogen and phosphorus (he called it nuclein) - importantly, it was NOT a protein

Frederick Griffith’s scientific discovery (early 1900s)

in trying to develop a vaccine, found that the nuclear material that Miescher discovered had the ability to transform non pathogenic bacteria into pathogenic bacteria (transforming principle)

• injected mice with live and dead forms of strep pneumoniae to trigger antibody response

  • Rough (R) strain did not kill the mice

  • Smooth (S) strain killed the mice

• something holding onto the trait for lethality was transferred

Oswald Avery, Colin MacLeod, & Maclyn McCarthy’s scientific discovery (1940)

concluded that the “transforming principle” was a nucleic acid

• chemically separated deadly smooth strain bacterial cells by destroying different components

  • mixed with non-lethal rough strain and injected into mice

Erwin Chargaff’s scientific discovery

researched the nitrogenous bases in DNA by pooling and separating them, concluding with two “rules”

1. for all species, the percentages of A=T and G=C

2. between different species there can be some variation in A, T, G, C amounts

Wilkins and Rosalind Franklin’s scientific discovery (1940s-50s)

used x-ray crystallography to find that DNA was a long, thin molecule in a double helix shape, the rungs of the ladder were .34 nm, one twist was 10 rungs (3.4 nm) long, and width was consistent at 2 nm

Watson and Crick’s scientific discovery (1953)

used Franklin’s x-ray results and other scientists’ discoveries as a basis to build a wood and metal model for the ground-breaking discovery of the “double-helix” structure for DNA (put all together into a one page Nature article):

• consists of 2 strands of nucleotides (not 3 as Linus Pauling suggested)

• sugars and phosphate make up the sides of the ladder

• nitrogenous bases make up the rungs of the ladder (some had suggested they were on the outside)

• nucleotide rung held together by hydrogen bonds (weak)

• the bonds form when each strand runs in opposite directions (anti-parallel) of the other (an a-Ha discovery)

Structure of DNA

double helix

• two strands twist around each other to form a double helix, a spiral staircase shape

  • sides run anti-parallel

backbone

• dioxyribose sugar and phosphate groups form the backbone of each strand, running along the outside of the helix

bases

• attached to each sugar molecule are one of the four nitrogenous base (A, T, G, C)

base pairing

• the two strands are connected by hydrogen bonds between specific base pairs: A-T and G-C

complementary strands

• because of base pairing, one strand of DNA can be used to determine the sequence of the other, making it possible to copy and replicate DNA

  • 5 end has phosphate group

  • 3 end has hydroxyl group

genetic information

• the sequence of the bases encodes the genetic information, carrying instructions for making proteins and other biological molecules

chromatin

DNA in its un-condensed form; the complex of DNA and proteins, primarily histones, that makes up the genetic material of eukaryotic cells

• exists in different forms within the nucleus

• thin wispy, threadlike

• in a non-dividing cell

• usually too thin to be visible

DNA packaging

• primary function is to package DNA into a compact form that fits within the nucleus

structure

• basic unit is the nucleosome, where DNA is wrapped around histone proteins

dynamic nature

• structure can change during the cell cycle, including condensation into chromosomes during mitosis

gene expression regulation

• plays a role in regulating the gene expression by controlling access to DNA

cell cycle regulation

• histone modifiers, which regulate chromatin structure, are also regulated by the cell cycle

chromosome

DNA in a fully condensed form; a structure within a cell’s nucleus that carries DNA

• made up of DNA tightly coiled around histone proteins

• can be single stranded or double stranded (X shape)

• DNA in preparation for cell division

DNA packaging

• DNA is a very long molecule, so its tightly packed and organized into chromosomes to fit within the cell’s nucleus

structure

• threadlike structures, visible under a microscope during cell division when DNA is most condensed

genes

• contain genes, which are specific DNA sequences that code traits

human chromosomes

• humans have 23 pairs of chromosomes, 46 total chromosomes

cell division

• are essential for accurate DNA replication and distribution during cell division, ensuring that each daughter cell receives a complete set of genetic information

histone protein

basic proteins that play a crucial role in organizing DNA within the nucleus of eukaryotic cells and regulating gene expression

