2Chromosomes and Cellular Reproduction

• unicellular/no membrane bound organelles

• its DNA does not exist in highly ordered and packed arrangement; typically no histones

• circular DNA/typically only one chromosome/small amount of DNA/double stranded

• made of eubacteria and arachaea

• Nucleus absent

• membrane bound organelles absent

Prokaryote

• can be unicellular and multicellular with membrane bound organelles

• DNA is surrounded in a nuclear envelope to make nucleus

• DNA is closely associated with histones to form tightly packed chromosomes

Eukaryote

What is the role of histone proteins in eukaryotic chromosomes?

• helps regulate accessibility of DNA to enzymes and other proteins that copy and read the DNA

• enables the DNA to fit in to the nucleus of a eukaryotic cell

Why do Eukaryotic cells require mechanisms to ensure that a copy of each chromosome is faithfully transmitted to each new cell?

Because unlike prokaryotes, eukaryotes have MULTIPLE linear DNA molecules (chromosomes) which can result in many errors if copying is not done correctly

• do not posses the structure of a cell

• composed of an outer protein coat surrounding DNA or RNA

• can only survive with a host cell

Viruses

What are the 3 fundamental events that must take place for a cell to reproduce successfully?

• genetic info must be copied

• copies must be separate from each other

• the cell must divide

What is the process that occurs when a prokaryotic cell reproduces?

Binary Fission

In binary fission, the specific place on a circular chromosome where replication begins

Origin of Replication

In binary fission how many origins of replication are there?

only one

First step of binary fission

replication of the circular chromosome begins at the origin of replication making two newly replicated chromosomes

Second step of binary fission

Origins of the two newly replicated chromosomes move away from each other and toward opposite ends of the cell.

Third step of binary fission

A new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome

Proteins that encircle the DNA and help keep the two newly synthesized chromosomes from getting tangled as they are replicated

Structural maintenance of chromosomes (SMC) complexes

Maintains precise spatial relations among the components of the nucleus and takes part in DNA replication, the expression of genes, and the modification of gene products before they leave the nucleus.

Nuclear Matrix

Explain a homologous pair

a set of chromosomes (one from each parent) in which they are alike in size and structure and carry information. the genes on each chromosome are in the same order, but these genes from both chromosomes can have different alleles based on the parent.

• THEY ARE NOT IDENTICAL ONLY THE ORDER OF THE GENES ARE THE SAME

How many chromosomes does a human have? How many pairs? How many sets?

• 46 total chromosomes

• 23 pairs (23 from one parent and 23 from the other)

• 2 sets of chromosomes from each parent

Cells that carry two sets of genetic information/chromosomes (2n n=number of chromosomes)

Diploid

A single set of chromosomes (n) typically found in gametes

Haploid

Cells of some other eukaryotes contain more than two sets of genetic information

Polyploid

Diploid cells have ____

a. 2 chromosomes

b. 2 sets of chromosomes

c. one set of chromosomes

d. two pairs of homologous chromosomes

b (2 sets meaning a set from each parent)

True or False: Each chromosome is a single molecule of DNA

True

What three elements consider a chromosome functional?

• centromere

• pair of telomeres

• origins of replication

Serves as the attachment point for spindle microtubules

Centromere

A multiprotein complex that assembles on the centromere and later attaches to spindle microtubules

Kinetochore

Chromosomes lacking a _______ cannot be drawn into the newly formed nuclei

Centromere

Submentacentric centromere

Metacentric centromere

Telocentric centromere

Acrocentric centromere

The specific DNA sequences and associated proteins located at the tips of whole linear chromosomes

Telomeres

What is the purpose of telomeres?

Protect and stabilize the chromosome ends. If a chromosome end breaks it will be degraded

In preparation for cell division, each chromosome replicates, making an IDENTICAL (same exact alleles) copy of itself called

Sister chromatids (2 molecules of DNA)

What would happen if a chromosome did not have a kinetochore?

Kinetochores are needed for spindle fibers to attach to the centromere and separate chromosomes. Without kinetochore, there would be no separation and there would be a missing chromosome in daughter cells.

The cell cycle consists of what?

