MCAT REPRODUCTION BIOLOGY | KEY CONCEPTS
EXPLAINED
1. Autosomal Cells: - Diploid (2n) - they contain two copies of each chromosome.
2. Germ Cells: - Haploid (n) - contain only one copy of each chromosome.
3. Cell Cycle: - For actively dividing cells, the cell cycle consists of four stages: G1, S, G2 and M.
- The first three stages are known collectively as interphase, the longest part of the cycle.
- During interphase, individual chromosomes are not visible with light microscopy. They are in a less condensed form
known as chromatin. This is because the DNA must be available to RNA polymerase so that genes can be transcribed.
4. G0 Stage: - Cells that are not actively dividing.The cell is simply living and serving its function.
- An ottshoot of G1 phase.
5. G1 Stage - Presynthetic Gap: - Cells create organelles for energy and protein production (mitochon-
dria, ribosomes and ER), while also increasing their size.
- Passage into the S stage is governed by a restriction point. Certain criteria such as containing the proper complement
of DNA must be met.
6. S Stage - Synthesis of DNA: - The cell replicates its genetic material so that each daughter cell will have
identical copies.
- After replication, each chromosome consists of two identical chromatids that are bound together at a specialized
region known as the centromere.
- The ploidy of the cell does not change even though the number of chromatids has doubled. Humans in this stage
still only have 46 chromosomes even though 92 chromatids are present. Cells entering G2 have twice as much DNA as
cells in G1.
7. G2 Stage - Postsynthetic Gap: - The cell passes through another quality control checkpoint. DNA has
already been duplicated and the cell checks to ensure that there are enough organelles and cytoplasm to divide between
two daughter cells.
- Furthermore, the cell checks to make sure that DNA replication proceeded correctly to avoid passing on an error to
daughter cells.
8. M Stage - Mitosis: - Consists of mitosis itself along with cytokinesis.
9. G1/S Checkpoint - Restriction Point: - The cell determines if the DNA is in good enough condition
for synthesis.
- If there has been damage to the DNA, the cell cycle goes into arrest until the DNA has been repaired.
- The main protein in control of this is p53.
10. G2/M Checkpoint: - The cell is mainly concerned with ensuring that the cell has achieved adequate size
and the organelles have been properly replicated to support two daughter cells.
- p53 also plays a role here.
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, MCAT REPRODUCTION BIOLOGY | KEY CONCEPTS
EXPLAINED
11. Cyclins/Cyclin Dependent Kinases (CDK): - Molecules responsible for the cell cycle.
- In order to be activated, CDKs require the presence of the right cyclins. During the cell cycle, concentrations of the
various cyclins increase and decrease during specific stages. These cyclins bind to CDKs, creating an activated CDK-cyclin
complex. This complex can then phosphorylate transcriptions factors.
- Transcription factors then promote transcription of genes required for the next stage of the cell cycle.
12. Meiosis: - Occurs in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes).
13. Similarities between Mitosis & Meiosis: - Genetic material must be duplicated.
- Chromatin is condensed to form chromosomes.
- Microtubules emanating from centrioles are involved in dividing genetic material.
14. Differences between Mitosis & Meiosis: - Mitosis consists of one round each of replication and
division, while meiosis consists of one round of replication followed by two rounds of division.
- Meiosis I results in homologous chromosomes being separated, generating haploid daughter cells - reductional
division.
- Meiosis II is similar to mitosis, in that it results in the separation of sister chromatids - equational division.
15. Numbers - Human Genome: - Composed of 23 homologous pairs of chromosomes, each of which
contains one chromosome inherited form each parent.
- Homologous pairs are considered separate chromosomes, while sister chromatids are identical strands of DNA
connected at the centromere.
- After S phase, there are 92 chromatids organized into 46 chromosomes, which are organized into 23 homologous
pairs.
16. Prophase I - Synapsis: - The chromatin condenses into chromosomes, the spindle apparatus forms and
the nucleoli and nuclear membrane disappear.
- First major ditterence between meiosis and mitosis occurs at this point: homologous chromosomes come together
and intertwine in a process called synapsis. Each chromosomes consists of two sister chromatids, so each synaptic pairs
contains four chromatids and is referred to as a tetrad.
- The homologous chromosomes are held together by a group of proteins called the synaptonemal complex.
17. Prophase I - Crossing Over: - Chromatids of homologous chromosomes may break at the point of
contact, called the chiasma and exchange equivalent pieces of DNA.
- This process is called crossing over and can be characterized by the number of crossover events that occur in one
strand of DNA, including single crossovers and double crossovers.
