Chapter 16 - Cellular Reproduction

What is the cell cycle?

The sequence of stages in which a cell passes from one division to the next

What are the two stages of the cell cycle called?

Interphase and Cell Division

What is interphase?

Known as the longest phase in the cell cycle, it is the time period in which cells grow in mass, roughly doubling it’s cytoplasmic contents, duplicating it’s chromosomes and prepares for the next division

What is cell division?

The process in which a cell divides into two or more daughter cells, including the division of the nucleus and more cell components

What are the three stages of interphase?

G1 Phase, S Phase and G2 Phase

What occurs during G1 phase? 

During the G1 phase

• Cellular organelles double (such as the mitochondria, ribosomes, etc),

• The cell grows in size

• Accumulates structures that will be needed to synthesize more DNA as well as proteins

• Gene expression

What occurs during the S Phase

During S phase:

• DNA in the chromatin replicates to create a second identical set of DNA, creating two identical chromosomes called sister chromatids 

• For humans, 46 single stranded (unduplicated) chromosomes in a somatic cell get replicated to form 46 double stranded chromosomes (duplicated) 

• These sister chromatids are joined at the centromere 

• Daughter cells are genetically identical to each others clones as well as to the original parent cell

What occurs during the G2 Phase

• Due to the great deal of energy needed for DNA replication during G1 phase, the G2 phase is time used by the cell to rebuild it’s reserves of energy in order to prepare for mitosis 

• Grows larger 

• Produces energy

• Produces proteins which will help in the process of mitosis 

• Duplicating organelles such as the centrosome 

What is defines a somatic cell?

Any cell in the body of a multicellular organism except sex cells/gametes

Why is DNA replication important during S phase?

It is important so that way when a cell completes the cell division stage, each new daughter cell will have 46 identical chromosomes and receives a complete identical copy of the parent cell’s genetic material

What are homologous chromosomes?

Paired chromosomes that are similar in shape, size, genetic arrangement and and genetic information

What are autosomes?

Any chromosome that is not a sex chromosome

What is a karyotype?

It is an organized profile of a person’s complete set of chromosomes

How do scientists prepare a karyotype?

• Collect a cell sample by using chemicals to stop the cell cycle when the condenses chromosomes are most clearly visible under a light microscope 

• Stain the cells for individual chromosome identification

• Photograph the stained chromosomes and transfer the images on a new background

• Organize the images into a series of homologous pairs 

What are the two main processes of cell division?

Mitosis and Cytokinesis

What is mitosis?

The process in which a parent cell divides into two genetically identical daughter cells, meaning the daughter cells receive the same amount of chromosomes as the parent cell 

What is cytokinesis?

The separation of the cytoplasm and organelles, resulting in the formation of two daughter cells 

After cytokinesis occurs, what two materials help reinforce a new cell wall?

Cellulose and Proteins

Why do cells grow?

Organism growth, efficiency for more cells, replacement of dead or older cells as well as obtaining a high surface area to volume ratio 

What does the term “diploid” mean?

It describes a cell that contains two pairs of every chromosome

• Somatic cells 

What does the term “haploid” mean?

It describes a cell containing half the number of chromosomes that the diploid parent cell contains, which occurs in gametes (ovum and sperm) 

What is the diploid value represented as?

2n = 46

What is the haploid value represented as?

n = 23

What are the 3 important functions that mitosis and cytokinesis have in regards to cell division?

• Growth - allowing organisms to grow from a single celled zygote into a multicellular organism with trillions of cells 

• Maintenance - production of new cells to take over the old and deteriorated ones 

• Repair - regeneration of damaged tissue 

How is DNA organized in a cell?

• During interphase, DNA is in a form known as chromatin, meaning the DNA is unwound and genes are being expressed 

• When mitosis begins, the DNA becomes highly wound up and condensed to form visible chromosomes 

What are the stages of mitosis?

• Prophase

• Metaphase

• Anaphase

• Telophase 

Describe what occurs during prophase of mitosis

• The chromatin condense into tiny into tightly packed chromosomes

• Cell structural change such as the nuclear membrane breaking down resulting in the release of chromosomes into the cytoplasm as well as the nucleolus disappearing 

• Centrioles within the cell move to opposite poles of the cell 

• Fibres known as spindle apparatus form between the two centrioles

• Each spindle fibre is made of microtubules, which are hollow tubes of protein that allow chromosome movement within a cell

Describe what occurs during the metaphase process of mitosis

• Spindle fibres guide the chromosomes to the equator/centre of the cell 

• Spindle fibres from each pole attach to the centromere of each chromosome 

• Spindle fibres attach in such a way that one sister chromatid faces one pole which the other faces the opposite pole 

Describe what occurs during the anaphase process of mitosis

• Each centromere splits apart and the sister chromatids separate from one another 

• The spindle fibres that link the centromeres to the poles of the cell shorten 

• Sister chromatids are pulled to the opposite poles 

• Other microtubules in the spindle apparatus lengthen and force the poles of the cell away from one another 

• At the end, one complete diploid set of chromosomes has been gathered at each pole of the elongated cell

Describe what occurs during the telophase process of mitosis

• Chromatids have reached the opposite poles of the cell

• Chromatids begin to unwind into the longer and less visible strands of chromatin 

• Spindle fibres break down 

• Nuclear membrane forms around each new set of chromosomes

• Nucleolus forms with each new nucleus 

What is a chromatid?

