genetics week 1 extra Introduction to Chromosome Structure, the Cell Cycle, and Meiotic Variation

Telomere Structure and Function

Telomeres help tie the end of a chromosome in a knot to protect the genetic material.
Each telomere creates a structure called a $t$ -loop, which is composed of telomeric repeats.
A single strand of DNA, which also contains these repeats, tucks itself away into the double-stranded DNA group through hydrogen bonding.
This process is facilitated by the repeat sequence, which allows the DNA tail to fold back and secure itself.
The telomere acts metaphorically like an aglet, which is the plastic tip at the end of a shoelace, preventing the end from fraying by tucking the tail into the side of the structure.

The Centromere and Chromosome Classification

Centromeres are essential for understanding the mechanics of mitosis and tracking the movement of chromosomes.
The position of the centromere is used to differentiate between four major types of chromosomes:
- Metacentric Chromosomes: These possess a centromere located in the direct middle of the chromosome.
- Submetacentric Chromosomes: The centromere is positioned just off-center.
- Acrocentric Chromosomes: The centromere is located near the telomeres; however, the short arms of the chromosomes remain visible.
- Telocentric Chromosomes: The centromere is so close to the telomere that the short arms are barely visible, if they can be seen at all.
Centromere position is a key identifier used to distinguish homologous chromosomes.

Chromatin, Mitosis, and DNA Replication

Chromosomes serve as the primary storage units for genetic information, passed from cell to cell and person to person.
The term chromatin was coined by a researcher who used various stains and dyes on cells, describing it as a "colorful substance" found inside the nucleus.
The term mitosis originates from the Greek word mitos, meaning "thread," referring to the thread-like structures observed in cells during this stage of the cell cycle.
Unreplicated Chromosomes: Consist of one end-to-end DNA molecule, representing a single chromatid.
Replicated Chromosomes: Occur after DNA replication splits the DNA and creates daughter strands. Although there are now two chromatids per chromosome, they remain attached at a single centromere.
Because they are attached to the same centromere, they move as "one body" or a "chromosome follicle body."

The Stages of the Cell Cycle

The cell cycle is divided into four primary phases:
- $G_1$ (Gap 1): A phase of cell growth following division. At this stage, each homologous pair contains one chromatid per chromosome.
- $S$ phase (Synthesis): The cell replicates its DNA. It begins with one chromosome consisting of one chromatid and ends with one chromosome consisting of two sister chromatids.
- $G_2$ (Gap 2): A second phase of growth preceding division.
- Mitosis ( $M$ ): The sister chromatids are split in half, resulting in two cells, each containing one chromatid per chromosome.

Homologous vs. Non-Homologous Chromosomes

Homologous Chromosomes: These pairs have the same size, the same centromere position, and carry the same genes at the same loci.
- While they carry the same genes, they may contain different alleles (e.g., a green chromosome might carry allele $A$ while its homolog carries allele $a$ ).
Sister Chromatids: Identical copies of a chromosome held together by a centromere.
Non-sister Chromatids: Chromatids present on separate homologs within a homologous pair. This distinction is critical for the processes involved in meiosis.
Non-homologous Chromosomes: Chromosomes that do not share the same size, centromere position, or genes.

The Mechanics of Mitosis

Prophase: The first phase of mitosis where chromatin condenses into visible chromosomes.
Prometaphase: The nuclear envelope breaks down, and the mitotic spindle forms to connect to the chromosomes for movement.
Metaphase: Individual chromosomes line up along the metaphase plate. Homologous chromosomes do not line up together here; every individual chromosome aligns independently.
Anaphase: Sister chromatids attached by the centromeres are pulled apart, ensuring exact copies of DNA move toward the two future daughter cells.
Telophase and Cytokinesis: The final stages where the two daughter cells are formed.