Interphase (G1, S, G2) and m phase (mitosis and cytokinesis)

Phase in the cell cycle undergoing a period of growth, development between cell divisions as well as protein/biochemical reactions. Chromosomes are relaxed but never uncoiled

• nuclear membrane is present and chromosomes are relaxed

Interphase

The cell grows and proteins necessary for cell division are synthesized. This stage typically lasts several hours.

G₁ (Gap 1)

A phase during interphase that a cell can choose to go into before the G_¹/S checkpoint. Cells exit the active cell cycle in response to regulatory signals and pass into a nondividing phase. Cells usually maintain in size and can stay in this phase for a while.

G₀

A checkpoint that holds the cell in G1 until the cell has all the enzymes and proteins necessary for the replication of DNA. After this checkpoint is passed then the cell is committed to divide.

G₁/S checkpoint

The phase in during interphase where each chromosome is duplicated

S Phase

Before: each chromosome is unreplicated

After: each chromosome is replicated making 2 sister chromatids and are connected by a centromere

True or False: DNA synthesis must take place before the cell can proceed to mitosis

True

If DNA synthesis is blocked (by drugs or by a mutation), the cell will not normally be able to undergo mitosis

The stage in interphase where several additional biochemical events necessary for cell division take place

G₂

A checkpoint near the end of G₂ that makes sure all of the cell’s DNA is replicated and undamaged. If cell does not pass, then some additional proteins may be activated in order to pass so that it can divide and enter the M phase

G₂/M checkpoint

Part of the cell cycle in which the copies of the cell’s chromosomes (sister chromatids) separate and the cell undergoes division

M Phase

State the 5 stages of mitosis and the last part of the M phase

• prophase

• prometaphase

• metaphase

• anaphase

• telophase

cytokinesis

• chromosomes condense forming chromosomes

• each chromosome has 2 sister chromatids that have identical alleles

• mitotic spindle forms

Prophase

• nuclear membrane disintegrates

• spindle microtubules attach to kinetochores on centromeres of chromosomes/sister chromatids

Prometaphase

• chromosomes line up on the plate

• spindle fibers extend from the centromeres to the centrosomes

Metaphase

• the sister chromatids move towards opposite poles

• previously known 2 sister chromatids are now known as 2 individual unreplicated chromosomes

Anaphase

• chromosomes arrive at spindle poles

• centrosomes are at the poles with few remaining spindle fibers

• nuclear membrane re-forms

• chromosomes relax

Telophase

Encircle DNA within chromosomes and bring about condensation. A type of structural maintenance of chromosomes complex that plays a role in chromosome segregation

Condensins

During prometaphase, ______ molecules are added to and removed from the microtubules, causing them to undergo repeated cycles of growth and shrinkage

Tubulins

cytoplasm divides

Cytokinesis

True or False: Mitosis ensures that one of the two sister chromatids from each replicated chromosome passes into each new cell.

True

True or False: Each of the cells produced contains a full complement of chromosomes: there is no net reduction or increase in chromosome number.

True

What process produces genetically variable cells?

Meiosis

What process causes the chromosome number in the newly formed cells to be reduced by half?

Meiosis

Process that comes at the end of meiosis I resulting in the number of chromosomes per cell to be reduced by half

Reduction Division

Process that comes at the end of meiosis II

Equational Division

What are the substages of Prophase I?

• leptotene

• zygotene

• pachytene

• diplotene

• diakinesis

What are the stages of meiosis I?

• Prophase I

• Anaphase I

• Telophase I

What are the stages of meiosis II?

• prophase II

• metaphase II

• anaphase II

• telophase II

Stage of prophase I: chromosomes condense and become visible.

Leptotene

Stage of prophase I: chromosomes continue to condense; homologous chromosomes pair up and begin synapsis

Zygotene

Close pairing of chromosomes

Synapsis

A homologous pair of synapsed chromosomes consisting of 4 chromatids

Bivalent/Tetrads

Stage of Prophase I: Chromosomes become shorter and thicker, and a three-part synaptonemal complex develops between homologous chromosomes

Pachytene

Crossing over where homologous chromosomes exchange genetic information occurs when? Are these diploid or haploid?

Prophase I and diploid (92)

Stage of Prophase I: Centromeres of the paired chromosomes move apart, but the two homologs remain attached at each chiasma.