• package DNA into structural units called nucleosomes

• influence how tightly DNA is packed

• chromatin wrap around to help 3m of DNA pack into a small nucleus (“spool of thread”)

DNA packaging

• responsible for wrapping DNA around themselves, forming nucleosomes. which are then further organized into higher-order structures (ie. chromosomes, chromatin)

• this process compacts DNA, allowing it to fit within the nucleus and enabling efficient cell division

gene expression regulation

• packaging of DNA by histones can influence whether genes are accessible for transcription

• histones can be modified to change accessibility of DNA, thereby influencing which genes are turned on or off

chromosome structure

• provide structural support for chromosomes, ensuring proper organization and function during cell division

DNA protection

• help protect DNA from damage by physical forces and chemical agents

nucleosome

the fundamental structural unit of chromatin, the DNA-protein complex that makes up eukaryotic chromosomes

• consists of DNA wrapped around a core of histone proteins - this packaging allows for the efficient organization and compaction of DNA within the cell nucleus

components

• made up of approx. 147 base pairs of DNA wrapped around a complex of 8 histone proteins

DNA wrapping

• DNA is wrapped around the histone complex

structure

• arranged in a beadlike structure along the DNA, with the DNA between nucleosomes called linker DNA

function

• play a crucial role in DNA packaging and gene expression regulation

  • by organizing DNA into compact structures, nucleosomes help protect the genome from damage and regulate access to genetic information

dynamics

• are dynamic structures that can be moved, assembled, and disassembled in a regulated fashion to allow for DNA replication and gene expression

sister chromatids

identical copies of a single chromosome that are formed during DNA replication and remain joined together by a centromere

• essential for ensuring each daughter cell receives a complete set of chromosomes during cell division

formation

• during the S phase of the cell cycle, DNA replication duplicates each chromosome, resulting in 2 identical copies

attachment

• are attached to each other at the centromere, a constricted region of the chromosome

role in cell division

• are crucial for accurate chromosome segregation during mitosis and meiosis

segregation

• during anaphase, spindle fibers attach to the centromeres and pull the sister chromatids apart, ensuring each daughter cell receives one complete set of chromosomes

DNA repair & chromatin structure

• play a role in DNA repair and maintain chromatin structure

centromere

a constricted region on a chromosome that serves as the attachment point for spindle fibers during cell division, ensuring accurate chromosome segregation

• a specialized structure composed of both DNA and proteins

• crucial for chromosome movement and control of gene expression

attachment site for spindle fibers

• provide the anchor point for the microtubules of the spindle, which are responsible for pulling chromosomes apart during cell division

sister chromosome cohesion

• holds sister chromosomes together until they are separated during anaphase

chromosome segregation

• are essential for proper segregation of chromosomes into daughter cells

karyotype

a pictorial array of all the chromosomes in an organism’s cell during mitosis

• arranged in pairs by length and centromere location and banding pattern

• “map” of an individual’s chromosomes

homologous chromosomes

matching pairs of chromosomes of approximately the same length, centromere position, and staining pattern that hold the same genes

• in a diploid organism, where one chromosome is inherited from each parent

• carry the same genes in the same order, but may have different versions of those genes (alleles)

chromosome number

the sum of all chromosomes in a cell of a given organism

autosome

chromosomes that are not involved in determining the sex of an organism

• come in pairs - chromosome pairs #1-22

• each carries numerous genes that control various traits and functions in an organism

sex chromosome

chromosomes that are responsible for determining the sex of an organism

• one pair - chromosome pair #23

• primarily determines biological gender

• XX = female

• XY = male

Okazaki fragments

short, newly synthesized DNA sequences on the lagging strand during DNA replication

• formed because the lagging strand is synthesized discontinuously in a direction opposite to the replication fork