Meiosis and Genetic Diversity

Mitosis produces two identical daughter cells (clones), whereas meiosis starts with one diploid parent cell and results in four haploid daughter cells.
Diploid ( $2n$ ): A cell containing two of every chromosome (one from each parent).
Haploid ( $n$ ): A cell, such as a sperm or egg (gamete), containing half the genetic information of a typical organism ( $n = 1/2 \times 2n$ ).
Meiosis consists of two separate cell divisions:
- Meiosis I: Known as the reduction division. Homologous pairs (one from the mother, one from the father) pair up and then separate. This reduces the resulting cells to one copy of every chromosome.
- Meiosis II: Known as the equational division. The sister chromatids are separated, similar to the process in mitosis.
Crossing Over: Occurs during Meiosis I when homologous chromosomes link together and swap genetic information between non-sister chromatids. This double-stranded DNA break and exchange creates recombinant chromosomes.
This recombination produces a combination of alleles that did not exist in either parent cell, driving genetic diversity. In humans, crossing over occurs across all $46$ chromosomes at an average rate of three crossover events per chromosome.

Questions & Discussion: Genetic Variation and Cell Cycle

Question: Is there genetic variation produced in mitosis?
Answer: No.
Question: Is there genetic variation produced in Meiosis I?
Answer: Yes, due to crossing over.
Question: Is there genetic variation produced in Meiosis II?
Discussion: There is a debate between yes and no. While crossing over does not occur in Meiosis II, and there is no further reduction in chromosome number, the separation of chromatids ensures they are not identical, contributing to the final distribution. This involves random distribution rather than genetic "swapping."
Question: If a normal somatic cell of an organism has $22$ chromosomes, how many telomeres are present during $G_2$ ?
Discussion: In $G_1$ , there would be $22 \times 2 = 44$ telomeres. After replication in the $S$ phase, entering $G_2$ , each of the $22$ chromosomes has two sister chromatids. Since each chromatid has two telomeres, there are four telomeres per replicated chromosome, totaling $22 \times 4 = 88$ telomeres.

Case Study: The "Fatherless" Timber Rattlesnake

A timber rattlesnake named Marsha Jones was kept in isolation for $16$ years, starting from when she was only $4$ days old.
Despite being alone, she gave birth to one healthy live male baby and two stillborns.
Timber rattlesnakes typically produce live births.
Sperm Retention: While some snakes can hold sperm in their fallopian tubes for up to $7$ years, Marsha Jones had been isolated for $16$ years, ruling this out.
Sex Determination: Snakes use the $ZW$ sex-determination system. Females are $ZW$ and males are $ZZ$ .
Hermaphroditism (Monoecious): A question was raised if the snake was a hermaphrodite (producing both egg and sperm). Marsha Jones was confirmed to have a typical female reproductive tract.
Mitosis vs. Meiosis Hypothesis: If the birth were a result of mitosis, the offspring would be an exact clone of the mother ( $ZW$ female). Since the baby was male ( $ZZ$ ), the birth must result from a meiotic process rather than simple mitotic cell division. This suggests a form of asexual reproduction involving meiosis where the offspring receives a different combination of chromosomes than the mother's identical genotype.

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genetics week 1 part 2

Slide 2 - Meiosis and Gametogenesis

Slide 3 - Eukaryotic chromosomes have telomeres and one...

Centromere

Telomere

Fluorescence in situ hybridization (FISH)

Probe is for the

centromere

Probe is for the

telomere

Note that each

chromosome is a pair of sister chromatids

Telomeres and

centromeres are classified as heterochromatin

(transcriptionally

silenced). They are highly repetitive DNA, clusters of 100,000s to millions of tandem repeats < 10 bp length, also called satellite DNA

Eukaryotic chromosomes have telomeres and one centromere

eukaryotic chromosomes are opposed to prokarotic with a begin middle and end with caps that prevent being degraded or lost to time

the centromere are where chromosomes are pulled. they are tubules which pull them away

telomeres are caps at the end of chromosomes that help everything stay nice and secure

Slide 4 - Each eukaryotic chromosomes has telomeres at...