Diplotene

Stage of Prophase I: chromosome condensation continues, the nuclear membrane breaks down, and the spindle forms, setting the stage for metaphase I

Diakinesis

• Initiated when homologous pairs of chromosomes align along the plate.

• A microtubule from one spindle pole attaches to one chromosome of a homologous pair, and a microtubule from the other pole attaches to the other member of the pair

• diploid 92

Metaphase I

• homologous chromosomes separate and move toward opposite poles

• sister chromatids remain attached and travel together

Anaphase I

• chromosomes arrive at the spindle poles

• homologous chromosomes are now considered sister chromatids

• cytoplasm divides

• haploid (n) 46

Telophase I

• chromosomes recondense and the events of interkinesis are reversed

• sister chromatids form

• spindle re-forms

• nuclear membrane once again breaks down

• haploid 46

Prophase II

• individual chromosomes/sister chromatids line up on the equatorial plate (horizontal)

• haploid 46

Metaphase II

• sister chromatids separate and move toward opposite poles

• each chromatid is now considered a chromosome

• haploid 46

Anaphase II

• chromosomes arrives at the spindle poles

• cytoplasm divides again

• nuclear membrane reforms around chromosomes

• 4 genetically non-identical haploid gametes form

• haploid 23

Telophase II

The period between meiosis i and meiosis II in which the nuclear membrane re-forms around the chromosomes clustered at each pole, the spindle breaks down, and the chromosomes relax

Interkinesis

Which of the following events take place in metaphase I?

a. crossing over

b. chromosomes condense

c. homologous pairs of chromosomes line up on the metaphase plate

d. individual chromosomes line up on the metaphase plate

C

What two processes result in genetic differences that are unique to meiosis?

• crossing over (occurs in Prophase I)

• separation of homologous chromosomes (In anaphase I)

After crossing over, will the sister chromatids be identical still?

No

The creation of new combinations of alleles on a chromatid

Recombination

A protein that holds chromatids together and is another type of structural maintenance of chromosomes (SMC) complex

Cohesin

During anaphase of mitosis, where does cohesin lie?

Along the entire length of the chromosome

What breaks down cohesin in mitosis that allows the sister chromatids to separate?

Separase

During anaphase I of meiosis how does cohesin lie?

Along the length of the chromosome but at the centromere they are protected by shugoshin

What is the role of shugoshin?

Prevents the separation of the two sister chromatids during anaphase I. It is broken down by the end of metaphase II, allowing cohesin to be broken down

Production of gametes in a male animal and takes place in the testes

Spermatogenesis

• diploid primordial germ cells that divide mitotically to produce diploid cells

• can undergo repeated rounds of mitosis

• can also initiate meiosis and enter prophase I and make a primary spermatocyte

Spermatogonia

• formerly a spermatogonia

• cell is still diploid since chromosomes have not yet separated

• completes meiosis I and becomes secondary spermatocyte

Primary spermatocyte

• formerly primary spermatocyte

• undergo meiosis II making two haploid spermatids

Secondary spermatocyte

• a secondary spermatocyte that underwent meiosis II

Spermatids

How many haploid spermatids does each primary spermatocyte produce?

4

Production of gametes in a female animal

Oogenesis

• Is produced when diploid primordial germ cells divide mitotically in the ovaries

• can undergo rounds of mitosis or can also go through meiosis

• If undergoes meiosis, turns into primary oocytes in prophase I

• diploid

Oogonia

• formerly oogonia

• completes meiosis I

• still diploid

• produces 2 products

Primary oocytes

• the name when most of the cytoplasm is allocated to the chosen haploid cell out of the two primary oocytes/cell

• now haploid and completed meiosis II

• cytokinesis is unequal

• produces two products

Secondary Oocyte

• The smaller cell, which contains half of the chromosomes but only a small part of the cytoplasm

• doesn’t have a purpose and disintegrates

First polar body

• one of the products from the secondary oocyte

• cytokinesis is unequal again

• acquires most of the cytoplasm

• capable of being fertilized

Ovum

• the smaller cell after production of the ovum

• disintegrates

Second polar body

How many gametes are produced form each primary oocyte?

One