01 - Early Discoveries in Genes and DNA

imp discoveries on the nature of the gene

• charles darwin: incorrect view of herebility

• mendal pea plant: see how traits are passed

• discovery of chromosome: realized heretible material

• discovery of homologous chromosmes

• discovery of crossing over

• discovery that gene could be mapped in order along length of chromosome

• discovery of dna as genetic material

• discovery of dna structure

Before we understood the structure of DNA, we acknowledged it as

the vessel for heritability and change

1858, Rudolf Virchow

Every cell comes from another cell

1879, Walther Flemming

• Cytoplasmic material distributed randomly to the daughter cells during cell division

• Nuclear material (of eukaryotes) became organized into visible threads and divided equally among the daughter cells.

Avery, MacLeod, McCarty

• Identified DNA as the "transforming principle."

Hershey-Chase experiment

Confirmed DNA (not protein) is genetic material using radioactively labeled bacteriophages.

DNA carries

heritable information

Meiosis vs Mitosis: DNA replication

meiosis: Once, before division

mitosis: Once, before meiosis I

Meiosis vs Mitosis: Cell division

meiosis: Once

mitosis: Twice

Meiosis vs Mitosis: The sister chromatids separate in…

meiosis: Anaphase II

mitosis: Anaphase

Meiosis vs Mitosis: The homologous chromosomes separate…

meiosis: Anaphase I

mitosis: never

Meiosis vs Mitosis: Final cell

meiosis: Diploid (2n → 2n)

Gamate

mitosis: Haploid (2n → n)

Somatic

Meiosis vs Mitosis: Genetic variation

meiosis: None, identical to the parent cell

mitosis: Yes, crossing over & independent assortment

Meiosis vs Mitosis: Function

meiosis: Growth, repair, asexual reproduction

mitosis: Sexual reproduction (gamete formation)

Gregor Mendel’s pea plant breeding experiments resulted in

• Laws of inheritance

• Concept of units of inheritance (genes!)

why was mendal lucky

Most traits/diseases have complex genetics (multiple genes involved) and do not follow simple rules of dominance. We more often discuss “penetrance” or “probability”

Which letter indicates one circled pair of sister chromatids?

c

Laws of inheritance

Each diploid individual has two copies of each gene, one copy from each parent

alleles

Alternate forms of a gene

same spot on chromosome

Homologous chromosomes

carry the same genes but can have different alleles

How many pairs of homologous chromosomes do humans have?

22

Dominant Allele

The allele that determines the phenotype of an individual

phenotype

(observable trait)

Homozygous

AA or aa

Two identical alleles

Heterozygous

Aa

Two different alleles

If the dominant allele determines the phenotype, when would it be possible to observe the phenotype for the recessive allele?

when homogyous ressecive

Law of Segregation

An individual’s maternal and paternal chromosomes segregate from one another during gamete formation (meiosis).

One gamete carries one allele for each gene

Law of Independent Assortment

Segregation of a pair of alleles for one trait (gene) has no effect on the segregation of alleles for another trait i.e. alleles do not influence each other when it comes to sorting into gametes.

Do homologous chromosomes separate in meiosis I or II?

I

What would happen if alleles didn’t sort independently?

less gentic variation

If some groups of genes are contained on the same chromosome, how can the Law of Independent Assortment apply during production of gametes?

cuz of crossing over, can have multiple crossing over events

Chromosomes in meiosis

A Tetrad or bivalent pair is aligned in Meiosis I

The first meiotic division separates the pair of homologous chromosomes into different cells

Meiosis II separates sister chromatids into separate cells

Crossing Over - what is it

• Homologous chromosomes pair up to form structures called tetrads (4 chromatids total).

• At points called chiasmata, non-sister chromatids (one from each homolog) physically exchange equivalent segments of DNA.

• This creates new combinations of alleles on each chromosome.

when does crossing over occur

• Occurs during meiosis I, specifically in prophase I

how does crossing over relate to patterns of inheritances

• Increases genetic variation and combination 

• Genes that are physically close together on the same chromosome tend to be inherited together — this is called genetic linkage. However, crossing over can break linkage

• Explains why not all traits follow Mendel's independent assortment

Crossing Over importance

Crossing over (genetic recombination) allows reshuffling of genes

What do you think happens if genes are located very close to one another on the same chromosome?

genetic linkage

Building blocks of DNA

determined in the late 1800’s:

deoxyribose sugar, phosphate, and one of 4 nitrogenous bases

DeoxyriboNucleic Acid structure

Double stranded molecule i.e. it consists of two strands of DNA

Double helix held together by nucleotides that base pair

What type of bond connects a base pair?

hydrogen

Nucleosides:

