Lecture 14: Organization of DNA into Chromosomes, Binary Fission, and the Eukaryotic Cell Cycle

Flashcards covering DNA organization in prokaryotes and eukaryotes, binary fission, the eukaryotic cell cycle (G1, S, G2, M phases), mitosis stages, chromosome and DNA counting, ploidy, and homologous chromosomes versus sister chromatids.

What are the three main reasons cells divide?

Reproduction (producing gametes), growth, and repair/tissue renewal.

What two main processes are involved in cell division?

1) Replication of DNA, and 2) segregation of DNA copies into two daughter cells.

How do the outcomes of mitosis and meiosis differ regarding daughter cells and DNA content?

Mitosis produces $2$ daughter cells with the same DNA as the parent cell (except for any new mutations), whereas meiosis produces $4$ gametes whose DNA is different from that of the parent cell.

How is DNA organized in prokaryotes (Bacteria and Archaea)?

Prokaryotes lack a nucleus or other cellular compartments; they have circular DNA organized into a single chromosome, along with plasmids.

What are plasmids and what do they typically carry?

Plasmids are circular DNA molecules smaller than the main chromosome that usually carry non-essential genes capable of giving a fitness advantage.

In this diagram of a prokaryotic cell, what two forms of circular DNA molecules are identified?

A single main chromosome and a smaller plasmid.

What environmental factor regulates bacterial and archaeal binary fission?

Nutrient availability (increased nutrients lead to increased cell division).

What are the three sequential steps of binary fission in prokaryotes?

1) DNA replication (creates $2$ copies of the genome), 2) segregation of DNA (one copy moves left and the other right), and 3) cell division / cytokinesis (produces $2$ daughter cells that are genetically identical to the parent cell barring mutations).

Where is DNA housed inside eukaryotic cells?

The nucleus contains multiple linear chromosomes, while the mitochondria and chloroplasts contain circular DNA.

What does the endosymbiotic theory state regarding eukaryotic organelles?

An early eukaryote engulfed a bacterium, which ultimately led to the evolution of mitochondria and chloroplasts.

What two functional classes of eukaryotic DNA are distinguished in the lecture?

Coding DNA (which codes for proteins) and non-coding DNA (regulatory sequences that turn genes on or off).

What are the four primary phases of the eukaryotic cell cycle?

$G_1$ phase (Gap 1), $S$ phase (DNA synthesis), $G_2$ phase (Gap 2), and $M$ phase (Mitosis and cytokinesis).

What characterizes cells in the $G_0$ phase of the cell cycle?

Cells in $G_0$ phase are not actively dividing.

What events occur in a eukaryotic cell during $G_1$ phase?

The cell grows, copies organelles, and prepares to replicate its DNA, with chromosomes remaining unreplicated.

What happens during the $S$ phase (synthesis phase) of the eukaryotic cell cycle?

DNA replication occurs, converting each unreplicated chromosome into a replicated chromosome made of $2$ identical sister chromatids.

What main functions take place during $G_2$ phase?

Proofreading and checking the replicated DNA, accompanied by continued cell growth.

What happens during $M$ phase of the cell cycle?

Mitosis and cytokinesis occur, wherein replicated chromosomes separate into different cells, dividing one parent cell into daughter cells.

Why are chromosomes not microscopically visible during interphase?

Because genetic information is actively being accessed by the cell, leaving the DNA molecules unspooled rather than tightly condensed.

What structural change happens to chromosomes during prophase of mitosis?

Mitosis begins and chromosomes condense, becoming visible like tangled strands of spaghetti.

What occurs during metaphase of mitosis?

Replicated chromosomes align along the metaphase plate in the middle of the cell.

What critical separation takes place during anaphase of mitosis?

Sister chromatids pull apart and move toward opposite poles of the cell.

What specific events occur during telophase and cytokinesis in animal cells?

In telophase, nuclear membranes reform around separated chromosomes; in cytokinesis, the cell divides its cytoplasm to form two separate daughter cells.

How many chromosomes and DNA molecules are present in this cell during metaphase of mitosis?

$3$ chromosomes and $6$ DNA molecules (each of the $3$ centromeres defines one replicated chromosome, and each replicated chromosome consists of $2$ sister chromatids/DNA molecules).

If this cell containing $5$ replicated chromosomes in $G_2$ divides through mitosis, how many chromosomes will each daughter cell possess?

$5$ chromosomes (each daughter cell receives one sister chromatid from each chromosome, maintaining the same chromosome number as the parent cell).

What is the genetic relationship between two sister chromatids of a replicated chromosome?

Assuming no mutations have occurred, sister chromatids are genetically identical and carry the exact same allele for each gene.

What are homologous chromosomes, and how do their alleles compare?

Homologous chromosomes are pairs of chromosomes (one maternal, one paternal) that carry the same genes, but can have different alleles for those genes.

During which type of cell division do homologous chromosomes pair up?

Homologous chromosomes only pair in meiosis I, never in mitosis.

What is the difference in chromosome copies and alleles between haploid and diploid cells?

Haploid cells ($n$) contain $1$ copy of each type of chromosome and $1$ allele per gene; diploid cells ($2n$) contain $2$ copies of each type of chromosome and $2$ alleles per gene.

Is the cell shown in metaphase haploid or diploid, and why?

Haploid, because it contains $3$ chromosomes of different sizes/types with no matching homologous pairs present.

What does this human karyotype show regarding human somatic cells?

Human somatic cells are diploid ($2n$), consisting of $22$ pairs of homologous autosomes and $1$ pair of sex chromosomes ($XX$), for a total of $46$ chromosomes.

How many total alleles per gene exist in a diploid eukaryotic cell during $G_1$ phase?

$2$ total alleles per gene (one on each homologous chromosome).

Before $S$ phase, what is the physical structure of a single unreplicated chromosome?

A single, double-stranded DNA molecule containing many genes with one allele per gene.