Chapter 4.5

The Nucleus

AP BiologyFall 2010

Objectves

• Describe the structure and function of the components of the nucleus

Nuclear Functions

• Keeps the DNA from getting tangled with cytoplasmic machinery

• Isolates DNA from potentially damaging interactions • Outer membranes are a boundary – Cells control the movement of substances to and from the

cytoplasm

Nuclear Envelope

• Nuclear Envelope: double membrane system , encloses the semifluid interior of nucleus (nucleoplasm)

• Made up of two lipid bilayers

Nuclear Envelope

• The ribosome-bound outer surface of the nucleus is loaded with pores – Envelope contains many proteins – Ions and small, water soluble molecules cross

envelope only at pores

• The outer surface is continuous with the endoplasmic reticulum membrane

Nucleolus

• Nucleolus: located within the nucleus – Appears as a darker globular mass– Subunits of ribosomes are prefabricated before

shipment out of nucleus – Constructed from ribosomal RNA and proteins

Nucleolus

• Every nucleus has at least one nucleolus, which is responsible for producing the large and small subunits of ribosomal RNA

• Subunits don’t join together until after they have left the nucleus– One large and one small subunit join as an intact

ribosome during protein synthesis

Chromosomes

• Chromatin: cell’s total collection of DNA and associated proteins

• Chromosome: double stranded DNA molecule and its associated proteins (histones)

Chromosomes

Chromosomes

• When eukaryotic cell is not dividing, you can’t see individual DNA molecules

• When cell is preparing o divide, it copies all of its DNA

• Duplicated molecules become visible, condense into compact structures

Review

1. What is the function of the nuclear envelope?

2. What are the differences between chromosomes and chromatin?

Answers

1. The nuclear envelope’s function is to enclose and keep separate the semifluid interior of the nucleus and to allow selective passage in and out of the nucleus for materials.

2. A chromosomes is a double stranded DNA molecule and its associated proteins while chromatin refers to the cell’s total collection of DNA and associated proteins.