Sugar + nitrogenous base

NO phosphate

Nucleotides:

Nucleoside + phosphate group(s)

example of nucleosides

Adenosine, Thymidine, Guanosine, Cytidine

example of nucleotides

Adenine, Thymine, Guanine, Cytosine

a pairs with what and how many hydrogen bond

• A pairs with T (2 H-bonds)

g pairs with what and how many hydrogen bond

• G pairs with C (3 H-bonds)

purine vs pyrimidines: how many rings

purine: double

pyrimidines: single

purine vs pyrimidines: bases in dna

purine: a, g

pyrimidines: c, t

purine vs pyrimidines: bases in rna

purine: a, g

pyrimidines: c, u

purine vs pyrimidines:size

purine: larger

pyrimidines: smaller

Nucleotides are joined together by

3’ – 5’ phosphodiester linkages

3 prime end

hydroxyl end

5 prime end

phosphate end

direction of dna

5 to 3

Each turn of the helix is how many bases

10

The major/minor grooves of the helix are

protein binding sites

why are protein binding sites important

provide a way for proteins to check to see where to bind

can hydrogen bonds between bases be broken

Strands can come apart (denature) or come together (renature, anneal, or hybridize)

DNA structure: Functional requirements

1. Storage of genetic information:

2. Replication and inheritance:

3. Expression of genetic message:

How does DNA structure allow for Storage of genetic information

Information is stored as the linear sequence of the bases; this codes for genes contributing to cellular processe

How does DNA structure allow for Replication and inheritance

• 2 Strands of DNA would come apart and each would then serve as a template for making new complimentary strand

How does DNA structure allow for Expression of genetic message

• The ability to break hydrogen bonds to access a single DNA strand also allows DNA to serve as a template for RNA synthesis (transcription) for expression of gene.

which organisms use DNA as the keeper of their heritable information

Bacteria, archea, eukaryotes

Changes in DNA mark the passage of time as species form and diverge

The central dogma of biology

Directionality in the flow of information;

DNA to RNA to Protein

Transcription

DNA → mRNA (in nucleus)

Translation:

mRNA → Protein (in ribosome)

Germline mutation:

In gametes → inherited by offspring.

Somatic mutation:

In body cells → affects only the individual (e.g., cancer).

backbone of dna

• Each strand of DNA has a sugar-phosphate backbone, formed by alternating deoxyribose sugars and phosphate groups. These are linked by phosphodiester bonds.

base pairing of dna

• Nitrogenous bases stick out from the sugar-phosphate backbone and pair with bases from the opposite strand

Antiparallel Strands of dna

• The two DNA strands run in opposite directions

Mitotic spindle - what is it

Organizes and pulls chromosomes apart using microtubules.

Mitotic spindle in Meiosis I

• Forms between two centrosomes at opposite poles.

• Pulls homologous chromosomes (not sister chromatids) apart to opposite ends of the cell

Mitotic spindle in Meiosis II

• Forms again after interkinesis.

• Now it pulls apart sister chromatids, just like in mitosis.

Microtubules - what is it

Fibers that connect to kinetochores and pull chromosomes/chromatids.

connect at centromere

Microtubules in Meiosis I

• Attach to kinetochores of homologous chromosomes.

• Align tetrads (homologous pairs) at the metaphase plate.

• Separate homologs, not sister chromatids.

Microtubules in Meiosis II

• Attach to kinetochores on sister chromatids.

• Separate sister chromatids into daughter cells.

Kinetochores - what is it

Protein structures on centromeres where spindle fibers attach.

Kinetochores in Meiosis I

• Kinetochores of sister chromatids act together and face the same pole (they do not split).

• Microtubules from opposite poles attach to homologous chromosomes.

Kinetochores in Meiosis II

• Kinetochores of sister chromatids face opposite poles, allowing separation of sister chromatids.

Cohesins - what is it

Protein complexes that hold sister chromatids and homologous chromosomes together.

Cohesins in Meiosis I

• Cohesins hold sister chromatids together along the entire length.

• Enzyme separase removes cohesins from arms, but centromeric cohesins are protected by shugoshin, preventing premature separation of sister chromatids.

Cohesins in Meiosis II

Centromeric cohesins are removed, allowing sister chromatids to finally separate and move to opposite poles

what if cohesin fails

if failed = no disjuction, lots or none chromosmes

cohsin in female breaks down as you age

Centromeres - what is it

Region where sister chromatids are joined and where kinetochores form.

Centromeres in Meiosis I

• Centromeres of sister chromatids do not split; they stay together as homologous chromosomes are separated.

Centromeres in Meiosis II

• Centromeres split, allowing sister chromatids to separate and move to opposite ends.