• these fragments are later joined together by DNA ligase to form a continuous strand

lagging strand

• during DNA replication, one DNA strand is synthesized continuously in the 5-3 direction, while the other strand (the lagging strand) is synthesized discontinuously

discontinuous synthesis

• the lagging strand synthesis is discontinuous because DNA polymerase, the enzyme that builds new DNA can only move in the 5-3 direction, and the lagging strand runs in the opposite direction of the replication fork

formation

• as the replication fork moves forward, DNA polymerase must detach and reattach at multiple points to synthesize the lagging strand in short fragments

RNA primers

• each fragment is initiated by a short RNA primer, which is later replaced by DNA polymerase and the fragments joined together by DNA ligase

genome stability

• proper processing of fragments is crucial for maintaining genome stability and preventing DNA damage

  • defects in this process can lead to DNA breaks and chromosome abnormalities

spindle fibers

array of microtubules that move chromosomes around the cells

• crucial for cell division in eukaryotic cells

• responsible for segregating chromosomes during mitosis and meiosis, ensuring each new cell receives the correct number of chromosomes

structure

• formed by microtubules

function

• chromosome movement

  • attach to chromosomes and use their motor proteins to pull them apart during cell division

• cell division

  • help to push the cell poles apart, which is necessary for cytokinesis

formation

• during prophase, spindle fibers start to form from centrosomes, which are located at the poles of the cell

centrosome (with centrioles)

organelle responsible for making microtubules (spindle fibers) that are used to move chromosomes around during cell division in animal cells

• acts as the main microtubule-organizing center, regulating cell shape, polarity, and cilia formation

• during cell division, centrosome duplicates and migrates to opposite poles of the cell, where it nucleates the mitotic spindle, ensuring chromosomes are evenly distributed to daughter cells

microtubule organization

• primary site for microtubule nucleation, meaning it initiates the growth of microtubules

mitotic spindle formation

• during cell division, the duplicated centrosomes move to opposite poles of the cell and act as the organizing centers for the mitotic spindle, a structure that separates chromosomes

asexual reproduction

form of reproduction in which offspring arise from one parent and inherit that parent’s genes only

• offspring are genetically identical to single parent

sexual reproduction

form of reproduction in which genetically variable offspring are produced by the fusion of two parents

• offspring have unique combination of genes from both parents

genetic variation

the differences in DNA sequences between individuals within a population or species

• differences are essential for evolutionary change and are caused by various mechanisms

• mechanisms:

  • mutations

  • gene flow

  • genetic recombination

• importance

  • evolutionary change

  • adaptation

  • unique individuals

* crossing over (Prophase 1) and random alignment (Metaphase 1) ensure that no two reproductive cells are alike

interphase

longest phase of the cell cycle where a cell increases in mass, roughly doubles the number of components (organelles) in its cytoplasm, and duplicated its DNA

• 3 stages:

  • G1 (Gap 1)

  • S (Synthesis)

  • G2 (Gap 2)

G1 (Gap 1)

first interval where cellular contents, excluding the chromosomes are duplicated

• cell growth and functioning (doing its job)

• lasts 10-12 hours

S (Synthesis)

DNA replication in preparation for division

• each of the 46 chromosomes is duplicated by the cell

• lasts 4-6 hours

G2

second interval where cell makes proteins that will drive cell division

• the cell “double checks” the duplicated chromosomes for error, making any needed repairs

• lasts 4-6 hours

G0

resting phase of the cell cycle

• cell cycle arrest

* not all cells are rapidly proliferating; the heart muscle, skeletal muscle, and nerve cells are “arrested” in G1 early in human development and never proceed past that point

proto-oncogene

genes that code for normal cell division that can turn into oncogenes - gene that transforms a normal cell into a cancer cell

checkpoints

the cell cycle has built-in checkpoints where specific proteins, products of checkpoint genes, monitor the structure of chromosomal DNA

• two categories:

  • growth factors - stimulate cell division

  • tumor suppressors - control/slow/stop cell division

• Spindle assembly checkpoint

  • check for chromosome attachment to spindle

• G1 checkpoint

  • check for cell size, nutrients, growth factors, DNA damage

• G2 checkpoint

  • check for cell size and DNA replication

* if a checkpoint gene mutates and its protein product no longer functions, the cell can lose control of the cell cycle (cell can become stuck in mitosis → tumors)

apoptosis

a natural, programmed form of cell death that is crucial for development and tissue maintenance in multicellular organisms

