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Summary MEDICAL GENETICS Jorde 5th edition
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This document is a summary of chapters 1-2-3-4-5-6-12-13 from the book Medical Genetics 5th edition
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Test Bank for Medical Genetics, 5th Edition by Jorde, 9780323188357, Covering Chapters 1-8 | Includes Rationales
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Test Bank for Medical Genetics, 5th Edition by Jorde, 9780323188357, Covering Chapters 1-8 | Includes Rationales
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Test Bank for Medical Genetics, 5th Edition by Jorde, 9780323188357, Covering Chapters 1-8 | Includes Rationales
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Chapter 1Background and History
Medical genetics is the science of human biological variation as it relates to health and
disease.
Although people have long been aware that individuals differ, that children tend to
resemble their parents and that certain diseases tend to run in families, the scientific basis
for these observations was only discovered during the past 140 years.
The clinical applications of this knowledge are even more recent, with most progress
confined to the past 50 years
In particular, the rapid sequencing of the entire human genome, completed in 2003, has
greatly accelerated the process of gene mapping for genetic conditions and a vast quantity
of valuable and continuously updated information has become readily accessible via the
internet.
Medical genetics involves any application of genetics to medical practice and includes
studies of:
o the inheritance of diseases in families
o mapping of disease genes to specific locations on chromosomes
o analyses of the molecular mechanisms through which genes cause disease
o the diagnosis and treatment of genetic diseases
Medical genetics also includes genetic counseling, in which information regarding risks,
prognoses, and treatments is communicated to patients and their families.
There are several reasons health-care practitioners must understand medical genetics.
o All components of the human body are influenced by genes, genetic disease is
relevant to all medical specialties.
o Genetic diseases make up a large percentage of the total disease burden in pediatric
and adult populations. This percentage will continue to grow as our understanding
of the genetic basis of disease grows.
o In addition, modern medicine is placing increasing emphasis on prevention.
o DNA-based diagnosis is available for several thousand inherited conditions.
Genetics already plays an important role in the prevention and treatment of many
common diseases and disorders.
Our understanding of the contribution of genetic factors to disease occurrence and
progression is continuously growing up.
In 1990, the first attempts at human supplementation gene therapy for a single - gene
disorder (adenosine deaminase deficiency) were performed.
Different gene therapy methods have been devised since that day, depending on the
nature of the mutation, and several hundreds gene therapy trials are now underway.
,There are now many genetic conditions for which a precise diagnosis leads to significant
benefits in terms of clinical management. The detection and early treatment of other
complications such as diabetes and heart block in myotonic dystrophy.
Chapter 2
Basic cell biology
The nuclear genome in each human germ cell, ovum and sperm, is organized into a set of
23 separate chromosomes known as the haploid genome, which represents the unit
genome of humans.
The haploid genome of the ovum consists of 22 chromosomes that do not participate in sex
determination and referred to as the autosomes, and one sex-determining X chromosome.
Similarly, the haploid genome of the sperm consists of 22 autosomes, and one sex-
determining X or Y chromosome.
The zygote and all somatic cells contain a diploid genome. Upon fertilization, both
haploid genomes of the sperm and the ovum constitute a diploid genome consisting of
their 46 chromosomes, 44 autosomes and 2 sex determining chromosomes, either XX in
females or XY in males.
The diploid genome characterizes the nuclear genome of the zygote as well as all somatic
cells descendant from it.
The cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell leading
to its division and duplication of its DNA (DNA replication) to produce two daughter
cells.
During the course of development, each human progresses from a single-cell zygote (an
egg cell fertilized by a sperm cell) to a marvelously complex organism containing
approximately 100 trillion (1014) individual cells.
Because few cells last for a person’s entire lifetime, new ones must be generated to replace
those that die. Both of these processes— development and replacement—require the
manufacture of new cells. The cell division processes that are responsible for the creation
of new diploid cells from existing ones are mitosis (nuclear division) and cytokinesis
(cytoplasmic division). Before dividing, a cell must duplicate its contents, including its
DNA; this occurs during interphase. The alternation of mitosis and interphase is referred
to as the cell cycle.
The interphase is divided into three phases, G1, S, and G2.
o During G1 (gap 1, the interval between mitosis and the onset of DNA replication),
synthesis of RNA and proteins takes place.
o DNA replication occurs during the S (synthesis) phase.
o During G2 (the interval between the S phase and the next mitosis), some DNA
repair takes place, and the cell prepares for mitosis. By the time G2 has been
, reached, the cell contains two identical copies of each of the 46 chromosomes.
