The Cell Cycle and Mitosis (04:25)
In cell division a cell splits and becomes two cells identical to the original; this process happens in the nucleus. The cell nucleus has a nuclear membrane and contains rRNA in the nucleolus and DNA in the chromatin. Animal cells have 2 sets of chromosomes with 23 chromosomes each.
During interphase the cell grows and duplicates its chromosomes and organelles in three phases, G1, S, and G2.
In prophase, the nuclear membrane and nucleoli disappear, and chromatin becomes distinct chromosomes attached to their duplicates at centromeres. Mitotic spindles form outside of the nucleus then attach to the centromeres.
Metaphase, Anaphase, and Telophase (00:59)
In metaphase the chromatids line up in the center of the cell, and in anaphase the mitotic spindles pull them apart. In telophase a nuclear envelope forms around each set of chromosomes and the cell begins to divide.
Cytokinesis starts in telophase as the cytoplasm divides, and then it finishes the cell division process. Animal and plant cells have different cytokinesis processes: in plant cells, a cell plate forms to split the cell in two, while animal cells have a cleavage furrow that pinches the cell apart.
Conclusion and Review (01:14)
In mitosis cells divide to produce two identical copies of the original. This occurs in all cells except sex cells. A phrase may help students remember the phases: "Interesting Pro Met Anna on the Telephone."
Reproduction and Heredity (03:05)
Cell division in sex cells is called meiosis. In asexual reproduction one parent makes an exact copy of its genes and passes it to its offspring. In sexual reproduction two parents' genes combine to produce unique offspring.
Meiosis starts with one cell that has two sets of chromosomes, and it ends with four different cells, each with one set. During interphase the chromosomes duplicate and remain attached. Unlike in mitosis, the chromosome count remains the same; two chromatids attached are considered one chromosome.
Prophase I (02:17)
Before prophase I, the cell contains chromosomes which are two identical chromatids attached. Chromosomes from each parent group together in pairs based on the type of gene they contain. The chromatids mix some of their genetic information in this stage, and the nuclear envelope disappears.
Metaphase I, Anaphase I, Telophase I, and Cytokinesis (01:03)
The tetrads of homologous pairs arrange in a line in the center of the cell, and then spindle fibers pull them apart. Chromatids are still intact, but no longer identical after the exchange of genetic information. In telophase I, the nuclear membrane forms, and in cytokinesis the cell divides, leaving two cells with a mixture of genes from both parents.
Meiosis II (01:28)
During prophase II and metaphase II, chromosomes line up in each cell and spindle fibers connect to centromeres. In anaphase II, spindle fibers pull chromatids apart, and in telophase II and cytokinesis the two cells split apart to form four cells. Gamete cells have three chromosomes each, half of the original.
Conclusion and Review: Mitosis and Meiosis (01:33)
Gamete cells are haploid because they will join another gamete to reproduce and form a diploid cell. The combination of genes from homologous chromosomes leads to different traits in offspring. Duplication of DNA and cell division leads to two identical diploid cells in mitosis and four distinct haploid cells in meiosis.
Mendel's Laws of Inheritance (05:11)
Gregor Mendel was one of the first to explore genes and inheritance. He crossbred pea plants to study the inheritance of different traits. Through experimentation he discovered that for each gene there are two alleles, one of which is dominant, and these alleles separate during sexual reproduction so they can recombine with another allele from the other parent.
Inheritance Patterns (08:41)
With Mendel's laws scientists can predict the frequency with which a certain trait will be expressed. The genotype has either two of the same allele, or one dominant and one recessive allele, and this determines the physical trait, or phenotype. A Punnett square can help predict the phenotype of the offspring using the genotypes of the parents.
Sex-Linked Traits (02:14)
Humans have two sets of 23 chromosomes, one of which determines sex. Females have two X chromosomes and males have an X chromosome and a Y chromosome. Genes on these chromosomes are sex-linked, and inheritance of these in sons and daughters can be predicted using a Punnett square.
Credits: Heredity and Inheritance (00:17)
Credits: Heredity and Inheritance
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