BIOL 205 Study Guide - Fall 2018, Comprehensive Midterm Notes - Zygote, Zygosity, Y Chromosome

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12 Oct 2018
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BIOL 205
MIDTERM EXAM
STUDY GUIDE
Fall 2018
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Cell Division, Genetics, and Molecular Biology
Chromosomes and the Cell Cycle
o The characteristics of each individual are coded for by sections of DNA called genes
o Each gene codes for one trait, such as hair color
o Chromosomes are long strings of genes lined up in a specific sequence. The more genes
in a species the greater number of chromosomes needed
o Humans have approximately 20 000 genes which fit into 23 chromosomes. However
because humans are sexually reproducing organisms, the normal human chromosome
number is 46
o Two chromosomes in a nucleus that have the same gene sequence come from each
parent are called a homologous pair
o All cells that have homologous pairs are said to be diploid or have the 2n chromosome
number, which in humans is 46. All cells that have derived from the zygote by mitosis
are also 2n
o If all zygotes have the diploid number of chromosomes then egg and sperm cannot; they
must have only 23
o Diploid cells in the ovary or testis undergo meiosis to produce cells with the haploid or n
chromosome number, which in humans is 23
o When a 2n ovary or testis cell undergoes meiosis to halve the chromosome number,
only one chromosome of each homologous pair is allowed to enter a gamete. Whether
that chromosome is maternal or paternal is completely random
o Each haploid sperm will therefore be a unique combination of the man's maternal and
paternal chromosomes
o Each haploid egg will be a unique combination of the woman's parents' chromosomes
o When a sperm and egg unite in fertilization, the diploid chromosome number is restored
in the zygote and every cell that is derived from it in mitosis. The 20 000 genes
expressed in an offspring are a unique combination that is never repeated in any
individual except identical twins
o Mitosis is cell division that produces two cells that are genetically identical to each other
and to the mother cell
o Mitosis occurs when an organism grows, or needs to repair itself
o The rate of mitosis is mostly dependent on an individual's age. When one is very young,
the rate of mitosis is far greater than the rate of cell death. Extremely rapid rates of
mitosis are typical of cancers
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o The period from one cell division to the next is called a cell cycle, which is divided into
three stages: interphase, mitosis, and cytokinesis
o One cell cycle can be hours or years long depending on the type of cell
o Cells spend most of each cell cycle in interphase, during which the cell carries out its
normal functions and grows larger
o Interphase can be divided into three phases: G1, S and G2
o During Gap 1 (G1 phase), the cell grows. Chromosomes are not visible during
interphase; at this point they are long strings of uncondensed DNA called chromatin.
During the S, or synthesis phase, the cell begins to prepare for cell division by replicating
the DNA. The 46 strings of chromatin (23 from each parent) are replicated. Each
chromatin string and its replicate become buttoned together by centromere, and are
referred to as sister chromatids. In Gap 2 (G2 phase) the cell continues growing until its
volume to surface are ratio becomes too large, and cell division becomes necessary
o The goal of mitosis is to separate the 46 sets of sister chromatids into two complete sets
- one chromatid from each pair of sister chromatids should go to each daughter cell
o Mitosis has 4 stages: prophase, metaphase, anaphase, and telophase
o In metaphase the chromosomes migrate to the metaphase plate and line up along the
cell equator. A spindle fibre attaches to each centromere
o In anaphase, the centromeres divide so each sister chromatid has its own centromere.
Because the definition of a chromosome is any DNA held together by a centromere,
each sister chromosome is now its own chromosome. As anaphase continues the
spindle fibres attach to the centromeres and proceed to pull the sister chromatids apart,
forming a cluster of chromosomes at each pole
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