These identical chromosomes are referred to as sister chromatids. Sister chromatids
often exchange material during interphase, a process known as sister chromatid
exchange.
Cells divide in response to important internal and external cues. Before the cell enters
mitosis, for example, DNA replication must be accurate and complete and the cell must
have achieved an appropriate size. The cell must respond to extracellular stimuli that
require increased or decreased rates of division. Complex molecular interactions mediate
this regulation. Among the most important of the molecules involved are the cyclin-
dependent kinases (CDKs), a family of kinases that phosphorylate other regulatory
proteins at key stages of the cell cycle. To carry out this function, the CDKs must form
complexes with various cyclins, proteins that are synthesized at specific cell-cycle stages
and are then degraded when CDK action is no longer needed.
Mitosis
Mitosis is the process through which two identical diploid daughter cells are formed from
a single diploid cell. Although mitosis usually requires only 1 to 2 hours to complete, this
portion of the cell cycle involves many critical and complex processes.
Mitosis occurs in:
➢ Development and growth
➢ Cell replacement
➢ Regeneration
➢ Asexual reproduction
Mitosis is divided into several phases.
o During prophase, the first mitotic stage, the chromosomes become visible under a
light microscope as they condense and coil (chromosomes are not clearly visible
during interphase). The two sister chromatids of each chromosome lie together,
attached at a point called the centromere. The nuclear membrane, which surrounds
the nucleus, disappears during this stage. Spindle fibers begin to form, radiating
from two centrioles located on opposite sides of the cell. The spindle fibers become
attached to the centromeres of each chromosome and eventually pull the two sister
chromatids in opposite directions.
o The chromosomes reach their most highly condensed state during metaphase, the
next stage of mitosis. Because they are highly condensed, they are easiest to
visualize microscopically during this phase. For this reason, clinical diagnosis of
chromosome disorders is usually based on metaphase chromosomes. During
metaphase, the spindle fibers begin to contract and pull the centromeres of the
chromosomes, which are now arranged along the middle of the spindle (the
equatorial plane of the cell).
, o During anaphase, the next mitotic stage, the centromere of each chromosome splits,
allowing the sister chromatids to separate. The chromatids are then pulled by the
spindle fibers, centromere first, toward opposite sides of the cell. At the end of
anaphase, the cell contains 92 separate chromosomes, half lying near one side of the
cell and half near the other side. If all has proceeded correctly, the two sets of
chromosomes are identical.
o Telophase, the final stage of mitosis, is characterized by the formation of new
nuclear membranes around each of the two sets of 46 chromosomes. Also, the
spindle fibers disappear, and the chromosomes begin to decondense.
Cytokinesis usually occurs after nuclear division and results in a roughly equal
division of the cytoplasm into two parts. With the completion of telophase, two
diploid daughter cells, both identical to the original cell, have been formed.
Mitotic errors that occur early in the life of the embryo can affect enough of the body’s
cells to produce clinically significant disease.
Mitotic errors occurring at any point in one’s lifetime can, under some circumstances,
cause cancer.
Meiosis
When an egg cell and a sperm cell unite to form a zygote, their chromosomes are
combined into a single cell. Because humans are diploid organisms, there must be a
mechanism to reduce the number of chromosomes in gametes to the haploid state.
Otherwise the zygote would have 92, instead of the normal 46, chromosomes. The primary
mechanism by which haploid gametes are formed from diploid precursor cells is meiosis.
Two cell divisions occur during meiosis. Each meiotic division has been divided into
stages with the same names as those of mitosis, but the processes involved in some of the
stages are quite different. During meiosis I, often called the reduction division stage, two
haploid cells are formed from a diploid cell. These diploid cells are the oogonia in females
and the spermatogonia in males. After meiosis I, a second meiosis, the equational
division, takes place, during which each haploid cell is replicated.
The first stage of the meiotic cell cycle is interphase I, during which important processes
such as replication of chromosomal DNA take place.
o The second phase of meiosis I, prophase I, is quite complex and includes many of
the key events that distinguish meiosis from mitosis. Prophase I begins as the
chromatin strands coil and condense, causing them to become visible as
chromosomes. During the process of synapsis, the homologous chromosomes pair
up, side by side, lying together in perfect alignment (in males, the X and Y
chromosomes, being mostly nonhomologous, line up end to end). This pairing of
homologous chromosomes is an important part of the cell cycle that does not occur
in mitosis. As prophase I continues, the chromatids of the two chromosomes
intertwine. Each pair of intertwined homologous chromosomes is either bivalent
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