- Those chromatids involved are left with an altered but structurally complete set of genes. Such genetic recombination
can unlink linked genes, thereby increasing the variety of genetic combinations that can be produced via gametogen-
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EXPLAINED
1. Autosomal Cells: - Diploid (2n) - they contain two copies of each chromosome.
2. Germ Cells: - Haploid (n) - contain only one copy of each chromosome.
3. Cell Cycle: - For actively dividing cells, the cell cycle consists of four stages: G1, S, G2 and M.
- The first three stages are known collectively as interphase, the longest part of the cycle.
- During interphase, individual chromosomes are not visible with light microscopy. They are in a less condensed form
known as chromatin. This is because the DNA must be available to RNA polymerase so that genes can be transcribed.
4. G0 Stage: - Cells that are not actively dividing.The cell is simply living and serving its function.
- An ottshoot of G1 phase.
5. G1 Stage - Presynthetic Gap: - Cells create organelles for energy and protein production (mitochon-
dria, ribosomes and ER), while also increasing their size.
- Passage into the S stage is governed by a restriction point. Certain criteria such as containing the proper complement
of DNA must be met.
6. S Stage - Synthesis of DNA: - The cell replicates its genetic material so that each daughter cell will have
identical copies.
- After replication, each chromosome consists of two identical chromatids that are bound together at a specialized
region known as the centromere.
- The ploidy of the cell does not change even though the number of chromatids has doubled. Humans in this stage
still only have 46 chromosomes even though 92 chromatids are present. Cells entering G2 have twice as much DNA as
cells in G1.
7. G2 Stage - Postsynthetic Gap: - The cell passes through another quality control checkpoint. DNA has
already been duplicated and the cell checks to ensure that there are enough organelles and cytoplasm to divide between
two daughter cells.
- Furthermore, the cell checks to make sure that DNA replication proceeded correctly to avoid passing on an error to
daughter cells.
8. M Stage - Mitosis: - Consists of mitosis itself along with cytokinesis.
9. G1/S Checkpoint - Restriction Point: - The cell determines if the DNA is in good enough condition
for synthesis.
- If there has been damage to the DNA, the cell cycle goes into arrest until the DNA has been repaired.
- The main protein in control of this is p53.
10. G2/M Checkpoint: - The cell is mainly concerned with ensuring that the cell has achieved adequate size
and the organelles have been properly replicated to support two daughter cells.
- p53 also plays a role here.
1/9
, MCAT REPRODUCTION BIOLOGY | KEY CONCEPTS
EXPLAINED
11. Cyclins/Cyclin Dependent Kinases (CDK): - Molecules responsible for the cell cycle.
- In order to be activated, CDKs require the presence of the right cyclins. During the cell cycle, concentrations of the
various cyclins increase and decrease during specific stages. These cyclins bind to CDKs, creating an activated CDK-cyclin
complex. This complex can then phosphorylate transcriptions factors.
- Transcription factors then promote transcription of genes required for the next stage of the cell cycle.
12. Meiosis: - Occurs in gametocytes (germ cells) and results in up to four nonidentical sex cells (gametes).
13. Similarities between Mitosis & Meiosis: - Genetic material must be duplicated.
- Chromatin is condensed to form chromosomes.
- Microtubules emanating from centrioles are involved in dividing genetic material.
14. Differences between Mitosis & Meiosis: - Mitosis consists of one round each of replication and
division, while meiosis consists of one round of replication followed by two rounds of division.
- Meiosis I results in homologous chromosomes being separated, generating haploid daughter cells - reductional
division.
- Meiosis II is similar to mitosis, in that it results in the separation of sister chromatids - equational division.
15. Numbers - Human Genome: - Composed of 23 homologous pairs of chromosomes, each of which
contains one chromosome inherited form each parent.
- Homologous pairs are considered separate chromosomes, while sister chromatids are identical strands of DNA
connected at the centromere.
- After S phase, there are 92 chromatids organized into 46 chromosomes, which are organized into 23 homologous
pairs.
16. Prophase I - Synapsis: - The chromatin condenses into chromosomes, the spindle apparatus forms and
the nucleoli and nuclear membrane disappear.
- First major ditterence between meiosis and mitosis occurs at this point: homologous chromosomes come together
and intertwine in a process called synapsis. Each chromosomes consists of two sister chromatids, so each synaptic pairs
contains four chromatids and is referred to as a tetrad.
- The homologous chromosomes are held together by a group of proteins called the synaptonemal complex.
17. Prophase I - Crossing Over: - Chromatids of homologous chromosomes may break at the point of
contact, called the chiasma and exchange equivalent pieces of DNA.
- This process is called crossing over and can be characterized by the number of crossover events that occur in one
strand of DNA, including single crossovers and double crossovers.
- Those chromatids involved are left with an altered but structurally complete set of genes. Such genetic recombination
can unlink linked genes, thereby increasing the variety of genetic combinations that can be produced via gametogen-
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