6  •  Chromosomes and Cell Division 231

Continuity and Variety

There are different types of cell division. 232

6.1

Immortal cells can spell trouble: cell division in sickness and in health. 232

6.2

Some chromosomes are circular, others are linear. 234

6.3

There is a time for everything in the eukaryotic cell cycle. 236

6.4

Cell division is preceded by chromosome replication. 238

Mitosis replaces worn-out old cells with fresh new duplicates. 242

6.5

Most cells are not immortal: mitosis generates replacements. 242

6.6

Overview: mitosis leads to duplicate cells. 243

6.7

The details: mitosis is a four-step process. 245

6.8

Cell division out of control may result in cancer. 246

Meiosis generates sperm and eggs and a great deal of variation. 250

6.9

Overview: sexual reproduction requires special cells made by meiosis. 250

6.10

Sperm and egg are produced by meiosis: the details, step by step. 252

6.11

Male and female gametes are produced in slightly different ways. 255

6.12

Crossing over and meiosis are important sources of variation. 257

6.13

What are the costs and benefits of sexual reproduction? 258

There are sex differences in the chromosomes. 260

6.14

How is sex determined in humans? 260

6.15

The sex of offspring is determined in a variety of ways in non-human species. 262

6.16

This is how we do it: Can the environment determine the sex of a turtle’s offspring? 263

Deviations from the normal chromosome number lead to problems. 265

6.17

Down syndrome can be detected before birth: karyotypes reveal an individual’s entire chromosome set. 265

6.18

Life is possible with too many or too few sex chromosomes. 268

StreetBIO: KNOWLEDGE YOU CAN USE

Can you select the sex of your baby? (Would you want to?) 270