• involves a tightly regulated process where the cell’s own enzymes break it down into small, membrane-bound fragments, which are then removed by other cells without causing inflammation

• crucial role in various biological processes

  • controlling cell populations

  • eliminating damaged or infected cells

programmed cell death

• is a genetically controlled process, meaning it follows a specific set of biochemical steps

morphological changes

• during apoptosis, cells shrink, nucleus condenses, outer membrane forms small protrusions

cell fragmentation

• cell breaks down into small, membrane-bound fragments called apoptotic bodies

phagocytosis

• apoptotic bodies are engulfed and removed by neighboring cells

growth factor

a naturally occurring signaling molecule, often a protein, that stimulates cell growth, differentiation, survival, and sometimes tissue repair

• accelerator - a mutation that keeps division happening

stimulate cellular processes

• play a vital role in regulating cell growth and development by stimulating processes like:

  • proliferation - increase in cell number, often through mitosis

  • differentiation - cell becomes specialized to perform a specific function

  • survival - helping cells to resist apoptosis

  • tissue repair - wound healing and other repair processes

cell communication

• act by binding to specific cell surface receptors, which trigger signals that affect gene expression and cell behavior

tumor suppressor

a gene that normally helps control cell growth and division, preventing cells from growing out of control and developing tumors

• brakes - a mutation that slows down or stops division, maintaining cellular balance

normal function

• encode proteins that regulate cell cycle, ensuring that cells divide at the right time and die when they should (apoptosis)

• play a role in DNA repair, helping to correct errors that could lead to mutations

mechanism of action

• work by inhibiting cell growth, promoting cell death, and ensuring proper cell-cycle checkpoints

loss of function & cancer

• mutations in tumor suppressor genes can disrupt their function, leading to uncontrolled cell death, which is a hallmark of cancer

  • in many cancers, 1+ tumor suppressors are inactivated

tumor (neoplasm)

mass of cells dividing at an abnormally high rate that forms a mass or a lump

• two types:

  • benign - slow growing tumor that stays localized in home tissue (ie. moles)

  • malignant - tumor cells that rapidly divide, metastasize, and invade other places in the body where they may start new tumors

metastasis

the process where cancer cells from a primary tumor spread to distant parts of the body, forming secondary tumors

cytokinesis

the process of cytoplasmic division during cell division, resulting in the formation of two daughter cells

• follows mitosis or meiosis

• ensures each new cell receives its share of organelles and other cytoplasmic components

cell division

• is the second major phase of cell division, alongside mitosis or meiosis

  • Mitosis involves the division of the cell's nucleus, while cytokinesis separates the cytoplasm

cytoplasmic division

• the cell’s cytoplasm, including organelles and other cellular components, is divided into two separate portions

daughter cells

• result is two daughter cells, each containing a copy of the original cell's genetic material and its own set of cytoplasmic components

cleavage furrow

an invagination of the plasma membrane during cytokinesis

• begins as a shallow groove and gradually deepens, eventually pinching the cell membrane in two to form two separate daughter cells

*animal cytokinesis

contractile ring

a ring-shaped structure of proteins that forms beneath the cell membrane during cell division, specifically during cytokinesis

• constricts the cell, creating a cleavage furrow and physically separating the cell into two daughter cells

*animal cytokinesis

cell plate

a structure that forms to divide the cytoplasm and create a new cell wall between the two daughter cells

• disc-like structure that builds a partition between the two new cells 

*plant cytokinesis

haploid

a cell or organism containing only one complete set of chromosomes

• has half the usual number of chromosomes as full complement

• set represents one copy of each chromosome

• does not possess any homologous pairs of chromosomes

  ie. in humans, haploid cells are specifically the gametes (sperm and egg cells)

diploid

a cell or organism that has two complete sets of chromosomes

• has two copies of each chromosome

• in most sexually reproducing organisms, this second set of chromosomes is inherited from the other parent

