2.1 Cell Division

How long is the cell cycle for humans?

24 hours (but this is general—depends on the organism)

Why do cells divide?

Reproduction — unicellular organisms need to create offspring

Growth and repair — multicellular organisms going through cell division do not create new organisms but replace cells, create new cells, etc

Production of gametes — a different type of cell division/meiosis

What’s a genome?

Describes the entire genetic information of an organism

Can be measured by the amount of base pairs

The genome is organized into units called chromosomes

What’s a chromosome?

A single piece of DNA that includes many genes, two identical pieces of DNA that are attagced attached at the atmosphere

Number of chromosomes = number of centromeres

What does a prokaryotic chromosome consist of?

• single chromosome consisting of circular piece of DNA

• DNA is considered naked

• Small self replicating units called plasmids

What do eukaryotic chromosomes consist of?

• several linear DNA molecules

• wrapped around proteins (group of 8) called histones

• Two or more different chromosomes

• Do not have plasmids

What is heterochromatin?

DNA that is tightly packed and not expressed so it cannot make proteins — “silent”

What is euchromatin?

DNA is loosely packed and can be expressed (can create proteins) — “not silent”

The number of chromosomes is a characteristic feature of members of a species per which concepts?

biological species concept and genotypic species concept 

What is autoradiography?

a technique used to determine the length of a molecule of DNA

What does the term Ploidy(n) mean?

a term used to describe the number of sets of chromosomes in the nucleus of a biological cell (where n=the number of chromosomes in a gamete)

What does haploid(n) mean? 

a gamete (sperm or egg) that carries a single set of chromosomes (one of each)

What does diploid(2n) mean?

two gametes that come together to form a zygote which has two sets of chromosomes (pairs of chromosomes)

Which chromosome numbers are included in autosomes?

1-22

What are X, Y chromosomes? 

referred to as “sex chromosomes” because they determine biological sex 

What is a gene?

a heritable factor which is a section of DNA found on a chromosome that gives instructions for a protein

A gene occupies a ______ on a ________

specific position, particular chromosome

What is an allele? 

different “versions” of a gene. These versions are a result of mutations that lead to different proteins being made. 

If present, which one will be expressed no matter what? Dominant allele, recessive allele

Dominant allele!

If one of these alleles are absent, then this allele will be expressed

Recessive allele!

What are homologous chromosomes? 

pieces of DNA that carry the same genes but have different versions of those genes (different alleles)

What are sister chromatids?

DNA that has been replicated during DNA replication; are exactly the same, even carrying the same alleles for every gene. They are held together at the centromere, and only exist during cell division.

Homologous chromosomes can either be ______ or _________.

Homozygous, heterozygous

What are homozygous chromosomes?

HOMO = same 

They carry the same, identical allele on both homologous chromosomes 

What are heterozygous chromosomes?

HETERO = different

They carry different versions (different alleles) on both homologous chromosomes

How is the prophase stage of mitosis different in plants compared to animals? 

Prophase has no centrioles or centrosomes and instead have MTOC (Microtubule organizing centers) that produce and coordinate spindle fibre formation  

How is telophase stage different in plants compared to animals?

There is no cleavage furrow and a phragmoplast (made out of microtubules) forms well cell plate exists in the middle

How is the cytokinesis stage different in plants compared to animals?

plant cell vesicles assemble sections of membrane and cell wall to create the cell plate

Instead of forming a cell plate, animal cells…

a contractile ring of proteins pinches the cell membrane in two to split the cytoplasm 

In most cases, what happens during cytokinesis?

Cells must receive at least one mitochondrion and any other organelles that can only be made by dividing cell structures

What are two rare cases in which unequal cytokinesis occurs? 

• Budding in yeast cells 

• Oogenesis (formation of egg)

What happens in unequal cytokinesis?

one cell gets more organelles and cytoplasm compared to the other cell

What does the mitotic index measure?

measures cell proliferation (rate of cell division) or the % of cells that are currently undergoing mitosis

What does a high mitotic index indicate? 

Rapid cell division, which is commonly found in 

• cancer

• periods of rapid growth in plants and animals 

• cellular repair at a site of an injury  

What can the mitotic index help with? 

It is a prognostic tool that helps predict overall survival and response to chemotherapy in most types of cancer 

How to calculate mitotic index?

(cells in mitosis)/(total number of cells)

Why is meiosis considered to be the “reduction division”?

It reduces the # of chromosomes from diploid (germinal cells) to haploid (gametes)

What occurs during S-phase of meiosis? 

• germinal cell copies its genetic information 

• goes from 2n (single piece of DNA) to 2n (sister chromatid) 

What happens during Meiosis I? *rmbr numeral

• goes from 2n (2 pairs of homologous chromosomes) to n (1 pair of sister chromatids)

What happens during Cytokinesis I? *rmbr numeral

• forms two cells with a pair of sister chromatids (n)

What happens during Meiosis II? *rmbr numeral

• goes from n (pair of sister chromatids) to n (single chromosome)

What happens during Cytokinesis II? *rmbr numeral

• forms 4 haploid cells (n)

In Meiosis I, synapsis occurs. What is it?

Synapsis: in prophase I, four sister chromatids come together to form a temporary structure called a tetrad

What happens in crossing over in Meiosis I? 

After the tetrad forms, there is sometimes an exchange of genetic information between sister chromatids within the tetrad

What is a chiasma (pl. chiasmata)?

the name of a point at which paired chromosomes remain in contact during metaphase I of meiosis, and exchange of genetic material occurs between strands

What is spermatogenesis?

Formation of sperm

Where does spermatogenesis occurs?

Testes

What is spermatogonium? And what does it develop into?

Spermatogonium: most undifferentiated form part of the male germ line

Develops into: spermatocytes, male germ cells that undergo meiosis to produce sperm

How often is sperm released?

Daily basis

Every daughter spermatogonium produces _______ spermatids

4 equal

What is transformed into sperm that are motile? 

Haploid spermatid 

What is oogenesis?

Formation of egg within female body

Where does oogenesis occur?

Ovaries

What is oogonia, and what does it develop into? 

Oogonia: immature diploid cells in the ovaries of a female fetus 

Develops into: oocytes after meiosis, from fetal development until birth 

When is development of oocytes stopped? When does it continue again?

Stopped after meiosis I until puberty!

What is released monthly from the ovary during ovulation?

1 or few mature ovum (mature female reproductive cell that divides to create an embryo only after male fertilization)

Between what periods does ovulation last?

Puberty to menopause

Every daughter oogonium produces _________ and __________

1 mature ovum

3 polar bodies (non functional cells, byproduct of oogenesis, due to unequal cytokinesis)

How is meiosis II triggered?

• triggered during puberty

• by hormone called FSH (follicle stimulating hormone)

• ends with fertilization by sperm cell

What’s an autosomal trisomy?

Genetic condition where an individual has three copies of one of the 22 non-sex chromosomes (the autosomes) instead of the regular number of two

Which autosome does Down Syndrome affect? 

Chromosome 21 (Trisomy-21)

What autosome does Edward’s Syndrome affect?

Chromosome 18 (Trisomy-18)

What is a karyotype?

the number and type of chromosomes in a cell/individual

What is a karyogram?

A photo of an individual’s chromosomes

When are karyograms taken?

• Taken during mitosis when homologous chromosomes are condensed and easy to see

• Can be taken before or after the chromosomes separate

In what order are the chromosomes placed in a karyogram?

From largest (Chromosome 1) to smallest (Chromosome 22), followed by the sex chromosomes (X,Y) which determine biological sex