Chapter 6Chromosomes & Cell Reproduction
General Information
about 2 TRILLION cells are produced by an adult humanbody EVERY DAY
new cells are formed when old cells divide
cell division = cell reproduction
Examples: a cut that is healing, a fawn growing
Prokaryotic Cell Reproduction
o asexual reproductionthat produces identicaloffspring
o Binary Fission
o two stages:
1. DNA iscopied (so eachcopy with havegeneticinformation
2. cell divides
Eukaryotic Cell Reproduction
Gene – a segment of DNA that codes for a protein or RNAmolecule; determines how a person’s body develops andfunctions
Chromosome –DNA and proteinsassociated withDNA
As a cell prepares to divide…
1.Chromosomes becomevisible
2.DNA is copied
3.It coils up
Chromosome Parts:
•Chromatids – two exact copies of DNA that make up eachchromosome
•Centromere – the region of the chromsome that holds thetwo sister chromatids together during mitosis
Human Chromosome Labeled:
Chromatid
Chromatid
Centromere
Chromatids are separated and one goes into each newcell, ensuring that each new cell will have the samegenetics information as the original
How many chromosomes do humans have?
23 sets (pairs) of chromosomes -> 46 chromosomes total
Homologouschromosomes(homologues) =chromosomes similarin size, shape, &genetic content
•You have 23 pairs of homologous chromosomes, one of thehomologues in each pair is from your mother and one is fromyour father
•Think: who do I look like?
Species have different diploid numbers
Important Terms
• When a cell contains two sets of chromosomes (one set of23 from the individual’s mother and a second set of 23 fromthe individual’s father), we call these cells diploid cells
•“di” means two
• Cells that only contain one set of chromosomes (23) arecalled haploid cells
•“ha” can start off the word half (or half the number ofchromosomes)
• Somatic cells are all the cells in the body other thangametes
• Gametes are reproductive cells; ex. Sperm cell, egg cell
n = 23 chromosomes
(23)
(23)
2(23) = 46
haploid
+
haploid
diploid
=
Fertilization = fusion of 2 haploid gametes; forms a diploid
Fertilization
•A fertilized egg is called a zygote, it’s the first cell ofa new individual
Of the 23 pairs of chromosomes inhumans:
•22 pairs (or 44 chromosomes) are autosomes,or chromosomes that are not directly involvedin determining the gender of an individual
•1 pair (or 2 chromosomes) contain genes thatwill determine the sex of the individual andthese are called sex chromosomes (usuallyreferred to as X & Y chromosomes)
XX = FemaleXY = Male
Would this be a boy or girl?
The structure and number of sex chromosomes vary indifferent organisms.
• Some insects (grasshoppers) no Y so O stands for absenceof chromosome
• female is XX
• male is XO
• In birds, moths, & butterflies
• males have XX and females are X
Karyotype
•A photo of thechromosomes in adividing cell thatshows thechromosomesarranged by size
•Can alert that thereis an abnormality inan individual…why?
The presence of all 46 chromosomes is essential for normaldevelopment and function
Trisomy – humans withmore than 2 copies of achromosome will notdevelop properly
Polydactyly
Down syndrome =condition with extra copyof chromosome 21 (trisomy21)
How does this happen?
1.All the egg cells in a female are present at birth, shewill never make more. Males, on the other hand,produce new sperm throughout their adult lives.
Therefore, as a female gets older her eggsaccumulate damage.
2.Mutations can occur.
Doctors are able to perform prenatal testing to checkfor certain genetic disorders.
Mutations = changes in an organisms’ structures
Four types of mutations:
1. Deletion – piece of chromosome breaks offcompletely; new cell will lack a certain set of genes;usually proves fatal to zygote
2. Duplication – a chromosome fragment thatattaches to its homologous chromosomes; carries 2copies of a certain gene
3. Inversion – a chromosome piece reattaches tothe original chromosome but in a reverse orientation
4. Translocation – if a chromosome piecereattaches to a non-homologous chromosome
Deletion – piece ofchromosome breaks offcompletely
Duplication – chromosomefragment attaches to itshomologous chromosome
Inversion – chromosomepiece reattaches to theoriginal chromosome but inreverse orientation
Translocation – piecereattaches to a non-homologous chromosome