• possesses 2 homologous pairs of each chromosome, one inherited from each parent

somatic cell

all the cells in a multicellular organism except for gametes (sperm and egg cells) and the cells that produce them

• essentially, any body cell that is not a germ cell

• make up the body tissues and organs

diploid

• typically contain two copies of each chromosome, one inherited from each parent

mitosis

• divide through mitosis, a process that produces two identical daughter cells

not reproductive

• not involved in the production of new offspring

differentiated

• are specialized and can perform different functions within the body

sex cell

a reproductive cell (sperm or egg) that contains half the normal number of chromosomes (haploid)

• aka gametes - specialized cells involved in sexual reproduction

• combine during fertilization to form a zygote with the full diploid number of chromosomes

haploid

• have a single set of chromosomes, while other cells in the body have two sets

fertilization'

• when a sperm and egg cell fuse, their haploid nuclei combine to form a diploid zygote

meiosis

• are produced through meiosis, which reduces the chromosome number by half

gonad

a reproductive organ that produces gametes (sex cells) and hormones

• females: ovaries, produce eggs and female hormones

• males: testes - produce sperm and male hormones

testes

the paired male reproductive glands responsible for producing sperm and testosterone

ovary

the female reproductive organ responsible for producing eggs and estrogen & progesterone

synapsis

the pairing of homologous chromosomes during early meiosis

• ensures that each resulting cell has the correct number of chromosomes after cell division

• allows for the exchange of genetic material between homologous chromosomes, leading to the formation of tetrads, which then undergo crossing over

• occurs in Prophase 1

tetrad

the structures formed by the paired homologous chromosomes after synapsis

crossing over (genetic recombination)

the exchange of genetic material between homologous chromosomes that happens during/after synapsis which leads to increased genetic diversity in offspring

• during Prophase 1 crossing over, a tetrad forms and sections of non-sister chromatids break off, switch places, and reattach

• during Meiosis 1 Prophase 1 the homologous chromosomes line up closely in a process called synapsis

• the 2 aligned chromosomes are called tetrads (four chromatids)

• during Prophase 1, an enzyme called Recombinase acts as scissors and cuts sections from one chromosome and pastes it on another - the results are recombinant chromosomes (cut, switched, recombined)

* chiasma is the site of the crossing over (appears as an X)

random alignment

the second mixing of genes that happens during Metaphase 1

during Metaphase 1 paternal/maternal pairs randomly position themselves along the midline of the cell

spermatogenesis

sperm producing process

• in males, the spermatagonium are arrested before they begin Meiosis and will not start Meiosis until puberty

• takes about 100 days from start to finish

• millions of spermatagonium cells in the testes are in various stages of Meiosis on any given day

oogenesis

ova (egg) producing process

• primary oocytes (immature eggs) in women are produced in the fetus but are halted at Meiosis 1 Prophase 1 until puberty

  • this is why women are said to have a limited number of eggs determined at birth

  • of the millions of primary oocytes present in a woman’s ovaries, only about 400 will mature during a woman’s lifetime

• only 1 egg is needed so 3 additional egg cells (polar bodies) that are produced will eventually decay

  • polar bodies are smaller

  • all of the “good stuff” - cytoplasm, organelles, etc. - are found in the designated egg

• the egg is produced in day 14-18 of oogenesis

• roughly 12 days after ovulation, if the egg hasn’t been fertilized, the unfertilized egg and uterine lining in released in the menstrual cycle (monthly)

• at menopause, hormones close down the process of oogenesis (Meiosis)

gamete

haploid reproductive cells produced as a result of meiosis

• two types of animal:

  • sperm

  • ovum

zygote

the first cell in a new organism when a sperm and ovum (egg) cell fuse

sperm

haploid male sex cell

ovum (egg)

haploid female sex cell

polar bodies

the cells produced in oogenesis that are targeted to decay and not become eggs

mitosis

purpose:

• organism growth

• cell size limitation (SA:V ratio)

