Chapter 6Chromosomes & Cell Reproduction

General Information

about 2 TRILLION cells are produced by an adult human body 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 Reproductiono asexual reproduction that produces identical offspringo Binary Fissiono two stages:

1. DNA is copied (so each copy with have genetic information

2. cell divides

Eukaryotic Cell ReproductionGene – a segment of DNA that codes for a protein or RNA molecule; determines how a person’s body develops and functions

Chromosome – DNA and proteins associated with DNA

As a cell prepares to divide…1. Chromosomes become

visible2. DNA is copied3. It coils up

Chromosome Parts:• Chromatids – two exact copies of DNA that make up each

chromosome• Centromere – the region of the chromsome that holds the two

sister chromatids together during mitosis

Human Chromosome Labeled:

Chromatid

Chromatid

Centromere

Chromatids are separated and one goes into each new cell, ensuring that each new cell will have the same genetics information as the original

How many chromosomes do humans have?23 sets (pairs) of chromosomes -> 46 chromosomes total

Homologous chromosomes (homologues) = chromosomes similar in size, shape, & genetic content

• You have 23 pairs of homologous chromosomes, one of the homologues in each pair is from your mother and one is from your father

• Think: who do I look like?

Species have different diploid numbers

Important Terms• When a cell contains two sets of chromosomes (one set of 23 from the individual’s mother and a second set of 23 from the individual’s father), we call these cells diploid cells• “di” means two• Cells that only contain one set of chromosomes (23) are called haploid cells• “ha” can start off the word half (or half the number of

chromosomes)• Somatic cells are all the cells in the body other than gametes• 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 of

a new individual

Of the 23 pairs of chromosomes in humans:

• 22 pairs (or 44 chromosomes) are autosomes, or chromosomes that are not directly involved in determining the gender of an individual

• 1 pair (or 2 chromosomes) contain genes that will determine the sex of the individual and these are called sex chromosomes (usually referred to as X & Y chromosomes)XX = Female XY = Male

Would this be a boy or girl?

The structure and number of sex chromosomes vary in different organisms.

• Some insects (grasshoppers) no Y so O stands for absence of chromosome • female is XX• male is XO• In birds, moths, & butterflies• males have XX and females are X

Karyotype• A photo of the

chromosomes in a dividing cell that shows the chromosomes arranged by size

• Can alert that there is an abnormality in an individual…why?

The presence of all 46 chromosomes is essential for normal development and functionTrisomy – humans with more than 2 copies of a chromosome will not develop properly

Polydactyly

Down syndrome = condition with extra copy of chromosome 21 (trisomy 21)

How does this happen?

1. All the egg cells in a female are present at birth, she will never make more. Males, on the other hand, produce new sperm throughout their adult lives.

Therefore, as a female gets older her eggs accumulate damage.

2. Mutations can occur.

Doctors are able to perform prenatal testing to check for certain genetic disorders.

Mutations = changes in an organisms’ structures

Four types of mutations:1. Deletion – piece of chromosome breaks off

completely; new cell will lack a certain set of genes; usually proves fatal to zygote

2. Duplication – a chromosome fragment that attaches to its homologous chromosomes; carries 2 copies of a certain gene

3. Inversion – a chromosome piece reattaches to the original chromosome but in a reverse orientation

4. Translocation – if a chromosome piece reattaches to a non-homologous chromosome

Deletion – piece of chromosome breaks off completely

Duplication – chromosome fragment attaches to its homologous chromosome

Inversion – chromosome piece reattaches to the original chromosome but in reverse orientation

Translocation – piece reattaches to a non-homologous chromosome