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What is a chromosome?

A long DNA that has been folded / packaged, containing genes

• DNA wraps around proteins called histones

• Those folded structures are called nucleosomes

• The nucleosomes can be folded further and further, allowing DNA to becomes highly packaged

• The packaging is reverable

What is a gene?

A gene is a specific segment / sequence of nucleotides within DNA

• Genes act like blueprints / codes stored in DNA

What is DNA?

Packaged into chromosomes

Chromosomes contain genes

Genes are specific sequences of nucleotides

What are histones?

Histones are proteins that DNA wraps around to form nucleosomes, which help package DNA into chromosomes

What are nucleosomes?

Folded structures of DNA wrapped around histone proteins.

They can be folded further and further, allowing DNA to become highly packaged.

Is chromosome packaging permanent?

No, it is reversible

What makes up the DNA structure?

It is a double helix structure made of sugar + phosphate = sugar phosphate backbone

It has four nitrogenous bases: Adenine (A), Guanine (G), Thymine (T), Cytosine (C)

What is the difference of Purines vs Pyrimidines?

Purines have 2 carbon-nitrogen rings : Adenine (A) + Guanine (G)

• “ PURe As Gold = Purines A + G ”

Pyrimidines have 1 carbon-nitrogen ring : Cytosine (C) + Thymine (T)

• and Uracil (U) in RNA

Explain the difference of Nucleoside vs Nucleotide

Nucleoside = Base + Deoxyribose sugar

Nucleotide = Base + Deoxyribose sugar + phosphate

DNA strands are held together by ________

hydrogen bonds

A ↔ T have ____ hydrogen bonds

G ↔ C have ____ hydrogen bonds

A ↔ T have TWO hydrogen bonds

G ↔ C have THREE hydrogen bonds

What is it called when the DNA strands run in opposite directions?

Antiparallel

• one runs 5’ to 3’

• while the other runs 3’ to 5’

If one DNA strand is 5’ TGCA 3’ what is the complementary strand?

3’ ACGT 5’

What is DNA replication?

Making a copy of DNA

Where does DNA replication occur?

In the Nucleus

When does DNA replication occur?

During the S phase of the cell cycle

What happens during DNA replication?

1. The hydrogen bonds between the DNA bases are seperated

2. The two original strands separate

3. Each original strand acts as a template

4. A complementary strand is made

5. The result is two DNA molecules identical to the original

What is Transcription?

When you are going to produce an RNA copy of one of the strands

RNA is produced from the ______ strand

RNA is produced from the antisense strand

Explain the Sense strand & the Antisense strand

The sense strand of DNA is the coding strand that has the same base sequence as the resulting messenger RNA (mRNA),

while the antisense strand is the template strand used by the cell to build that mRNA.

Explain the association of mRNA and Ribosomes

The ribosome reads the mRNA sequence and uses that information to specify amino acids to make a protein

All genes code for ____

All genes code for RNA

What is a genome?

A genome is the sum total of the genetic material / chromosomes

What is this showing?

It is a graph of all the different segments of each of these chromosomes that are being transcribed throughout the genome

• the green regions are where the genes are actively being transcribed, making RNAs

• the red regions are the genes that are essentially silent

Humans have ___ chromosome pairs

23

Somatic cells have ___ total chromosomes

46

____ chromosomes come from mother

23

____ chromosomes come from father

23

What is the longest chromosome?

Chromosome 1

What is the shortest chromsosome?

Chromosome 22

What is chromosome 23?

The chromosome that contains the sex determining genes

It can be either X or Y

What is the order of the phases of the cell cycle?

G1 phase → S phasve → G2 phase → M / mitosis phase

What happens in G1?

The growth phase

• The cell grows, produces mRNA, produces proteins needed for DNA synthesis, duplicates cellular components in preparation for division

It is checking for “Should this cell actually continue dividing?”

What happens in the S phase?

DNA replication

What happens in G2?

It is another growth phase

• The cell continues growing, produces proteins, produces organelles needed for mitosis, reorganizes contents, and checks that DNA replication went correctly

That happens in M phase?

Where mitosis occurs ; Division of genetic material

• has 5 subphases

and where cytokinesis occurs ; Division of cytoplasm

What are the subphases of mitosis?

Prophase → Prometaphase → Metaphase → Anaphase → Telophase

• “Please Pee on the MAT”

What happens during Prophase?

1. Chromatin condenses

2. Centrosomes move

3. Mitotic spindle forms

4. Nucleolus dissolves

Chromosomes become visible / condensed

Sister chromatids are joined at the centromere

What happens during Prometaphase?