One of the two identical halves of a replicated chromosome

What is a chromatin?

Long fibres that form chromosomes and contain DNA, very few amounts of RNA and various proteins 

What is a centromere?

The point at which two sister chromatids of a chromosome are joined and to which the spindle fibres are attached to during mitosis 

What is the spindle apparatus? 

Assembly that carries out the separation of chromosomes during cell division (mitosis)

• Composed of spindle fibres and are assembled during prophase

Where is DNA found?

In each chromosome of a cell

What are sister chromatids

Two chromatids in a chromosome that are genetically identical and are held together by a centromere

Cell division in animals is very different from cell division in plants. Because plants do not contain centrioles, how do they perform cytokinesis in terms of “furrowing” 

A cell plate—which is a membrane that separates the two daughter cells’ nuclei—is formed and is then reinforced by cellulose and proteins to create a new cell wall

What is meiosis?

The cellular division process that produces haploid gametes from diploid cells in the ovaries and testes 

• Commonly known as sexual cell reproduction 

Meiosis occurs under which two reproductive processes?

Gametogenesis and Spermatogenesis

What are the two key outcomes of meiosis?

• Reduction division - due to it being a form of cell division that produces daughter cells that have fewer chromosomes than the parent cells 

• Recombination - the products of meiosis have different genetic combinations, giving rise to offspring that are genetically distinct from their parents

What are the two phases of meiosis?

Meiosis I and II

What are the stages of meiosis I?

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I 

Describe what occurs during prophase I

• Each pair of homologous chromosomes align side by side in a process called synapsis 

• Crossing over occurs

Define the term synapsis

Synapsis - the alignment of each pair of homologous chromosomes side by side during prophase I

Define the term “tetrad”

Tetrad - a homologous pair formed during prophase I of meiosis, so named because it contains 4 chromatids

Describe what occurs during metaphase I

• Spindle fibres attach to the centromere of each chromosome 

• A spindle fibre from one pole attaches to one pair of sister chromatids in the tetrad, and a spindle fibre from the opposite pole attaches to the other pair of sister chromatids 

• Spindle fibres guide each tetrad to the equator of the cell 

• Chromosomes line up in homologous pairs 

• One homologous chromosome is positioned on one side of side of the cell’s equator and the other homologous chromosome is positioned on the other side 

Describe what occurs during anaphase I

• Spindle fibres shorten, causing the homologous chromosomes to separate from one another 

• Homologues move to the opposite poles of the cell 

• Sister chromatids are held together, meaning the centromeres do not split 

Describe what occurs during telophase I

• Some cells move directly from anaphase I to meiosis II

• Those who do go through telophase I have the homologous chromosomes beginning to uncoil and spindle fibres disappear 

• Cytoplasm is divided 

• Nuclear membrane forms around each homologous chromosome 

• Each of these new cells contain one set of sister chromatids and are now haploid 

What are the stages of meiosis II

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Describe what happens during prophase II

• The chromatin condense into tiny into tightly packed chromosomes

• Cell structural change such as the nuclear membrane breaking down resulting in the release of chromosomes into the cytoplasm as well as the nucleolus disappearing 

• Centrioles within the cell move to opposite poles of the cell 

• Fibres known as spindle apparatus form between the two centrioles

• Each spindle fibre is made of microtubules, which are hollow tubes of protein that allow chromosome movement within a cell

Describe what occurs during metaphase II

• Spindle fibres guide the chromosomes to the equator/centre of the cell 

• Spindle fibres from each pole attach to the centromere of each chromosome 

• Spindle fibres attach in such a way that one sister chromatid faces one pole which the other faces the opposite pole 

Describe what occurs during anaphase II

• Each centromere splits apart and the sister chromatids separate from one another 

• The spindle fibres that link the centromeres to the poles of the cell shorten 

• Sister chromatids are pulled to the opposite poles 

• Other microtubules in the spindle apparatus lengthen and force the poles of the cell away from one another 

• At the end, four haploid cells are produced as a result and are single stranded chromosomes 

Describe what occurs during telophase II

• Chromatids have reached the opposite poles of the cell

• Chromatids begin to unwind into the longer and less visible strands of chromatin 

• Spindle fibres break down 

• Nuclear membrane forms around each new set of chromosomes

• Nucleolus forms with each new nucleus 

What occurs in the crossing over stage of meiosis?

