each chromosome and the duplicate piece of DNA that was just made are held in the center by this region
How many chromosomes does the cell have?
we still describe the cell as having 46 chromosomes, each consisting of two chromatids
Describe the process of "prophase"
the centrioles move away from each other to opposite sides of the cell, form a bunch of fibers (mitotid spindle) that attach to the chromosomes at their centromeres and help to push/pull them around during mitosis, chromosomes condense, and nuclear membrane begins to break up
Describe the process of metaphase
the chromosomes line up (moved by spindle fibers) at the equator of the cell
Describe the process of anaphase
the centromere that joins each pair of chromatids splits in two so that each chromatid separates from its partner (each chromatid now chromosome); newly separated chromosomes move toward opposite poles of the cell; the cell physically begins splitting in two
Describe the process of telophase
a nuclear membrane forms in each new cell and two daughter cells result (each 46 chromosomes); then divides in cytokinesis
What happens after a cell undergoes mitosis?
the two daughter cell that it produces enter interphase
What is the "one-gene-one-protein" theory?
each gene corresponds to a single protein; when we say "gene," referring to some portion of a chromosome that gives rise to one protein molecule
What happens during meiosis?
1. cell undergoes DNA replication during interphase; all chromosomes replicate and left with a cell that still has 46 chromosomes each made up of two chromatids joined by a centromere
2. replicated chromosomes are split up in the course of two sets of divisions: prophase I, metaphase I, anaphase I, telophase I, and prophase II, metaphase II, anaphase II, and telophase II
3. the differences between mitosis and meiosis found during the first set of divisions
Describe "Meiosis I"
the first set of four phases: prophase, metaphase, anaphase, and telophase I; chromosomes already replicated and found as two chromatids held together at the centromere
What is the difference between Meiosis I and the four phases of mitosis?
at the very beginning, homologous chromosomes pair up in a process called synapsis
Describe "prophase I"
synapsis occurs; all chromosomes have to find their homologous partner and pair up, longest phase of meiosis; spindle formed, chromosomes condense, nuclear membrane disintegrates
What happens after synapsis (Prophase I)?
"crossing over" takes place, where the like segments on homologous chromosomes are exchanged
because each pair of replicated chromosomes consists of four chromatids (two chromatids per replicated chromosome , and two replicated chromosomes) it can be called a "tetrad"
Describe "metaphase I"
the chromosomes line up on the equator of the cell; chromosomes stay in their homologous pairs
Describe "anaphase I"
unlike anaphase of mitosis, the centromeres in meiosis do not divide; instead, homologous pairs separate, with one entire replicated chromosome (a pair of chromatids and a centromere) moving to opposite poles of the cell
Describe "telophase I"
the two cells finish cytokinesis and nuclear membranes reform around the chromosomes
What is the result of "telophase I"?
because there are no homologous pairs (only 23 replicated chromosomes) the cells are considered haploid
What is the difference between Meiosis I and Meiosis II?
in meiosis I, we start off with 46 chromosomes, but in meiosis II, we start off with 23 chromosomes
Describe Meiosis II.
prophase II, spindle forms, nuclear membrane disintegrates, DNA condenses, but not pairing of chromosomes this time; metaphase II, chromosomes line up individually along equator; anaphase II, centromere splits and the chromatids divide (called chromosomes again); telophase II, nuclear membrane forms around the newly split chromosomes and left with four haploid cells
What happens to the haploid cells after telophase II?
the four haploid cells do not replicate any further unless fertilization triggers new cell cycles
What do spermatogonia live in?
tiny tubules called seminiferous tubules (located in testes; male gonads)
Describe the first three steps of "spermatogenesis"
1. spermatogonium replicates all chromosomes during interphase (46 chromosomes) and each chromosome is made of two chromatids joined by a centromere
2. cell undergoes prophase I, homologous chromosomes undergo synapsis and crossing over occurs
3. cell undergoes metaphase I where paired chromosomes lind up on spindles at the equator; two centromeres on each spindle fiber
Describe the next two steps of "spermatogenesis" after the first three steps.
4. cell undergoes anaphase I, but the centromeres don't divide; homologous chromosome pairs separate
5. cell finishes dividing during telophase I and now have two cells; each cell 23 chromosomes and each chromosome made up of two chromatids still joined by centromere; cells are haploid
Describe the last step of spermatogenesis
6. each of cells then go through meiosis II; chromosomes condense but do not pair up during prophase II; line up individually along spindle fibers during metaphase II, centromeres divide during anaphase II, cells finish telophase II and have four cells of 23 unreplicated chromosomes (haploid)
How often does spermatogenesis occur?
occurs on a daily basis, beginning at puberty and lasting the entire lifetime of the male
Where are primary oocytes found?
found in ovaries in the female reproductive system; ovary is the female gonad
What happens to the two daughter cells from meiotic division in oogenesis?
the two daughter cells simply disintegrate
How often does oogenesis occur?
occurs on a monthly basis, beginning at puberty and ending at menopause
end of regular menstrual cycles, an event that usually occurs between the ages of 46 and 54; a single ovum is produced per month