Compare DNA copying, chromosome separation, mitosis, meiosis, and sources of genetic variation.
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Sister chromatids are copies of one chromosome joined after DNA replication. Homologous chromosomes are a maternal and paternal pair with the same types of genes, often carrying different alleles.
A replicated chromosome still counts as one chromosome until its sister chromatids separate. Chromosome number is usually tracked by centromeres.
During which part of the cell cycle is DNA replicated: G1, S, G2, or mitosis?
S phase of interphase.
G1 includes growth and ordinary cell activity. S copies DNA. G2 prepares for division, and the mitotic phase separates the copies and usually divides the cell.
During mitosis, chromosomes condense, the spindle attaches, chromosomes line up, sister chromatids separate, and new nuclei form. Cytokinesis divides the cytoplasm.
The familiar order is prophase, metaphase, anaphase, telophase. Some courses separate prometaphase. Animal cells pinch inward; plant cells form a cell plate.
A diploid cell with 12 chromosomes completes mitosis and cytokinesis normally. How many chromosomes are in each daughter cell?
12.
DNA is copied before division, then sister copies are separated so each daughter receives the same chromosome number as the starting cell.
Meiosis follows one DNA replication with two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II. The chromosome number is halved.
A typical diploid starting cell produces four haploid products. These are genetically varied because of crossing over and independent assortment.
Name two processes during meiosis that generate genetic variety.
Crossing over and independent assortment of homologous chromosomes.
Crossing over exchanges corresponding DNA segments between nonsister chromatids of paired homologs. Independent assortment comes from random orientation of homolog pairs at metaphase I.
A species has a diploid number of 20. What is the normal chromosome number in a gamete, and what number is restored at fertilization?
10 in a gamete; 20 in the fertilized zygote.
A haploid gamete has one member of each homologous pair. Fertilization joins two haploid sets and restores the diploid number.
Cell-cycle checkpoints help determine whether division should proceed. They respond to conditions such as DNA damage and proper spindle attachment. Cancer can develop when division-control systems fail.
A tumor is a mass of abnormal cells. Malignant cells invade surrounding tissues and can spread to distant sites, a process called metastasis.
What is nondisjunction, and what can it do to gametes?
It is failure of homologous chromosomes or sister chromatids to separate correctly, producing gametes with abnormal chromosome numbers.
After fertilization, the zygote may have an extra or missing chromosome. The exact pattern depends on whether the error occurred in meiosis I or II.