• Genetic material between non-sister chromatids of homologous chromosomes is exchanged due to the chromatids overlapping one another during synapsis

• As a result, genetic variation is increased

• Not all homologous chromosomes cross over

When does crossing over occur?

Prophase I

Why is crossing over necessary?

It is necessary so that genetically different gametes are formed at the end of meiosis, thus increasing genetic variation in future offspring

What is independent assortment?

It is the random assortment of maternal and paternal chromosomes along the equatorial plate during metaphase I

What two processes increase genetic variation during meiosis?

Crossing over and independent assortment

What is nondisjunction?

It is when homologous chromosomes or sister chromatids don’t separate properly during meiosis I or II, resulting in some gametes containing an improper number of chromosomes 

When does nondisjunction in meiosis I occur? 

When one pair of homologous chromosomes don’t separate to opposite poles during anaphase I and instead, one entire pair is pulled to the same pole 

When does nondisjunction in meiosis II occur?

When one pair of sister chromatids don’t separate to opposite poles in anaphase II and are instead both pulled to the same pole 

What does the term “monosomy” mean?

It means a chromosome is lost due to nondisjunction

What does the term trisomy mean?

It means the gain of an extra chromosome

What is a monosomy disorder?

It is when a gamete containing “n-1” number of chromosomes is fertilized by a gamete containing the normal “n” number of chromosomes, resulting in one less of a chromosome

What is Turner’s Syndrome? 

It is a genetic condition only affecting females and occurs when a female is born with a missing or even partial X chromosome, resulting in an XO chromosome and not an XX 

What is a trisomy disorder?

It is when a gamete containing a “n+1” number of chromosomes is fertilized by a gamete containing the normal “n” number of chromosomes, resulting in an extra chromosome 

What is Down Syndrome?

When an individual has 3 copies of chromosome #21, resulting in the chromosome count to be 47 and not the normal 46 

Describe the process of spermatogenesis specified under meiosis

• Starts with a diploid germ called a spermatogonium

• Spermatogonia are stimulated to divide by mitosis to form two daughter cells

• One of these daughter cells replenishes the spermatogonia cell population while the other develops into what is called a primary spermatocyte

• The primary spermatocyte undergoes meiosis I and form two secondary spermatocytes, which then undergo meiosis II and form four spermatids

• Spermatids undergo the final stages of maturation which include the nucleus and enzymes organizing into the head region, the midsection holding many mitochondria and flagella formation 

Describe the process of oogenesis specified under meiosis

• Starts with a diploid germ cell called a oognium which undergoes mitosis to form two primary oocytes  

• After about 3 months of conception, about two million primary oocytes are found in the ovaries

• Are “arrested” in prophase I and remain that way until puberty

• Every month after puberty, one primary oocyte undergoes meiosis I

• After meiosis I, the cytoplasm between the two daughter cells is not equally split and so as a result, the cell with the most cytoplasm content is called the secondary oocyte while the other is called the first polar body which may or may not go through division to turn into a second polar body. Eitherway, the polar bodies are not functional and will degenerate

• The secondary oocyte undergoes meiosis II and is unequally split. The cell with the most cytoplasm eventually becomes a matured egg and the other becomes a second polar body which is not a viable gamete 

• The secondary oocyte is arrested at metaphase II until fertilization occurs and if it does not come in contact with a sperm, it will not complete a second meiotic division. If it does, it will undergo meiosis II which results in an ovum and secondary polar body 

• The haploid nucleus of the egg cell then fuses with the haploid nucleus of the sperm cell to complete fertilization and create a diploid zygote

How are fraternal twins conceived?

They are conceived when two oocytes are released. If both of these oocytes get fertilized, and are implanted into the uterus, fraternal twins may be born

How are identical twins conceived?

They are conceived when a single zygote or blastocyst divides into two separate bodies in the first few days of embryonic development.

What is binary fission?

The asexual reproductive process of cellular division within bacteria

What differentiates asexual reproduction versus sexual reproduction?

Asexual Reproduction:

• is a process in which a parent organism is capable of producing genetically identical offspring without the use of gametes and another parent organism

• produces offspring through the mechanism of mitosis

Sexual Reproduction

• is a process in which two parent organisms must breed with one another to produce genetically different offspring through processes such as gamete formation and fertilization

• produce offspring through the mechanism of meiosis

What is budding?

It is a form of asexual reproduction in which a complete but mini version of the parent grows from out of the parent’s body. As a result, the new organism then separates and becomes a completely independent organism

What is vegetative reproduction?

It is a form of asexual reproduction in which a new plant grows/emerges from a modified stem

What is fragmentation?

It is a form of asexual reproduction in which a new organism is created from a fragment of the parent organism

What is parthenogenesis?

It is a form of asexual reproduction in which an unfertilized egg develops into an adult