The nuclear envelope breaks down

• Spindle microtubules form / expand

• Kinetochore microtubules attach to chromosomes

• Centrosomes / centrioles move toward opposite poles

What happens during Metaphase?

Chromosomes line up alone the middle of the cell = this is the Metaphase Plate

• Centrosomes are at opposite poles

• Chromosomes are completely condensed

• Kinetochores are attached to spindle microtubules

What happens during Anaphase?

Sister chromatids separate

ana = apart

• They move toward opposite poles

• The cell elongates

• Microtubules shorten / depolymerize and pull chromosomes toward the poles

What happens during Telophase?

Two nuclei

telo = two

• Nuclei & Nucleoli reform

• Chromosomes decondense

• Spindle microtubules dissolve

• Mitosis phase finishes

• Cytokinesis occurs

What happens during Cytokinesis?

The cytoplasm divides

in animal cells: a cleavage furrow forms

• A contractile ring of actin microfilaments contracts with the help of myosin

• The cell is pinched into 2 daughter cells

  • The daughter cells are genetically identical to the parent cell

Homologous chromosomes

Maternal + paternal versions of the same chromsosome

Sister chromatids

Duplicated copies of the same chromsome

Mitosis separates …

Sister chromatids

Meiosis 1 separates …

Homologous chromosomes

Meiosis 2 separates …

Sister chromatids (into four cells)

Why do cells need checkpoints during the cell cycle?

To make sure the cell is ready & to prevent damaged / improper cells from continuing

Where are the three major checkpoints?

G1, G2, M

Explain the G1 Checkpoint

The most important checkpoint!

• Because once the cell passes G1, it has committed to division

What are growth factors?

Protein messengers that are released by neighboring cells that stimulate nearby cells to divide

If the cells conditions are not good at the checkpoint, what can happen?

The cell can leave the cell cycle and enter:

G0 = non dividing / resting state

• some cells can stay in G0 permanently

What is Density Dependent Inhibition?

A cell regulation mechanism so that normal cells don’t just keep growing on top of eachother

• when cells become crowded they stop dividing

What happens with cancer cells for density dependent inhibition?

Cancer cells ignore these “stop” signals and continue dividing

What happens during Anchorage Dependence?

Where cells need to be anchored to a surface in order to proliferate

• Examples include attachment to a surface or extracellular matrix

Cancer cells may not respond to this control and can grow unattached

Explain the G2 Checkpoint

It evaluates newly duplicated DNA

Asks “Was the DNA copied completely and correctly?”

• If an error is detected: the cell cycle can stop, or

  • if damage is too severe to repair: apoptosis / cell death

Explain the M Checkpoint

It makes sure the spindle microtubules are properly attached to kinetochores

It checks chromosome alignment

• If a chromosome isn’t properly aligned / attached: the cell pauses

  • once everything is properly attached: cellular progression continues

Then separase can be activated to separate sister chromatids

What are Cyclins?

Proteins whose concentrations change throughout the cell cycle

These consist of:

• G1 cyclin

• G1 / S cyclin

• S cyclin

• M cyclin

What are CDKs?

Cyclin dependent kinases

Their concentrations remain relatively constant, but they require cyclins to become active

• Cyclin binds CDK:

  • Cyclin + CDK → activated CDK

    • kinases phosphorylate target proteins, driving the cell cycle forward

What is M-Cyclin?

It forces the cell to evaluate what’s going on in prometaphase

• it is crucial for the progression of mitosis during the cell cycle

It increases during G2 and binds CDK to form MPF

What is MPF?

Maturation promoting factor

It helps push the cell into M phase

• It decreases as anaphase approaches → during anaphase: cyclins are degraded → MPF activity decreases

• The CDK remains and can be reused

Explain Ras mutation with cancer

It is a gain of function mutation the can cause the pathway to remain active even without a growth factor, leading to increased cell division

What is p53?

A tumor suppressor

• It helps prevent damaged DNA from proceeding through the cell cycle

If DNA is damaged:

• p53 → cell leaves / stops cell cycle → DNA repair can occur

If p53 is mutated:

• Damaged DNA may remain in the cycle, mutation can be passed along, increased cell division can occur

When does cancer develop?

When normal controls over cell growth / division are lost

• A single cell can: avoid cell cycle control, escape destruction, begin dividing excessively

  • This can produce a tumor

Compare Benign vs Malignant tumors

Benign tumors: Does not spread to other areas & can usually be surgically removed

Malignant tumors: Grows without restraint & invades / spreads to nearby tissues

What is Metastasis?