• replace dead cells

• repair damaged/injured tissue

done by: somatic (body) cells of multicellular eukaryotes

#divisions: 1

location: occurs actively in all cells of the body (except nerve cells and skeletal muscle cells)

sexual or asexual: asexual

#genetically identical cells produced: 2

Diploid cells are created; produces cells with full complement of chromosomes as parent cell

meiosis

purpose:

done by: sex cells (gametes)

#divisions: 2

location: occurs only in the reproductive organs (testes and ovaries)

sexual or asexual: sexual

#genetically identical cells produced: 4

Haploid cells are created; produces cells with half the chromosomes as the parent cell

* cells do not re-enter interphase between Meiosis 1 and Meiosis 2

* crossing over and random alignment cause genetic variation

* all genetic variability occurs in Meiosis 1

At the end of meiosis I, are the daughter cells 2n or n?  How many chromatids does each chromosome have at that point?

at the end of meiosis 1. the daughter cells are haploid (n) and each chromosome still consists of two sister chromatids

At the end of meiosis II, are the daughter cells 2n or n?  How many chromatids does each chromosome have at that point?

The daughter cells at the end of meiosis II are haploid (n) and each chromosome has one chromatid

how many homologous pairs are there in the human body?

23; the last determining biological gender

cytokinesis plant cell versus cytokinesis plant cell

plant cell

• plant cells divide using cell plate formation = vesicles packed with cell wall building materials (from the Golgi bodies) form along the midline and fuse with one another forming the cell plate

animal cell

• animal cells divide by partitioning their cytoplasm using a contractile ring mechanism = a thin band of contractile filaments just below the plasma membrane contracts forming a cleavage furrow, and then pinches the cytoplasm in two

Mitosis plant cell versus Mitosis animal cell

plant cell

animal cell

What does “reduction” mean in Meiosis?

In Meiosis, "reduction" refers to the reduction of the chromosome number from diploid (2n) to haploid (n). This is achieved through the two stages of Meiosis I and Meiosis II, where homologous chromosomes separate in the first division, and sister chromatids separate in the second division, resulting in four haploid daughter cells. 

meiosis 1 (reduction division)

• During Meiosis I, homologous chromosomes separate, resulting in two daughter cells each with half the number of chromosomes as the original cell.

meiosis 2 (equational division)

• Meiosis II separates sister chromatids, similar to mitosis. While it does not further reduce the chromosome number, it does result in four daughter cells. 

purpose

• This reduction in chromosome number is crucial for sexual reproduction. When a sperm (haploid) and egg (haploid) fuse, they combine their genetic material to form a diploid zygote, ensuring the correct chromosome number for the next generation. 

What role do spindle fibers play in Mitosis/Meiosis?

Spindle fibers, made of microtubules, are crucial for separating chromosomes during both mitosis and meiosis. They attach to chromosomes at the centromeres, ensuring each new daughter cell receives an accurate number of chromosomes.

• mitosis: pull sister chromatids apart to opposite poles

• meiosis: separate homologous chromosomes in meiosis I and sister chromatids in meiosis II

Human Genome Project (2003)

developed the complete genetic map (mapped the entire human genome)

• found that not just a single gene was causing a disease but the WHOLE THING

• helped determine what each of the chromosomes do

• there are over 20,000 genes in the human genome that are responsible for traits or disorders

• 3 billion base pairs

nucleotide

monomer of nucleic acids (DNA) that contains:

• a deoxyribose

• a phosphate group

• a nitrogen-containing base (Adenine, Guanine, Thymine, Cytosine)

Purine

the bigger, double carbon ring nitrogenous bases: Adenine & Guanine

Pyrimidine

the smaller, single carbon ring nitrogenous bases: Thymine & Cytosine

Base pairing rules

• A pairs with T (2 hydrogen bonds)