Each eukaryotic chromosomes has telomeres at each end

Telomeres (also described in Chapter 12) consist of specific repetitive DNA sequences and don't contain genes

• Tandem repeats of A or T followed by several G's (e.g. human telomere repeat is 5’-TTAGGG-3’, 100-1000 repeats per telomere

• Sequence of repeats is specific to each species

• Maintain integrity of chromosomal ends and protect the ends of the chromosomes to prevent chromosome fusion

Note that the two strands are not the same length:

• The G-rich strand has a 3'-overhang

Pierce 7e, Fig 11.10a

Slide 5 - Specific structures occur at the telomeres

Specific structures occur at the telomeres

The G-rich 3’ overhang folds over and forms a loop at the end of the telomere

(t-loop) by intermolecular H-bonding with the complementary strand

Slide 6 - Chromosomes are differentiated by their length...

Chromosomes are differentiated by their length and centromere position

Metacentric – centromere in the exact middle of a chromosome

Submetacentric – centromere is off-center

Acrocentric – centromere is almost to the telomere, but the small-arms are still visible

Telocentric – centromere is close enough to the telomere that the small arms are not visible

centromeres:

meta centric- centromeres are in middle of chromosome

subcentric- just off center centromere

acrocentric- centro almost to the teomere

Telocentric- centromere close to telomere that the small arms are not visible

Slide 7 - Eukaryotes have two major portions of their...

Eukaryotes have two major portions of their cell cycle

• Mitosis is the portion where chromatin condenses into thread-like chromosomes are visible, align and separate into the nascent cells

• Interphase is the time between one mitosis and the next. In interphase, chromatin is diffuse

Giphy

Mitosis is the portion where chromatin condenses into thread-like chromosomes are visble

interphase- the time between one mitosis and the next

Slide 8 - Chromosome morphology

Chromosome morphology

Centromere

Unreplicated

Replicated

Sister chromatids homologs

Same genes, can be different alleles

Identical except for mutational error

homologs

between cell cycle, we look at chromosome morphology

both are chromosomes, but the amount of chromatids

centromeres are still always attatched, but they're still one body moving together, even if they contain 4 chromatids now. they are identical since they are replicated

Slide 9 - The mitotic cell cycle

The mitotic cell cycle

G1 + S + G2 = interphase

Mitosis = Prophase + prometaphase + metaphase + anaphase + telophase

Cytokinesis = cell division

Synthesis

Growth

(Single pair of chromosomes shown)

G1+S+G2 = interphase

G1= gap 1- phase of cell growth. one chromatid per chromasome

S- one crhomosome wih two sister crhomatids

divdes again, repeats

Slide 10 - The cell cycle consists of interphase and mitosis

Interphase chromosomes are relaxed and cannot be visualized in a microscope

Mitotic and meiotic chromosomes are highly condensed and can be visualized in a microscope

S phase- DNA replication

(chromosome duplication

(Replication)

The cell cycle consists of interphase and mitosis

S phase- 1 chromasome w 1 chromatid to one chromosome with 2 sister chromatids

Slide 11 - After S-phase, chromosomes consist of pairs of...

After S-phase, chromosomes consist of pairs of sister chromatids

• After S, identical copies of each chromosome joined at centromeres

• Each chromatid consists of double-strand DNA

• A and a are two different alleles of the

A gene

• B and b are two different alleles of the

B gene

Centromere

Nonhomologous chromosomes

Sister chromatids

Nonsister chromatids

Homologous chromosomes

A A a a

B b b

Figure is from a different textbook (Hartwell 4e)

homologous chromosomes= same size, same centromere position, same genes

non sister chromatids- do not have specific exact chromatids on a chromosome

they can be homologous because of same shape and size, but they are not identical in terms of alleles, so they are would have non suster chromatids

Slide 13

Slide 14 - 2.10 (4) The cell cycle is divided into...

G1 loosey goosey s phase creates g2 bigger

prophase, metaphase, anaphase, telophase

A normal somatic cell of a particular organism has a total of 22 metacentric chromosomes. In this organism, what is the total number of telomeres per cell in G2?

