University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

303

Pages:

22
Author:

Kaitlynn Bridges

Chromosomal Basis of Inheritance Part 1 Ch. 3 1 INTRODUCTION ! In this chapter we will survey reproduction at the cellular level ! We will examine chromosomes at the microscopic level – This examination provides us with insights to understand the inheritance patterns of traits GENERAL FEATURES OF CHROMOSOMES ! Chromosomes are structures within living cells that contain the genetic material – They contain the genes ! Biochemically, chromosomes are composed of – DNA, which is the genetic material – Proteins, which provide an organized structure Animal cell Cytogenetics ! The field of genetics that involves the microscopic examination of chromosomes ! A cytogeneticist typically examines the chromosomal composition of a particular cell or organism – This allows the detection of individuals with abnormal chromosome number or structure – This also provides a way to distinguish between two closely-related species Cytogenetics ! Animal cells are of two types – Somatic cells " Body cells, other than gametes – Blood cells, for example – Germ cells " Gametes – Sperm and egg cells Cytogenetics ! In a cytogenetics laboratory, the microscopes are equipped with a camera ! Microscopic images can now be scanned into a computer ! There, they can be organized in a standard way, usually from largest to smallest ! A karyotype is the photographic representation of the chromosomes within a cell Karyotypes can be produced by cutting micrograph images of stained chromosomes and arranging them in matched pairs Human male karyotype 8 Female Karyotype 9 Cytogenetics ! Cytogeneticists use three main features to identify and classify chromosomes – – – 1. 2. 3. Size of arms Location of the centromere Banding patterns " Based on different staining dyes 10 Chromosomes Short arm; For the French, petite Long arm 11 Eukaryotic Chromosomes Are Inherited in Sets ! Most eukaryotic species are diploid (2n) – Have two sets of chromosomes ! For example – Humans " 46 total chromosomes (23 per set (n) ) – Dogs " 78 total chromosomes (39 per set (n) ) – Fruit fly " 8 total chromosomes (4 per set (n) ) Eukaryotic Chromosomes Are Inherited in Sets ! Members of a pair of chromosomes are called homologues – The two homologues form a homologous pair ! The two chromosomes in a homologous pair – Are nearly identical in size – Have the same banding pattern and centromere location – Have the same genes " But not necessarily the same alleles Eukaryotic Chromosomes Are Inherited in Sets ! The DNA sequences on homologous chromosomes are also very similar – There is usually less than 1% difference between homologues ! Nevertheless, these slight differences in DNA sequence provide the allelic differences in genes – Eye color gene " Blue allele vs brown allele Eukaryotic Chromosomes Are Inherited in Sets ! The sex chromosomes (X and Y) are not homologous – They differ in size and genetic composition ! They do have short regions of homology that allow for homologous pairing in meiosis…. Two homologous chromosomes labeled with 3 different genes The physical location of a gene on a chromosome is called its locus. Mitosis ! Eukaryotic cells that are destined to divide progress through a series of stages known as the cell cycle Figure 3.5 Synthesis Gap 1 Gap 2 Mitosis ! In actively dividing cells, G1, S and G2 are collectively know as interphase ! A cell may remain for long periods of time in the G0 phase – A cell in this phase has " Either postponed making a decision to divide " Or made the decision to never divide again – Terminally differentiated cells (e.g. nerve cells) Mitosis ! During the G1 phase, a cell prepares to divide ! The cell reaches a restriction point and is committed on a pathway to cell division ! Then the cell advances to the S phase, where chromosomes are replicated – The two copies of a replicated chromosome are termed chromatids – They are joined at the centromere to form a pair of sister chromatids Mitosis ! At the end of S phase, a cell has twice as many chromatids as there are chromosomes in the G1 phase – A human cell for example has " 46 distinct chromosomes in G1 phase " 46 pairs of sister chromatids in S phase ! Therefore the term chromosome is relatively confusing: – In G1 and late in the M phase " it refers to the equivalent of one chromatid – In G2 and early in the M phase " it refers to a pair of sister chromatids Mitosis ! During the G2 phase – the cell accumulates the materials necessary for " nuclear and cell division ! It then progresses into the M phase of the cycle – where mitosis occurs ! Purpose of mitosis is to distribute the replicated chromosomes to the two daughter cells – In humans for example, " The 46 pairs of sister chromatids are separated " Each daughter cell thus receives 46 chromosomes

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