• G pairs with C (3 hydrogen bonds)

semi-conservative replication

half of original DNA is conserved in the new DNA molecule

• DNA is 2 nucleotide strands held together by hydrogen bonds

• hydrogen bonds between 2 strands are weak and easily broken

• each single old strand then serves as a template for new complimentary strand

• every cell has a big pool of free nucleotides available to join the original strand “template”

main enzyme actors in DNA replication

topoisomerase - untwists the double helix

DNA helicase - breaks hydrogen bonds between DNA strands

DNA primase - catalyzes the synthesis of a short chain of RNA because DNA can not initiate the synthesis of itself (this starter is called a “primer”)

DNA polymerase - joins free nucleotides into a new strand of DNA

DNA ligase - joins DNA fragments on the discontinuous strand

eukaryotic DNA replication steps

1. initiator proteins attach to DNA at certain sites on the chromosome (replication can typically start at multiple points for eukaryotes)

2. these proteins attract 2 key enzymes to the chromosome:

   a) topoisomerase - unwinds the DNA

   b) DNA helicase - breaks the hydrogen bonds joining the base pairs (this area where the strands are split is called the “replication fork”)

3. another enzyme, DNA primase, makes short RNA primers that attach to the DNA where new nucleotides will be added

4. DNA polymerase attaches to these primers and begins to assemble the complementary strand in “its” 5-3 direction

5. there is a continuous (leading strand) and a discontinuous strand (lagging strand)

6. the segments of the lagging strand (called “Okazaki fragments”) are joined by a third enzyme, DNA ligase

* helps maintain chromosome number

replication fork

area where continual separation of DNA strands occurs

• leading (continuous) strand goes towards

• lagging (discontinuous) strand goes away

mutation

mistake that occurs during DNA replication

• those acquired during life can harm the organism

  ie. skin cancer

• inherited are already in cells, so gamete can pass them to offspring

  ie. Huntington’s disease, sickle cell anemia, hemophilia

genes

heritable information in chromosomal DNA

Cell Cycle

a series of events from one cell division to the next

• Interphase (95% time - 18-24 hours)

  • G1 (10-12 hours)

  • S (4-6 hours)

  • G2 (4-6 hours)

• Mitosis (5% time - 0.5-1 hour)

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

  • Cytokinesis

* Eukaryotic cells spend 95% of their time in interphase so only a small portion of time in actual mitosis

Mitosis Prophase

• duplicated chromosomes begin to condense and become more visible

• microtubules (spindle fibers) create a bipolar mitotic spindle that reaches from one pole to the other

• nuclear envelope breaks up

Mitosis Metaphase

• microtubules (spindle fibers) move chromosomes to the center (metaphase plate) of the cell

Mitosis Anaphase

• sister chromatids split and become independent chromosomes

• microtubules (spindle fibers) contract and drag sister chromosomes to opposite poles

• once chromatids separate they are considered two separate chromosomes

Mitosis Telophase

• chromosomes begin to decondense

• nuclear membranes begin to reappear

• 2 daughter cells form, each with the same chromosome number as the original parent cell

• cytokinesis occurs (division of the cytoplasm)

Cancer characteristics

• abnormal growth and division

• cytoplasm and plasma membrane become grossly altered

  • cytoskeleton becomes disorganized

  • membrane becomes leaky/missing proteins

• cancer cells have a weakened capacity for adhesion

  • don’t stay anchored to the proper tissues (metastasis)

• cancer cells can crowd out normal cells and also “infect” them, turning them into cancer cells

RNA primer

a short sequence of RNA that initiates DNA synthesis during DNA replication

• serves as a starting point for DNA polymerase to add nucleotides to the template strand

• are synthesized by an enzyme called primase

function

• are essential for DNA replication because DNA polymerase, the enzyme responsible for synthesizing new DNA strands, cannot start adding nucleotides without pre-existing 3'-OH (hydroxyl) group to attach to the 3'-OH group at the end of the RNA primer provides this necessary starting point

complementarity

• must be complementary to the DNA strand it will prime, meaning that its base sequence must match the base sequence of the DNA strand, according to the base-pairing rules (A-T) (G-C)

removal and replacement

• once DNA synthesis is complete, the RNA primers are removed and replaced with DNA by specialized enzymes, ensuring a continuous DNA strand

importance

• crucial for both leading and lagging strand DNA replication, especially on the lagging strand where multiple primers are needed to synthesize short Okazaki fragments