4 per chromosome

every chromosome has a beginning and end with telomeres on those sides. 22 metacentric chromosomes, then there are 44 telomeres. now after replication and s phase, there are now 88

Slide 16 - Briefly describe the stages of the cell cycle,...

Briefly describe the stages of the cell cycle, mitosis, and meiosis. Describe the major function of each step in each cycle, specifically with regards to the chromosomes (have they replicated, are they pairing with homologs, are the chromatids joined?).

Pierce 7e, Table 2.1

Slide 17 - Gamete fusion combines one set of human...

Gamete fusion combines one set of human chromosomes with another

• Humans have 2 sets of 23 chromosomes (diploid)

• 1 from mom, 1 from dad

• Sperm and egg each contain 1 set of the 23 chromosomes

(haploid)

• Meiosis is the process of separating homologous chromosomes to create gametes

Darryl Leja

MEIOSIS

haploid- state of egg and sperm with each of them containing half of the genetic information

diploid means two

meiosis starts with one parent cell and ends with 4 daughter cells that i sreductional division

Slide 18 - Mitosis vs. Meiosis

Mitosis vs. Meiosis

• Mitosis

• 2 identical daughter cells are produced from 1 parent cell;

• Daughter cells have full set of homologous chromosomes

• Meiosis

• 4 non-identical daughter cells from 1 parent cell;

• Daughter cells have half the parent cell’s chromosomes

• Haploid daughter cells have no homologous chromosomes

Slide 19 - Mitosis = Equational Division

Slide 20 - Meiosis = Reductional Division

Slide 21 - TopHat Practice – Sister Chromatid Origins

Slide 22 - Reminder: Homologs and chromatids

Reminder: Homologs and chromatids

• Identical copies of each chromosome present after S-phase and held together at their centromeres

• Each chromatid consists of double-

strand DNA

• A and a are two alleles of the A gene

• B and b are two alleles of the B gene

• The A and B genes are unlinked

(independently assorting)

Centromere

Nonhomologous chromosomes

Sister chromatids

Nonsister chromatids

Homologous chromosomes

A A a a

B b b

Figure is from a different textbook (Hartwell 4e)

Slide 23 - TWO cell divisions occur in meiosis

Slide 24 - Mitosis vs. Meiosis

Slide 25 - This image is the same as the image on the...

Slide 27 - Crossing-over takes place between non-sister...

the chromatids of parental and maternal sepearate, and then the non sister chromatids cross over and creates genetic diversity

Slide 28 - Crossing-over during meiosis I results in...

Crossing-over during meiosis I results in genetic variation

• Non-sister chromatids recombine (cross over) and create new combinations of alleles

• A dihybrid (AB/ab) for two linked genes → four gamete genotypes produced:

two nonrecombinant

(look like parental- AB, ab) and two recombinant (don’t look like parental- aB, Ab)

Nonrecombinant

Recombinant

Pierce 7e, Fig 2.16

S-phase

Meiosis I

Meiosis

Slide 32 - Comparison of mitosis, meiosis I, and meiosis II

what are you asking about? chromosomes vs chromatids?

Slide 33 - General life cycle of animals

General life cycle of animals

Sperm

(1n)

Egg

(1n)

Fertilization

Zygote (2n)

Embryo (2n)

Somatic cells (2n)

Germ cells

(2n)

Mitosis and differentiation

All cells and tissues (2n)

Mitosis and differentiation

Meiosis

Somatic cells are all the cells other than germ cells

• Most types of normal somatic cells are diploid, none are haploid

• Somatic cells NEVER go through meiosis

• Somatic cells can undergo mutations (including recombination) and CAN have abnormal chromosome segregation

(nondisjunction) during mitosis

Germ cells are precursors to gametes

• Develop from specialized diploid cells

• Located only in the gonads

(testis and ovary)

• Go through mitosis and meiosis only at specific stages of development

• Germ cells are the ONLY cells that go through meiosis

how do you submit that of music?

heterogametic- zw/xy