8-1 Cell Reproduction

Objectives

• Describe the structure of a chromosome.

• Compare prokaryotic chromosomes with eukaryotic chromosomes

• Explain the differences between sex chromosomes and autosomes.

• Give examples of diploid and haploid cells.

The Nucleic Acids history• Discovery of DNA

– named by biochemist Johann Friedrich Miescher (1844-1895) and his student

– isolated an acidic substance rich in phosphorus from salmon sperm

– believed it was heriditary matter of cell, but no real evidence

• Discovery of the double helix– by 1900:components of DNA were

known– by 1953: xray diffraction

determined geometry of DNA molecule

– Nobel Prize awarded in 1962 to 3 men: Watson, Crick and Wilkins but not to Rosalind Franklin who died of cancer at 37 from the xray data that provided the answers.

What’s DNA?

• Long thin molecule to store information on how to make proteins.

• If your cell was a basketball you’d have 40 miles of DNA crammed in there.

Nucleotide Structure• Nucleic acids like DNA are

polymers of nucleotides• Nucleotides consist of

– sugar• RNA - ribose• DNA - deoxyribose

– phosphate group– nitrogenous base

• next slide

Your cells grow and split

• You pack all that DNA in by coiling it up into chromosomes.– DNA & proteins that wrap up DNA (histones)

• We can stain and see chromosomes.

Chromosome

• Each half of the chromosome is identical– Chromatid: ½ of the chromosome

• Cells need these copies when they split in two.

• Centromere: Place in the middle where chromatids are joined– Like an anchor

Eukaryotes vs. Prokaryotes

• Prokaryotes don’t have as much DNA so they might only have 1 chromosome.

• Eukaryotes usually have more – up to more than 100.

Sex chromosomes and autosomes

• Animals have different kinds of chromosomes

• Sex chromosomes determine if you’re a boy or a girl

• Everything else is an Autosome

• Every cell in you has two copies of every autosome

• Two copies of chromosomes with the same size, shape, and genes are: Homologous chromosomes– Homo = the same

Chromosome Numbers in Different Species

• Buffalo 60• Cat 38• Cattle 60• Dog 78• Donkey 62• Goat 60• Horse 64• Human 46• Pig 38• Sheep 54

• If two species can interbreed what does that say about their chromosome ? It must be similar for example the horse & Donkey. How many chromosomes do you predict a mule would have? 63 since mules have an odd number of chromosomes they don’t match up; mating can’t happen no viable offspring

• Both lions and tigers have 38

Liger

Karyotypes

• Stained chromosomes lined up by size and a picture is taken

Extremes in Chromosome #

• The record for minimum number of chromosomes belongs to a subspecies of the ant Myrmecia pilosula, in which females have a single pair of chromosomes. This species reproduces by a process called haplodiploidy, in which fertilized eggs (diploid) become females, while unfertilized eggs (haploid) develop into males. Hence, the males of this group of ants have, in each of their cells, a single chromosome.

• The record for maximum number of chromosomes is found in found in the fern family. Polyploidy is a common conduction in plants, but seemingly taken to its limits in the Ophioglossum reticulatum. This fern has roughly 630 pairs of chromosomes or 1260 chromosomes per cell. The fact that these cells can accurately segregate these enormous numbers of chromosomes during mitosis is truly remarkable.

Myrmecia pilosula & Ophioglossum

Spectral Karyotype

• Fluorescent dyes are hybridized to the chromosomes

Diploid vs. Haploid Cells

• Cells with two sets of chromosomes; Diploid cells– Have two copies of each autosome and sex

chromosome– All of our cells except sperm and eggs are

diploid– Abbreviated 2n

• Haploid: only one set of chromosomes– Abbreviated 1n