Topoisomerases

enzymes that play a crucial role in managing the topology of DNA, essentially untangling and relaxing DNA strands during processes (replication, transcription, and cell division) by temporarily cutting and resealing DNA strands

• prevents DNA tangles and supercoils, which can hinder essential biological processes

DNA topology

• are essential for maintaining the proper structure of DNA

replication & transcription

• during replication and transcription, the DNA helix needs to be unwound and accessed, which can lead to tension and tangles → topoisomerases help to relieve this tension by introducing and resealing DNA breaks

cell division

• during cell division, chromosomes need to be separated → topoisomerases play a role in detangling and separating replicated sister chromatids

Helicases

enzymes that unwind double-stranded nucleic acids, like DNA and RNA, by breaking the hydrogen bonds between base pairs

• crucial for various cellular functions (DNA replication, repair, transcription)

unwinding DNA

• act as molecular motos, using energy from ATP hydrolysis to “unzip” the DNA double helix

  • this unwinding creates exposed single strands of DNA, which are then used as templates for DNA replication and transcription

DNA replication

• during DNA replication, helicases are essential for separating the two strands of the double helix, allowing DNA polymerase to synthesize new complementary strands

  • the process ensures that each daughter cell receives a complete copy of the genetic information

other cellular processes

• involved in DNA repair, where they help to identify and remove damaged DNA

• play a role in transcription, where they unwind DNA to allow RNA polymerase to access the DNA and synthesize RNA

DNA Polymerase

a vital enzyme that creates DNA molecules by assembling nucleotides, ensuring the accurate replication and maintenance of DNA

• essential to DNA replication

• usually work in pairs to create two identical DNA strands form one original DNA molecule

  • synthesizes new DNA strands by adding complementary nucleotides to a template strand, ensuring the accurate duplication of genetic information

DNA replication

• builds new DNA strands by adding nucleotides, ensuring accurate replication of the genetic code

proofreading & repair

• has proofreading capabilities, allowing it to detect and correct errors during replication, maintaining the integrity of the genome

Ligase

an enzyme that joins two DNA fragments together

• required for the repair, replication, and recombination of DNA

DNA replication

• crucial for joining together Okazaki fragments synthesized on the lagging strand

DNA repair

• crucial for repairing breaks in the DNA backbone caused by damage or other processes

Semi-conservative replication

a DNA replication process where each new DNA molecule consists of one original strands and one newly synthesized strands

• each daughter DNA helix contains half the original DNA, which is essential for maintaining genetic continuity

process

• during replication, the double helix of DNA unwinds, and the two strands separate

• each original strand then serves as a template for the synthesis of a new complementary strand

result

• the end result is two new DNA molecules, each with one original strand and one newly synthesized strand

importance

• ensures that genetic information is passed on accurately to daughter cells during cell division

Leading strand

in DNA replication, the DNA strand that is synthesized continuously in the 5-3 direction towards the replication fork

• made in the same direction as the replication fork is moving

• continuous synthesis - made without breaks or fragments

• requires only 1 primer - a single RNA primer is needed to initiate synthesis

• does not require ligase - since its synthesized continuously, it doesn’t require ligase to join fragments

• efficient replication - the continuous nature of leading strand synthesis makes for faster and more efficient replication compared to the lagging strand

Lagging strand

in DNA replication, the DNA strand that is synthesized discontinuously, in short Okazaki fragments in the 5-3 direction away from the replication fork

• made in the opposite direction as the replication fork is moving

• discontinuous synthesis - must be made in short fragments

• requires multiple primers - fragments are initiated by RNA primers and then extended by DNA polymerase leaving RNA primers that are later on removed and replaced with DNA by other enzymes

• requires ligase - ligase join together gaps between fragments

• inefficient replication - the discontinuous nature of lagging strand synthesis makes for slower and less efficient replication compared to the leading strand