The spread of cancer cells from one location to another in the body

• Cancer cells can leave the original tumor and enter the bloodstream or lymphatic system to form another tumor in other organs

Meta = Move

How can cancer cells escape normal controls?

1. Ignore external “stop” signals

2. Continue growing without growth factors

3. Produce their own growth factors

4. Lose responsiveness to cell surface signals

5. Develop abnormal internal signaling

6. Develop mechanisms allowing immortal growth

7. Accumulate genetic changes / mutations

8. Some have impaired DNA repair

What is Chemotherapy?

It targets actively and rapidly dividing cells

It interferes with stages of the cell cycle to kill or slow down cancer cells

Why does Chemotherapy cause side effects?

Because some healthy cells also divide rapidly

• Such as: Hair follicles, intestinal tract, esophagus, bone marrow / immune related cells

  • So, it can cause hair loss, digestive problems, and immune supression

What is Radiation therapy?

It uses high doses of radiation to kill or damage cancer cells by disrupting their ability to divide and grow

• Radiation → DNA damage → cancer cells unable to repair → cell death

Explain the differences between Mitosis vs Meiosis

Explain the difference between Diploid vs Haploid

Diploid = 2n (two sets of chromosomes)

• Human somatic cells: 2n = 46

  • one set from mom + one set from dad

Haploid = n (one set of chromosomes)

• Human gametes: n = 23

What are homologous chromosomes?

They are similar but not necessarily genetically identical

• contain the same genes

• are the same size

• look similar after chemical staining

• one came from mother & one came from father

Explain Autosomes vs Sex chromosomes

Autosomes are chromosomes that do not determine the sex of an individual, while

Sex chromosomes are involved in determining the sexual characteristics and reproductive functions

• In humans, there are 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females and XY for males).

What is a Karyotype?

An ordered display of homologous chromosomes

• Used to examine an individual's chromosomal composition and identify chromosomal abnormalities

What process separates homologous chromosomes?

Meiosis 1

What process separates sister chromatids?

Meiosis 2

What is Prophase 1?

The first stage of Meiosis 1 where chromosomes have already replicated so each chromosome has 2 sister chromatids

Homologous chromosomes pair together

This is where crossing over occurs

What happens during crossing over?

Non sister chromatids exchange DNA

• between homologous chromosomes

genetic information from both parents - increasing genetic diversity

What is a Synaptonemal complex?

A protein complex that helps bring homologous chromosomes together gene by gene during prophase 1 of meiosis

• This allows crossing over and Synapsis = the close association of homologous chromosomes through this process

What is Chiasmata?

Visible crossing over points

What are Cohesins?

Proteins that hold sister chromatids together

Why is crossing over important?

It creates genetic variation

• allows the resulting chromosomes to not be identical to the original parent chromosomes

What happens in Metaphase 1?

Homologous pairs line up

What happens in Anaphase 1?

Homologous chromosomes separate

• Sister chromatids stay together

What happens in Telophase 1 + Cytokinesis?

Two haploid daughter cells form, but chromosomes are still duplicated

How does Meiosis 2 start?

With the two cells produced by Meiosis 1

What happens during Prophase 2?

Spindle forms again

What happens during Metaphase 2?

Chromosomes line up in center

What happens in Anaphase 2?

Sister chromatids separate

What happens in Telophase 2 + Cytokinesis?

Nuclei reforms and cells divide

What is the final result of Meiosis 2?

4 genetically distinct haploid cells

What is Independent assortment?

During Meiosis 1, maternal and paternal homologous chromosomes are randomly shuffled into daughter cells to create different combinations of chromosomes

crossing over + independent assortment = genetic variation

What is Trisomy 21?

During Meiosis 2, sister chromatids of chromosome 21 fail to separate properly = nondisjunction

The resulting gamete can contain an extra chromosome 21

So after fertilization, there are 3 copies of chromosome 21 instead of 2, resulting in 47 total chromosomes

What did Gregor Mendel study?

Pea plants and their inheritance patterns

• He used true-breeding plants

What is a true-breeding organism?

One that consistently produces offspring with the same characteristic when self-pollinated

What were the three generations from the pea plant crossing?

P generation = parents

F1 = First generation offspring

F2 = Offspring produced from the F1 cross

• Example: purple x white

  • F1: all purple

  • F2: purple + white

    • the white trait wasn’t actually gone, it was masked

What are alleles?

Different versions of the same gene

• Example:

  • P= purple , p= white

    • Each gene has two alleles: one from each parent