section 8.1 chromosomes. chromosome structure chromosomes are visible during cell reproduction only...
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Section 8.1
Chromosomes
Chromosome structure
Chromosomes are visible during cell reproduction only
Chromatin – the same material in between cell divisions (uncoiled)
Each chromosome is made of 2 parts:
1. A single DNA molecule tightly coiled around
2. Proteins called histones
Each chromosome has:
Two identical halvesEach half is called a chromatidChromatids are made before cell divisionChromatids are joined close to the
center at a spot called the centromere
Chromosome structure
Types of chromosomes
Sex chromosomes – determine the sex of an organism; may also carry other information
In humans sex chromosomes are X and Y Normal males have XY; normal females have
XX Autosomes – all the other chromosomes
(body chromosomes) In humans there are 44 autosomes (46 total)
Homologous chromosomes
In sexually reproducing organisms, every cell has two copies of each autosome
One copy comes from each parentThese are called homologous
chromosomes or homologuesThey are the same size and shape, and
carry information about the same traits
Karyotypes
A picture of the chromosomes taken during cell reproduction
All the chromosomes are arranged in homologous pairs
Can be used to detect genetic defects, such as incorrect chromosome number
Chromosome number
Diploid cells – 2 sets of chromosomes 2 of each autosome + 2 sex chromosomes Represented with 2n (humans n=23; 2n=46) Haploid cells – 1 set of chromosomes Only one autosome of each homologous pair + only 1
sex chromosome Represented with 1n Egg and sperm cells are haploid after fertilization
the new cell is diploid All eggs have one X chromosome; sperm have either
X or Y sperm determine the sex of the child
Chromosome number
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