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Structure of Chromosomes
Next Generation: HS.LS3.1 HS.LS3.1

Structure of Chromosomes

Author: Amanda Soderlind
Description:

Understand the structure of DNA.

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Tutorial

Source: Video and Images Created by Amanda Soderlind

Video Transcription

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Welcome to this lesson today on the structure of chromosomes. Today, we will be discussing the structure, as well as the function, of chromosomes. Chromosomes are basically condensed DNA molecules that appear when the cell is preparing to divide. And our chromosomes contain our genetic information.

So basically, when a cell is getting ready to divide, our genetic information, which is in the form of DNA, will condense into this structure called a chromosome. And this is how genetic information is passed from parent to offspring. What you're looking at right here is a pair of homologous chromosomes. Homologous chromosomes are chromosomes that contain the same set of genes. They're the same length and the same shape.

One of these chromosomes is from our mom, and one of these chromosomes is from our dad. One chromosome is from each parent. And we call these homologous chromosomes. They contain the same information.

Basically, we have two types of chromosomes within our body. We have autosomes. And we have sex chromosomes.

Now, autosomes are all the chromosomes in our body except for the sex chromosomes. The sex chromosomes are the chromosomes we have that are associated with sex and gender. The chromosome number is the number of chromosomes in a species cells. For humans, our chromosome number is 46. This means that we have 46 chromosomes in our cells.

Of those 46 chromosomes, most of them are autosomes. Only two of those chromosomes are sex chromosomes. We have 46 chromosomes or 23 pairs of chromosomes.

Then the chromosome number is different for different species. For example, a mouse has a total of 40 chromosomes. This chromosome number is going to be different for different species. But for humans, our chromosome number is 46.

Now, chromosomes, as I mentioned, are only visible in this form, when the cell is preparing to divide. They're in this condensed form as a cell is preparing to divide. So the rest of the time, our genetic information can be found in the form of chromatin, which is kind of like this thread-like form which is found within the cell's nucleus.

Our genetic information is generally contained within our cell's nucleus in the form of chromatin, except for when the cell is getting ready to divide. And then, we can find it as chromosomes. But just for the sake of this lesson, you need to know that genetic information is found in the nucleus of our cells.

And chromosomes are composed, as I mentioned, of DNA. So let's take a brief look at the structure of DNA. DNA is said to be in the structure of a double helix. It's basically like a ladder that's kind of been twisted.

The outside part of the ladder is made up of a phosphate-sugar backbone. It's composed of phosphate molecules and also deoxyribose sugar molecules. Then, the rungs of the ladder are made up of nitrogen bases, adenine, thymine cytosine, and guanine.

These nitrogen bases compose to base pairs. One base pair is adenine with thymine. Adenine always pairs with thymine on the DNA structure. And cytosine always pairs with guanine. So this is our other base pair-- cytosine and guanine.

That is the structure of DNA. When the cell is getting ready to divide, that DNA will coil and condense to form this structure of chromosomes. Otherwise, as I mentioned, DNA is found in the form of chromatin within the nucleus of our cells. So this lesson has been an overview on the structure of chromosomes.

Terms to Know
Adenine (A)

A nucleotide building block of DNA and RNA, adenine is classified as a purine and complements thymine (T) in DNA and uracil (U) in RNA.

Autosomes

All of the chromosomes in the body except for sex chromosomes.

Base Pair

The way that nucleotides interact with one another. In DNA, A bonds with T, and C bonds with G. In RNA, C bonds with G, and A bonds with U (uracil). The sequence of base pairs creates the genetic code that is transcribed and translated into proteins.

Chromosome

A condensed DNA structure.

Cytosine (C)

A nucleotide building block of DNA and RNA, cytosine is classified as pyrimidine and complements guanine (G) in DNA and RNA.

Double Helix

The shape of the DNA molecule; often is referred to as the “twisted ladder” ("Double Helix" is the title to the book about Watson & Crick's discovery of DNA's structure).

Gene

A segment of DNA that codes for a specific protein, genes are a sequence of nucleotides.

Guanine (G)

A nucleotide building block of DNA and RNA, guanine is classified as a purine and complements cytosine (C) in DNA and RNA.

Homologous Chromosomes

Chromosomes that are paired together that are the same length and shape and contain the same sets of genes. Typically, one of the homologous pair is contributed by each parent.

Mutation

A change in the nucleotide sequence.

Nucleotide

Organic molecules that consist of a 5 carbon sugar (ribose in the case of RNA, and deoxyribose in the case of DNA), a phosphate group and a nitrogenous base; nucleotides are the building blocks of nucleic acids (DNA & RNA).

Nucleotide Sequence

The arrangement of nucleotides that form genes in strands of DNA.

Sex Chromosomes

Chromosomes associated with sex and gender.

Thymine (T)

A nucleotide building block of DNA, thymine is classified as pyrimidine and complements adenine (A) in DNA; thymine is not found in RNA.